CHAPTER VI MAIN FISH DISEASES AND THEIR CONTROL

1. The significance and principles of fish disease control in China

1) Significance

The purpose of preventing fish disease is entirely for bettering fishery industry, improving farming production and the proliferation of fish resources.

As fish live in water, which is a complicated environment, so they are susceptible to infections of virus, bacteria, fungus or invasions of parasites, etc.

If fish is sick, a malaise will affect their normal growth and development while a serious case can cause mortality, which will jeopardize the regular aquaculture and threaten the expected increase of fish yields. So the control of fish disease is one of the most vital steps towards fish culture.

2) The principles of fish disease control in China

The principles of fish disease control in China are “all-round prophylaxis and positive treatment, prevention is better than treatment” and the positive methods practised are “prevention during diseasefree, timely treatment upon early stage”. Because of aquatic ambience, it is not easy to get aware of the activities of fish. It is difficult to conduct a correct diagnosis and timely treatment after an affection. This is because the drugs for oral administration can only be effective through ingestion of fish. In other words, if fish fall ill and have lost the feeding ability, and then no medicine can help. Of course, we can conduct a parenteral administration instead, but dip treatments are confined to small containers or spread measures to fish ponds. It is not practical for large lakes, rivers or reservoirs; therefore, prevention is always more important than treatment in the control of fish diseases.

2. Major types of fish diseases

From different pathogenesis, fish diseases are simply classified into infectious diseases, invasive diseases and the diseases caused by other factors.

1) Infectious disease

This type of disease is mainly caused by the pathogens of virus, bacteria, fungi or unicellular algae, that is also called “ichthyoml crobial disease”. For instance, bacterial enteritis, bacterial gill rot and bacterial erytherma.

The loss of fish production from infectious diseases accounts for 60% of all diseased . It indicates that such a disease ranks a noticeable place among the other diseases. For this reason, the study of infectious disease is of primary significance to the development of aquaculture.

(1) Characteristics of infectious disease

1. Infectious disease can be divided into acute, subacute and chronic forms based on clinic picture, e.g. if enteritis of Grass carp or Black carp occurs in an acute form; it develops rapidly and soon reaches the high peak of mortality. It also goes off quickly. On the contrary, chronic enteritis kills only a few fish per day, but it lasts a long time.

2. Bacterial pathogens of infectious diseases are not strictly parasitic microorganisms. If the condition for parasitism is unsuitable, it will lead a saprophytic life. These pathogens have a high adaptability to the changes of environmental conditions, e.g. the pathogen of enteritis of Grass carp and Black carp will not cause any diseases if water temperature is below 20°C; whereas the temperature between 20°C and 25°C will enhance the virulence remarkably. That's why its main epidemic season begins from late spring to early summer, so zoospores of Saprolegnia often attach to intact fish skin and do not bring disease to fish unless the host has injured parts where the zoospore will rapidly grow and multiply, and finally fish will be infected.

3. Most pathogens of infectious diseases show a preference for certain spp. some fish and to certain organs (organotropism), e.g. bacteria of enteritis only affect Grass carp and Black carp, not Silver carp or Bighead; likewise, branchiomyces parasitize the gill instead of other parts of a fish body.

(2) The course and forms of infectious diseases

Throughout the whole course there is a latent period, a symptomatic period and in attacking period.

The infectious diseases can be divided into two forms: pure infection (a fish infected by one kind of causative agent) and mixed infection (infection with over two kinds of pathogens on a single fish): e.g. a sick Grass carp suffering both from saprolegniasis and gill rot; or a Black carp with a complication of enteritis and red skin disease.

(3) The origin of infectious diseases and the mode of infection

Regarding the origin, there are two kinds of sources: primary source and secondary source. Most sick fish as primary infective source are the carriers of pathogens of variant infectious diseases in natural world. The pathogen infects intact fish school by virtue of direct contact or the discharge of morbific agents into ambient water. Sometimes healthy fish themselves in pond are the “carriers”, the outbreak of fish disease will happen if conditions are favourable.

Disease-free pond could be polluted by contact with the pond water, coming from diseased ponds, with contaminated feeds, gears, etc. resulting in an occurance of disease. These are so called secondary source. Pathogens are widely spread through these approaches.

(4) Resistance of fish against infectious diseases:

Infectious diseases normally attack fish body through tissues and organs, such as skin, gill, intestine tract or excretory organ but fish have their own resistance against pathogenic microbes. Such a surface texture of skin and mucous membrane of fish functions as a screen to keep the infectious microorganisms out of it. Besides, the lysozyme which organs have can kill the bacteria. So a complete skin and membrane are the first protective screen. The pathogenic microbes entering digestive tract will be under the influence of high disinfecting secretions from the organs of digestive tract. White blood cell, lymphoid tissue cell, and reticuloendothelial cells of spleen, foreign body can not only catch pathogenic microbes but also pathogenic microorganisms. In addition, blood of fish contains bactericidin which can eradicate all kinds of pathogenic bacteria.

2) Invasive diseases

Such diseases are caused by animal parasites, like trichodinasis, ichthyphthiriasis, lernaesis, argulusis, etc.

Fish carrying parasites or corpse of diseased fish are the direct sources of invasive disease. It is called the primary source. And objects accompanied with direct source, such as contaminated feeds, gears, pond water and silt, etc. are called indirect source, or secondary source. For examples matured oocysts of Eimeria or matured myxosporidia may enter water in large numbers together with fish, and precipitate onto the pond bottom, so the pond silt is the secondary source.

The occurance and spreading of infectious and invasive diseases often appear in different seasons, because the pathogens and fish are influenced by outside factors (such as place, climate, physicochemical property of water and farming skills, etc.) and inner factors (such as growth and physiological status), e.g. Saprolegnia may spread in winter and spring but may not in summer or autumn. Besides, pathogens often show their special selectivity, e.g. Cryptobia only attacks Grass carp, nor Black carp, Silver carp or Bighead in the same pond; Lernae polymorpha parasitizes Silver carp, Bighead and Wuchang fish while Lernaea ctenopharyngodontis parasitizes Grass carp.

3) Diseases caused by other factors

Physical and chemical influence or other organism within the pond may retard the growth or even kill the fish, e.g. gasping and suffocation due to anoxia may direct lead to inbalance of physiological function inside fish body, or lead to mass mortality if it is serious. The harmfulness is no less than that of pathogens.

3) General knowledge of fish disease control

1) Prevention of fish diseases

(1) Special importance of prophylaxis

“Prevention is better than treatment” has been a radical theory in medical science. Fish are schooling animals in water which brings up difficulties in observation, diagnosis and treatment. Apart from this, some effective drugs and measres to cure certain fish diseases are still unknown up to now, e.g. diplostomulumsis. Therefore, perfect preventive measures must be taken since it is a key point to the control.

(2) General practices for control

(i) Clearing and repairing

Regulating pond is an effective measure to improve the pond environmental condition, to prevent diseases and raise fish yields. It includes two aspects: one is the pond trimming, the other is pond disinfection (see chapter 4 “nurture of fry and fingerlings” for details).

(ii) To better the rearing management

1. A special person should be assigned to take charge of daily management such as stocking, feeding, manuring, and disease prevention, etc.

2. Performing the “four fix” feeding which is a positive procedure both for fish farming and disease prevention. (see chapters of “nurture of fry and fingerlings” and “food fish culture”).

3. To improve environment of water body

   Carefully observe the variations of water quality. According to the observation, timely apply the fertilizers or fill the pond with fresh water or even change water all over. Not only does it minimize or avoid the disease outbreaks, but also benefits fish growth.

4. To strengthen the daily management work

   Pond inspection is strictly required once in the morning, particularly in dismal weather or mornings after torrential rain during the epidemic season from May to September. Besides, it is necessary to remove the weeds along the pond sides, clear the feeding platforms so as to prevent disease occurance.

5. Operation should be handled tenderly during netting, transferring and transporting.

(iii) Conduct medical prophylaxis yearly during the epidemic seasons from May to September.

1. Fingerling disinfection: It is helpful to the prevention of diseases to disinfect fingerlings for transference or just before restocking into larger water bodies. This performance can be done in a boat, cabin, jar, pail, cage, etc. in line with local conditions whatever possible.

   Feeding platform disinfection: Hang small bamboo baskets with bleaching powder or cloth bag with mixture of copper sulphate and ferrous sulphate (ratio:5:2) placed round the feeding place or platform. When fish come to the place or platform for feeding, their skin will be automatically disinfected.

   Method of bleaching powder hanging basket: the purpose of this is to prevent bacterial diseases of the fish such as Grass carp that mainly feeds on buoyant foods. Starting from the beginning of feeding in spring every year, put a triangular or square bamboo frame at shallow corner of the pond against the wind. Along the frame, hang 3--6 small bamboo baskets with the opening 3 cm out of water to prevent the chemicals from floating away, then put a small stone as a sinker inside each basket to make it standing vertically in the water. Put 100–150g of bleaching powder into each container every day, but the container ought to be cleaned before the following day's medication. After hanging the containers each time, put some duck weed or tender grasses onto the place to attract fish (see figure 6--1).

   

   Fig. 6--1 Demonstration of bleaching powder container to control bacterial red skin and gill rot diseases.

   To prevent bacterial diseases of Black carp, the containers with bleaching powder should be put in line, attached to a bamboo rod, sinking under the water but 15--18cm above the bottom near the feeding place. Each container is covered with a lid to keep the bleaching powder (see fig. 6--2).

   

   Fig. 6--2 Bleaching powder container to control bacterial red skin disease of Black carp

   The method of hanging bag (mixture of copper sulphate and ferrous sulphate) are similar to that of bleaching powder container. The chemicals ooze out of the bag of fine cloth slowly (the best duration of drug dissolving is 3--4 hours in the bag), hang the bags once a day. The numbers of bags and the chemical amount in them depend greatly on depth of water and the size of feeding place, e.g. if a triangular frame is three meter long on each side and 33cm in water depth, normally with 2 bags on each side, 100g of copper sulphate and 40g of ferrous sulphate each bag. If water depth is 49cm, then apply 150g copper sulphate, and 60g ferrous sulphate; with water depth of 66cm, use 200g copper sulphate and 80g ferrous sulphate or use 250g copper sulphate and 100g ferrous sulphate at water depth of 83cm. For the first time of hanging bags, whether or not the fish come to eat should be closely watched one hour later. If fish refuse to appear, it shows an overdose of chemicals, and then, it's better to take away some bags close to the center of the pond until fish would come before the chemicals fully dissolve. (Fig. 6--3)

   

   Fig. 6--3 The method of hanging bags with copper sulphate and ferrous sulphate to control parasitic gill disease of Grass carp

   Some drugs used in the disinfection of fingerlings are listed just for reference only. (table 6--1).

2. Disinfections of feeds, feeding platforms and gears

   Feeding dirty or spoiled feeds may bring in pathogenic bacteria to the pond. If the leftover of feeds decompose in water, they are good for the pathogenic bacteria to multiply rapidly, which will lead fish diseases to spread rapidly in pond. For this reason, feeds and manures must be disinfected before application; animal foods such as snails need to be washed and supplied when they are fresh and alive. As for application of plant feeds, bleaching powder solution at a concentration of 6 ppm is used to immerse the aquatic grass for 20--30 minutes before supplying, (not necessary for land grass); as for organic manures, blend 120g bleaching powder with 500kg manure before application.

   Apart from the above mentioned method (of hanging basket or bag) for feeding place disinfection, during the epidemic season the feeding places need to be cleaned frequently by spreading the solution (ratio, 250g bleaching powder : 12kg water) once or twice a month.

   Gear disinfection: to avoid the spread of fish disease, the nets, pails and dip nets, etc. that are once used during epidemic season need to be disinfected before using them again. Huge nets can be exposed to sunlight for 1--2 days and wooden pails can be sterilized by immersing in quick lime solution or 10 ppm copper sulphate solution for 5 minutes.

3. Pond treatment: Spreading the whole-pond with drugs is one of the most common practices to prevent diseases. Before or shortly after stocking fry, diluted 1g of 2.5% dipterex should be sprayed all over the pond. It is more particularly needed for those ponds with weeds. Three months later, treat the pond again with 0.7 ppm solution of copper sulphate and ferrous sulphate (ratio, 5:2). After June, spray bleaching powder to make the pond water at a concentration of 1 ppm once or twice a month.

   To improve the deteriorated pond water, it is recommended that 20–25 kg quick lime be used mu for a water depth of one meter. Such method can better the water quality and prevent diseases. The procedure is to liquefy quick lime chunks with a little water in a container, after it is fully dissolved, add more water and stir it, and then spray it evenly over the pond.

4. Prevention with medicated feeds: during epidemic season of Grass carp enteritis, medicated feed is more helpful. Use 1–2 kg garlic per 100 kg of fish once a day for six consecutive days. Pulp and blend the garlic with fish feeds for feeding, preventive result would be better if 40g of table salt is added for every 5kg of food. For adult fish, mix the pulped garlic with some adhesive, and spread onto tender grass which can be applied after it is dry, or medicated feed may be given in pellet form.

Table 1. Some medical solutions for fingerling's immersion

(iv) Application of Chinese medicine for disease control

Controlling diseases by using Chinese herbal medicines has many advantages, such as broad source, good effect, cheapness and easy to learn by the masses. In their production activities, farmers of different farms have been using many kinds of Chinese herbal medicines to control diseases and achieve good results, e.g. in Zhejiang province, euphorbia humifusa and acalypha australis are utilized for enteritis and in Guangdong province, Thysanospermum diffusum is used to cure bacterial skin disease, gill rot, enteritis. Hubei Hydrobiological Research Institute reports a promising result of application of Chinese tallow tree (Sapium sebiferum) with Chinese rhubarb (rheum officinale) for “white-head-and-white-mouth” disease and bacterial gill rot disease.

With a constant practice and summarizing, the control with Chinese herbal medicines will surely make some contribution to freshwater fisheries in China.

(v) Implement the quarantine regulations and prohibit the travelling of diseased fish.

As China possesses a vast area, the geographic and, climatic conditions vary from place to place. This produces some epidemic diseases within some regions, e.g. The disease of Grass carp yearling caused by Bothriocephalus gowkongensis and branchiomycosis of Mud carp used to occur in Guangdong and Guangxi Provinces only; likewise, oodiniosis was in Jiangxi Province and Liang country, Guangdong Province; Ichthyophthiriasis in Hunan and Hubei Provinces. Recently as the freshwater farming industry develops rapidly, allocation and transportation of fry and fingerlings between the regions are quite frequent. Attention must be paid to the quarantine before the allocation and transportation. Diseased fish are strictly prohibited to transport just to avoid the spread of epidemic diseases.

2) Diagnosis

Diagnosis of disease is the first step for effective treatment.

(1) Cautions before diagnosis

1. Diagnosis requires objects alive or newly dead diseased fish, and better to keep the body damp.

2. Try to keep the dissected organs as complete as possible.

3. Keep the instruments for autopsy clean to avoid intercontamination of pathogens among the organs.

4. Use distilled water for microscopic observation of the skin and use 0.85% normal saline for microscopic observation of internal organs.

5. Reserve the samples for further identification if there is a doubt about pathogens and clinic signs.

(2) Methods and steps

1. Survey on spot: make an overall investigation on the diseased pond.

   1. Make sure whether the water source is seriously polluted or not.

   2. Notice whether there is an effluent from industrial sewage or an inlet of water contaminated by agricultural chemicals in water supply.

   3. Observe the behaviour of the diseased fish.

   4. To understand the rearing status, e.g. pond clearing, stocking density, feeding, preventive methods and mortality, etc.

2. Examination of diseased fish

A. Observation with naked eye: mainly examine the skin, gill and internal organs.

1. Body: put diseased fish in an enamelware plate, closely observe it with naked eye from head down to eyes, gill cover, scale, fin, etc. to look for visible pathogens: such as Nematodes, Argulus, Glochidium, Saprolegnia, it is also possible to see the pathogens of bacterial erythrodermatitis, albinoderm, stigmatosis, furunculosis, etc.

2. Gill: inspect gill with an emphasis on gill filaments; observe the colour of gill lemella, quantity of mucus, congestion and putridity of filament tips after an opercular incision is done.

3. Internal organs: mainly check the intestines. Start to observe abdominal hydrops and visible parasite, e.g. Ichthyoxenus, Nematodes, cysts of Myxosporidia, Liqulos), then observe the appearance of other internal organs. And then extract the internal organs with a knife and scissors and separate liver, gall bladder, airbladder, etc. Finally open the intestine to search any pathological changes.

B. Microscopic exam (with a microscope)

This is a further step to examine the focus determined by naked eye examination. For normal cases, only the skin, gill, intestine, eye and brain are to be microscopically observed.

Skin: scrape a little tissue and mucus from skin in question and put them on a slide, with a drop of distilled water, and observe it under a microscope after slightly pressing it with a coverslip. One should always start with low power objectives. Normally, at least three tissues from different points of the same organ is to be inspected. Common parasites on skin are Trichodina, Ichthyophthirius, Chilodonella, Costia, Glochidium. and Myxosporidia, etc.

Gill: Tweezer some gill filaments and mucus onto the slide. Through microscopic observation, the following parasites are often found: Dactylogyrus, Gyrodactylus, Cryptobia and Myxosporidia.

Intestine: transfer a little mucus from anterior intestinal wall to the slide and observe whether there is Nematode, Eimeria, Myxosporidia.

Eye: press the entire ocular bulb or crystalline body on the slide. The existance of the cyst of Diplostomulums can be recognized as diplostomulumsis.

Brain: open the cerebral cavity of whirling disease fish, white cysts of Myxosporidia in lymphatic fluid beside the brain can be noticed. And then, tweezer them out onto the slide and we can see a number of spores after being crashed beneath the coverslip.

During the diagnosis, if complications are discovered, try to find out the primary and secondary cases and give an anticipatory treatment one after another or synchronous treatment.

4. Control of some major diseases

1) Infectious diseases

(1) Hemorrhagic septicemia (Fig. 6--4)

Pathogen: reovirus. The particle or this virus presents a spheric or hexangular shape, measuring 60--70 nm (nano meter 10^-9) with an average diameter of 69 nm. The nucleus of high electron density in the middle of the particle has a diameter of 32 nm on average. The nucleus is enveloped with a membrane which is 20 nm or so in width. Besides, there is a kind of viral particle without membrane, but with an even electron distribution. The particle has a diameter of 46–60 nm and 52 nm on average, which exists in the inclusion bodies in nucleus or cytoplasm. The viral particles observed always appear in sanguifacient tissues of kidney, and not in red cells or granular white cells. The virus is sensitive to ether, acid and alkali, but not sensitive to drugs of tetracycline family, and it shows a high resistance to heat. Have the virus treated for 18 hours at 41°C or 1 hr. at 55°C and then, infect the fish with the virus. As a result, the mortality is still 100%; accordingly, 3 hr. at 60°C, mortality 10% 1 hr. at 65°C, no death. So the last method just mentioned is an available way to destroy it.

Symptom and pathological changes: The main symptom of this disease is congestion. External appearance is usually dark and slight red. If you observe juvenile fingerling against light, you can see hypodermic hyperemia. Some fish show congestion around the mouth, lower jaw, skull or orbit, and even exophthalmos, congestion of operculum and fin base are visible, by peeling away the skin of the sick fish, its musculature presents punctiform or lump congestion. In serious case, entire musculature gives a colour of bright red and “white gill” or bright red patch congestion appear on operculum, but some diseased fish has no symptom on gill. As for the clinic picture of internal organs, intestinal congestion is common. The whole or part of the intestine is red due to congestion. Mesentery and its peripheral fat is often accompanied with punctiform congestion. In a few cases, the liver, spleen and kidney are pale or locally congested. The wall of airbladder and gallbladder are often bloodshot.

Entire musculature congestion

Musculature punctiform congestion

Red patch congestion on operculum

Fig. 6--4 Grass carp with symtoms of Hemorrhagic septicaemia

The disease can be classified into three types based on its symptom and the pathological changes:

1. Red-muscle type: Though no evident or only slight hemorrhagic symptom on skin, the muscles is heavily congested, usually all muscles are red, and gill la'mellae are white because of loss of blood. Such a phenomenon is common among fingerlings of Grass carp of 7--10cm in body length.

2. Red-fin-red-operculum type: Congestion on operculum, base of fins, skull, orbit, and in mouth cavity, are obvious, even beneath the scale, but not distinct in muscles except some spotted congestion. This happens among Grass carp fingerlings of over 13cm in body length.

3. Enteritis type: instead of the skin or muscles, the intestine is seriously congested. The whole or part of the intestine is bright red. Sometimes, there are some punctiform congestion in mesentery, fat and wall of swim-bladder. Such case can be seen among either large or small fingerlings of Grass carp.

   The above mentioned three types can't be definitely separated; more often they are mixed up.

   Epidemic situation: It is one of most common and harmful viral diseases during the fingerling nurturing period. The epidemic season lasts long, with a high incidence, often bringing about mass mortality to Grass carp fingerlings. Grass carp and Black carp are both susceptable to this disease, but with the former as the main victim. This disease is widely spread in many places in China, with a distinctive epidemic season from June to September every year. That is to say, the occurance reaches its high peak in Aug. when water temperature is over 27°C, and it is going to gradually diminish after mid-Sept. when water temperature lowers to 25°C. Control measures:

1. Inoculate the fingerling with deactivated vaccine to produce a strong special protective capability which may last 14 months in function.

2. Decoct or immerse 250--500g pulverised Chinese rhubarb or maple leaves over night and after blending it with fish food, start feeding for 5 consecutive days and then, the pond is sprayed with Dyrene to make the pond water to a concentration of 0.6 ppm or 0,7 ppm or either one of copper sulphate or copper acetate or copper chloride to a concentration of 0.7 ppm. If copper sulphate is used, 2 days of medication is needed for a course, and two courses are conducted, if needed, to achieve a good result.

3. Minimize the possible occurance of hemorrhagic septicemia by improving the farming techniques through seed, food and management.

(2) Erythroderma

Pathogen: Pseudomonas fluorescens. It has the shape of short rods with round ends; it measures 0.7--0.75 × 0.4--0.45 micron; existing either single or in pairs. It has a motile, single polar flagellum, but no gemma. Gram negative; agar colonies are circular in shape, semiopaque and greyish white, producing yellowish green pigment after 24 hours incubation.

Symptom and pathological changes: Inflammation, bleeding from skin, loss of scale, particularly on the sides of the abdomen; blood shot on fin base; necrosis of terminal of fins; red blotches around the upper & lower jaws; and occasionally, congestion and inflammation along the intestines (Fig. 6--5)

Epidemic situation: erythroderma is one of the important diseases of Grass carp and Black carp, and widely spread on all farming sites. The mechanical lesions in the process of stocking or netting make it possible for the pathogenic bacteria possible to invade the injured fish. Frostbites in severe winter can also be infected. Besides, fish, rubbing against solid objects in water may get wounds which induce the disease. That's why the disease happens all the year round.

Control:

1. Prevention: it is helpful to do the pond clearing and disinfection thoroughly, and a delicate operation is required to avoid any injures in netting, carrying or stocking. Apart from these methods, another promising way is to dip fingerling with bleaching powder solution at a concentration of 5--10 ppm for 30 minutes before stocking or inoculate the fingerlings with vaccine.

2. Treatment: as the pathogenic bacterium not only causes pathological changes in the skin and the muscle, but also invades the blood. Medicine should be therefore, administered both internally and externally.

Internally, Sulphathiazole (ST) should be taken orally, once a day for 6 consecutive days. The dose for the first day should be 1 gram per 100 kg of fish, and the dose for the remaining five days is cut in half. The medicine is mixed with feeds by rational adhesive for feeding.

Externally, spreading all over the pond with bleaching powder (containing 30% of available chlorine) to bring the pond water to a concentration of 1 ppm, or scattering the Chinese gall Galla Chinesis to reach a concentration of 2--4 ppm.

(3) Enteritis

Pathogen: It is still an issue. Some people believe that it is Aeromonas punctata f. intestinalis, which is short-rod-shaped with two round ends; 0.4--0.5 × 1--1.3 micron in size; existing mostly in pairs. Its polar monoflagelum has motile force; no gemma; Grame-negative; agar colony is round, and semiopaque brownish pigment is produced around the colony one or two days later. The bacterium is one of pathogen caused by certain conditions. The pathogen is more likely to bring about the disease when water temperature is suitable, water quality worsens, air pressure is too low, and fish are overfed.

Symptoms and pathological changes: the diseased fish shows an expanding abdomen on which red blotches are seen, fins are congested and decayed, the anus is red and swollen through which yellow mucus may be released out if a light pressure is applied on the abdomen. By dissecting the abdomen, we can see ascites. The hyperaemia and inflammation are found on the intestinal walls. Cells of mucous membrane ulcerate and drop off, becoming bloody mucus to block the intestine. The illed fish loses appetite and swims slowly in solitude and soon dies.

Fig. 6--6 Grass carp infected with enteritis

Epidemic situation: the disease is very common among Grass carp and Black carp, with few cases in Bighead & Common carp. Under yearlings of Grass carp and yearling of Grass carp and Black carp are more apt to catch such a disease with a high mortality, commonly ranging from 50 to 90%. So enteritis is one of the most harmful ones to the cultivated species in China. It can be found everywhere in farming sites. The season of prevalence and the degree of incidence in different places differs a little in terms of different climate and rearing management. Generally speaking there are two distinct epidemic seasons annually: May to June is the season for 1--2 years old Grass carp and Black carp, while the period from August to September is a terrible time for underyearlings of Grass carp. More often, the disease is complicated with bacterial gill rot disease.

Control:

1. Prevention: To keep a good water quality and strict performance of “four disinfection and four fix” (note) is a vital step to avoid the disease. During the prevalance, it is recommended to limit the feeding amount and conduct medical prophylaxis regularly.

2. Treatment: integrate the oral administration with external administration. Externally, bleaching powder is sprayed into the pond to make the pond water at a concentration of, 1 ppm or quicklime in a dose of 15--25 kg/1mu/1 meter water depth should be scattered over the pond.

   The following medicines can be selected to be mixed into medicated feeds.

   1. Sulphaguanidine (SG): for the first day, use the medicine in a dose of 1g for every 10 kg fish, and the dose for the next five days is reducd by half.

      Note: four disinfection	disinfection of feeding platforms, fingerlings, feeds and tools.
      four fix	fixed quality, fixed quantity, fixed time and fixed position.

   2. When enteritis and gill rot diseases come out at the same time, spray the of the Chinese gall over the pond, making the pond water at a concentration of 2--3 ppm. Meanwhile, feed the fish with furazolidone for six days with a dose of 1g/100kg fish, or double this dosage for a serious case in the first treatment.

   3. Treatment with garlic: use 1--2kg of garlic per 100kg of fish once a day for six consecutive days.

   4. Treatment with Euphorbia humifusa: use 500g dry herb or 2500g fresh herb for 100kg of fish once a day for 3 consecutive days.

   5. Treatment with Acalypha australis: use 500g dry herb or 2000g fresh herb per 100kg of fish once a day for 3 consecutive days.

   6. Treatment with water Polygonum hydropiper: use 500g dry herb or 2000g fresh herb for 100kg of fish once a day for 3 consecutive days. It is also effective to use two or three kinds of the herbs in (d), (e), (f), making a compound one.

   7. Treatment with creat Andrographis paniculata: use 2kg dry herb or 3kg fresh herb for 100kg of fish daily for 5--7 consecutive days.

   8. Fortifying the resistance against disease by inoculating vaccine before stocking fingerlings.

(4) Bacterial gill rot

Pathogen: Myxococcus piscicolus: It is slender in shape, soft and easy to coil. The length of the bacterium varies greatly, normally 2--37 micron Gram-negative. The colony on peptone agar diffused to pseudomycorrhiza, about 3 mm in diameter. The growth stops after five day's culture.

Symptom and pathological changes: Fish are black in appearance, especially the head. The gill filaments which often attached with mud and mucus are putrid and pale. In serious case, hyperemia and inflammation are found on the inside and outside opercula, the epidermis of which often rots away, leaving a transparent spot there.

Studies were made on histological pathology of bacterial gill rot of Grass carp. It was found that its pathological development can be divided into chronic and acute types. Chronic course lasts long with as its main character, and acute course is short, mainly with inflammatory dropsy or necrosis of the cell (Fig. 6--7)

Fig. 6–7 Head of Grass carp affected with gill rot (left operculum is removed)

Epidemic situation: This disease affects Grass carp, Black carp, Bighead, Common carp and other fishes as well, but chiefly affects Grass carp. It is one of the most serious diseases of Grass carp, occurring on all fish farms all the year round. It seldom appears when water temperature is below 15°C and begins to occur when water temperature is over 20°C. The optimum temperature for the prevalence is 28–35°C. Therefore, it is more popular in spring, summer and autumm than in winter. Such disease is concurrently accompanied by enteritis.

Control:

1. In the prevalent season, disinfect pond water along the pond sides once a week with dissolved bleaching powder at a rate of 0.25kg/mu. At early outbreak, bleaching powder can be hanged around the feeding platforms for prevention purpose, or bleaching powder can be spread to make the pond water at a concentration of 1 ppm in worse case.

2. Treatment with dry powder of Chinese tallow tree Sapium sebiferum leaves: the amount involved is to make the pond water at a concentration of 6.25 ppm. If the fresh leaves are used, the dosage should be four time as much as the dry amount. Usage: immerse the leave powder with 2% quick lime solution, the quantity of which is 20 times that of leave powder for 6–12 hours. Then boil it for 10 minutes to keep pH value upto 12 plus prior to spraying

3. Pond treatment with Chinese rhubarb Rheum officinale to bring the pond at a concentration of 2.5–3.7 ppm. Usage: immerse the rhubarb with 0.3% ammonia water, the quantity of which is 20 times rhubarb, for 12 hours before spraying.

4. Spray the pond with the erythromycin for poultry use to make the pond water at a concentration of 0.3 ppm. And again mix the medicine with fish feeds for feeding from the next day for six consecutive days. Apply 4g of the medicine for 100kg of fish for the first day and cut the dosage in half for the remaining five days.

5. Do a pond treatment with pulverized Chinese gall to make the pond water at a concentration of 2–4 ppm.

6. Spread the pond with maple leave solution. Usage: add water to 20kg of pulped maple leaves per mu before application.

7. Make the pond water at a concentration of 20 ppm by spraying quick lime.

8. Enhance the immunity by inoculating vaccine to Grass carp fingerlings before transferring or stocking.

(5) Verticle scale disease

Pathogen: It is preliminarily recognized as Pseudomonas punctata. In terms of Japanese data, the disease is caused by Aeromonas sp. or bacterial infection by such genus.

Symptom and pathological changes: the diseased fish looks rather rough on skin, and some scales (especially on posterior part of fish body) stretch out, resembling pinecone, so it is also named pinecone disease. The scale capsule contains semiopaque or sanguineous exudate which makes scale verticle. Therefore, it is called verticle scale disease. If the scale is lightly pressed, the esudate will eject out from the scale base and the scales will drop immediately. Other symptoms are congestion on fin bases; mild bleeding and inflammation on skin; reddish ulceration on information (loss & scale) exophthalmos and abdominal distension, etc. As the case develops, the morbid fish shows slow swimming movement, dyspnea, and abdomen turning upwards and then the fish dies 2–3 days later.

Fig. 6–8 Common carp affected with vertical scale disease (after diagnostic guide of fish disease)

Epidemic situation: It mainly injures Common carp, Crucian carp, Grass carp, Silver carp and occasionally goldfish (Cyprinus auratus) It usually occurs in the places of northeast, central and eastern part of China. Such disease has two prevalences. One is in spawning time of Common carp; and the other is in overwintering period of Common carp. Normally, the disease starts its prevalence from late April to early July. According to some data collected on fish farms, mortality of parent fish, resulting from this disease reaches 45%, the maximum mortality 85%. And the mortality rate of yearling Common carp accounts for over 50%. The outbreak of the disease is related with injured skin, contaminated pond water and poor disease resistance.

Control:

1. Save fish from injures during netting, transferring or stocking.

2. Drain and fix the spawning pond of parent Common carp in winter, and disinfect the pond with quick lime or bleaching powder.

3. Make pellet food by mixing the impure aureomycin or terramycin in a dose of 5% of the feeding amount.

4. Give an injection to abdominal cavity with 3–6mg chloromycetin.

(6) Saprolegniasis

Pathogen: They belong to Family Saprolegniaceae, Class Phycomycetes. The common genera are Saprolegnia and Achlya. Mould is a coenocytic mycelium without cell wall wall. One end of the mould resembling root attaches to the wounded part of fish body. It has many branches that penetrate into skin and muscle, which is called mycelia, sucking nutrition from its host. The rest of the mould tufting outside fish body is called hyphae, which may be 3cm long and looks like white floccule. The mould on a dead fish can spread and cover the whole body within 12–24 hours.

Symptoms and pathological changes: no abnormal sign could be seen in the initial stage of the infection. When it is visible, it indicates that the mould has already penetrated and stretched on the lesions. Mycelia go deep into muscles, permeating into the fissure of tissue cells, and branching out heavily. Hyphae grow into grey flocky substance. A certain kind of ferment secreted from the mould is able to decompose the tissues of the host and fish secretes a great deal of mucus because of the irritation. The diseased fish suffers from the fidget, shows an abnormal behaviour, and rubs itself against some solid materials. As the fish is overloaded, morbid muscle will rot fish will loose its appetite and will move slowly and eventually will die (Fig. 6–9)

Fig. 6–9 Bighead affected with Saprolegniasis

Epidemic situation: It is common in all farming areas in four seasons. It can affect fish from egg to any other stages of all farming species. Mould takes advantage of invading the wounded part caused by netting, transporting and cause the disease, particularly in over-wintering pond with high stocking density.

Control:

1. To minimize the possible outbreak, disinfect the pond with quick lime.

2. To prevent the disease, try to avoid any lesions caused by catching, carrying or stocking.

3. Select healthy fish as parent fish instead of injured fish. Smear the parent fish with 1% malachite green ointment or sulfa ointment.

4. If the disease occurs in an eel culturing pond, sprinkle the infected pond with malachite green solution or methylene blue solution to make the pond water to a concentration of 0.15–0.2 ppm and 2–3 ppm respectively. If the effect is not apparent, do a sprinkling again with the same dosage 3–4 days after the first application.

5. Prevention of saprolegniasis of eggs: First, manage to raise the rate of fertilization. Choose fine days to conduct artificial spawning, and put egg nests indoors for indoors hatching by showering water to increase hatchability.

6. Disinfect viscid eggs by immersing them with malachite green solution of 7 ppm for 10–15 minutes for two consecutive days. Afterwards, sprinkle the hatching shelf twice a day, in the morning and evening respectively with 10–15kg of malachite green solution at a concentration of 70–100 ppm until fry is hatched out.

7. Apply malachite green solution to the circular incubation tank to make the water light green at an intervals of 6–8 hours. Repeat this proceudre until hatchlings are out. This method helps to diminish the mould infection and improve the hatchability.

8. It is helpful to prevent the infection by immersing the egg nests for 2–3 minutes with 3–5% formalin solution or 1–3% table sult solution for 20 minutes.

2) Invasive diseases

(1) Cryptobiosis branchialis

Pathogen: Cryptobia branchialis is a flattened creature with a wide anterior end and a narrow posterior end like a willow leaf. The body length measures 7.7–10.8 microns. Two flagella with different length grow out from the anterior end. One that is stretching forward is named anterior flagellum. The other is called posterior flagellum which forms an undulating membrane along the surface of the body and stretches to the posterior. In the middle of the body there is a round nucleus, in front of which exists an avoid kinetonucleus. Around the nucleus there are chromatin granules. Endosoma is in the center of nucleus. The live parasite attaches itself to the gill of its host by penetrating its posterior flagellum into epidermal tissue of the host's gill. When leaving the host, it keeps wagging its anterior flagellum and undulating membrane so as to move its body forward (Fig. 6–10)

Fig. 6--10 Cryptobia brabchialis

Symptoms and the pathological changes: the parasite generally fixes on the gill of host, destroying the epithelia on gill lamella and producing thrombus that blocks the blood vessels of gill lamella. The inflammation of branchial organs retards the normal blood circulation. Meanwhile, much mucus secreted by irritation will cover the intact part of gill. Thus, the respiration is impeded. If the case worsens, fish will show dyspnea and die of suffocation.

Epidemic situation: this parasite has no particular selectivity to host. That's to say, it can invade any fresh water fish, especially pond fish. It can cause disease and bring mass mortality, mainly to Grass carp summer fingerlings. It is reported on all fish farms across the country. The epidemic season is from May to Oct. with a high peak from July to September often in accute form.

Control:

1. Bathe fingerlings with copper sulphate solution at a concentration of 8 ppm for 20–30 minutes before stocking.

2. Disinfect the feeding place with mixture of copper sulphate and ferrous sulpate by hanging method during the prevalence.

3. The infected pond can be treated with mixture of copper sulphate and ferrous sulphate to make the pond water at a concentration of 0.7 ppm.

4. It is effective to bathe fish with 2–3% table salt solution for 5 minutes.

(2) Myxosporidiasis

Pathogen: Many species belong to the order of Myxospor; the class, of Sporoza. Such parasites have the ability to parasitize any organs or tissues of all kinds of fish. Up to now, over 100 species which can parasitize the freshwater fishes have been found in China. Some species will cause harmful epidemic.

A spore of myxosporidia is formed by two Chitin shells of same size and thickness, holding the character of plasmodium. The line that two shells joins is called sutural line. The sutural line, because of thickness or protuberance, shows the ridge texture, which called sutural ridge. The side with sutural ridge is called sutural side (or lateral side). The side without sutural ridge is called shell side, (or front side). The spore contains polar capsules and sporoplasm. The amount of polar capsules that a spore has is not the same because of different species, generally from one to four. Each polar capsule has a polar filament that curls spirally. The number of nuclei inside the sporoplasm may differ in different developmental stages of the spore. Some species contain a iodinophilous vacuole which can be stained in brown with Lugol's solution (Fig. 6--12).

A shell view of the spore	A sutural view of the spore
A top view of the spore

Myxosporidia parasitize host generally in the form of cytocyst. The most dangerous Myxosporidia that parasitize fish skin are Myxobolus koi, Myxobolus ellipsoides, Thelohanellus rohitae, etc. (Fig. 6--12) Myxosporidia parasitizing fish gills are Sphearospora amurensis, Myxobolus dispar (Fig. 6--13). Myxosporidia attacking intestine are Myxosoma lieni, M. artus (Fig. 6--14). A great amount of Myxobolus lieni (Fig. 6--15) that parasitize the central nerves and sensory organs of Silver carp and Bighead will lead to whirling disease.

Fig. 6--12 Common Myxosporidia on the skin of cultivated fishes (after PATHOGENIC FAUNAGRAPHY OF FISH DISEASE IN HUBEI PROVINCE)

Fig. 6--13 Myxosporidia commonly seen on gills of cultured fishes (after PATHOGENIC FAUNAGRAPHY OF FISH DISEASE IN HUBEI PROVINCE)

Fig. 6--14 Myxosporidia easily found in the intestines of cultured fish (after the PATHOGENIC FAUNAGRAPHY OF FISH DISEASE IN HUBEI PROVINCE)

Fig. 6--15 Myxobolus lieni in the brain of Silver carp (after the PATHOGENIC FAUNAGRAPHY OF FISH DISEASE IN HUBEI PROVINCE)

Control:

1. Eradicate the spore inside the pond silt by sterilizing the pond with 100kg of nitro lime (CaN₂) /mu or 125kg of quick lime/mu. In the fry and fingerling stages of Silver carp (June-Sept.), sprinkle dipterex powder to make the pond water at a concentration of 5 ppm twice a month so as to prevent whirling disease of Silver carp.

2. Bathe Mud carp with 1% crystal dipterex (of 90% effective element) for 3--10 minutes, and at the same time, sprinkle the pond with crystal dipterex to make the pond water at a concentration of 0.2--0.3 ppm. It is effective to control Myxosporidiasis of Mud carp.

3. Feed every 10000 fry with 2g of paludrine 5 days after fry nursing for six consecutive days to control the Myxobolus artus.

(3) Ichthyophythiriasis

Pathogen: Ichthyophthirius multifiliis. Its bodily form and size differ much in different developmental stages (Fig. 6--16)

Fig. 6--16 Ichthyophthirius multifiliis

Syptoms and pathological changes: the skin, fin rays operculum are covered with many white protuberant pustules, for this reason, it is also named “white dot disease”. If a fish is seriously sick, its skin is smeared with a layer of white membrane. The sick fish is slow in swimming and in response, floating on surface. Occasionally, they are running around the pond in groups. The sick fish keeps rubbing itself against other objects, or jumping out of water surface. This disease will cause mass mortality.

The white dot on the skin is formed by the proliferation of epidermal cells together with enormous mucus secreted due to irritation which is caused by the drilling of parasites on the epidermis. Because of the damage caused by parasites and secondary bacterial infection, epidermal phlogosis happens; local necrosis appears; scales are easily . Fin rays are rotten and split open. The presence of parasite on branchial tissues destroys the gill lamella, and stimulates the secretion of mucus. The branchial epithelia around the parasites proliferate. The congestion of gill vein or histological anemia of local gills are caused. The invasion of parasite into cornea of fish may cause phlogosis and blindness. The diseased fish will die due to dyspnea and no appetite.

Epidemic situation: It is widely spreaded across the country, and it is one of the major protozoa diseases. Freshwarer fishes of all developmental stages are apt to the infection with fingerlings as a main victim. Ornamental fish in aquarium or petty cement tanks also die of this disease. The optimum water temperature for the proliferation of the parasite is 15--25°C. So early winter and late spring is its prevalent season. The fish in high-density-overwintering ponds is more susceptable to the disease.

Control:

1. Prevent the transmission of Ich. by disinfecting the pond with quick lime, rearing with a rational density and conducting quarantine and sterilization of fingerlings before stocking.

2. According to reports, bathe fingerlings with a mixed solution of malachite green at a concentration of 0.05 ppm and formalin at a concentration of 25 ppm twice a day. It is not to be used for food fish.

3. It is effective to immerse fingerlings with malachite green solution at a concentration of 0.2--0.4 ppm for 2 hours. This method was used by Beijing Fishery Research Institute.

4. In Europe, some people deal the disease with methylene blue or prevent the white dot disease by treating fish with sea water (salinity is over 1%).

(4) Trichodinasis & Trichodinelliasis

Pathogen: A number of species of the genus Trichodina and the genus Trichodinella. Lateral view of the parasite looks like a hat, and in aboral view, it resembles a round plate. It moves like a rolling wheel. The convex part of the parasite is called adoral end. And the opposite of adoral end is called aboral end on which there is counterclockwise adoral groove stretching to the cytostome. On each side of the adoral groove there is a line of cilia, forming an oral zone that extends to vestibule. The cytostome is linked with cytopharynx and in the proximity of cytopharynx there is a contractile vacuole. The shape of macronucleus varies in species: some, horseshoe-shaped; some, sausage-shaped, etc. Micronucleus is rod-shaped or sphere-shaped, generally in the proximity of outer margin of the macronucleus end. On the aboral and there is a posterior girdle of cilia. There are two lines of rather short cilia, one called upper marginal cilia and the other called lower marginal cilia. Some species have a thin and transparent membrane called border membrane, behind the lower marginal cilia. The aboral end is concave and its most apparent structure is denticulating ring and chitin striated ring. The denticulating ring is formed by many denticles that join together. The number and shape of denticules and the number of radiant rays on each denticle differ in species. A live parasite attaches to the skin or gill of host with its adhesive disc. Sometimes it contracts its border membrane, moves its posterior girdle of cilia and keeps sliding on the skin and gill. When swimming freely after leaving its host, the parasite spins like a wheel with its aboral end forward. The reproduction includes asexual longitudinal division and sexual reproduction by conjugation. The optimum temperature for reproduction of Trichodina is 20--28°C (Fig. 6--17).

Fig. 6--17 Structure of Trichodina (lateral view)

Symptoms and pathological changes: Trichodina can cause infection to fish of any developmental stages, but mainly jeapodise juvenile fish, especially the juvenile fish of merely 5 cm in body length and the infestation will cause severe mortality. Generally, the parasite do not harm adult fish too much. Trichodina of larger species mainly invade the skin of juvenile fish and live on the tissue cells of fish and destroy the delicate fish skin. Trichodinella chiefly parasitize the gill, and concentrate along the branchial periphery or between the gaps of gill filaments to destroy the gill tissues. When it is serious, the gill tissue rots and the cartilage exposed. The respiratory function of fish is thus seriously impeded and finally the fish is bound to die.

Epidemic situation: Trichodinasis is one of the rather dangerous diseases during fry and fingerlings stage. It is prevalent in all fish culture places of our country, especially on farms along Yangtze River Valley and the West River Valley. Every year from May to August when fry is nurtured to summer fingerlings, serious trichodinasis often occurs and causes high mortality to fry and fingerlings. The disease is more apt to occur in small ponds with shallow water, poor water quality, high density and continuous rain.

Control:

1. Disinfect fingerlings with copper sulphate solution at a concentration of 8 ppm for 20--30 minutes or with table salt water 1--2% for 2--10 minutes.

2. Inhibit the reproduction of Trichodina by soaking 15--20kg Chinaberry Melia azederach tree leaves per mu and renew the leaves once a week. And it is also effective to sprinkle the pond with 25--30 kg of decoction of fresh branch and leaves of China berry.

3. It is promising to treat the pond with mixture of copper sulphate and ferrous sulphate at a concentration of 0.7 ppm (ratio 5:2)

4. It is reported in Japan that formalin solution at a concentration of 30 ppm can kill the Trichodina on juvenile eel gill, but it brings in a disadvantage of deteriorating water quality and affecting the appetite.

(5) Dactylogyrosis

Pathogen: many species of the genus Dactylogyrus. The pathogenic species parasitizing the cultured fish in China are as follows:

1. D. lamellatus, parasitizing the gill, skin and fin of Grass carp. The shape of its body is flat, 0.192–0.529mm long, 0.072–0.136 mm wide. (Figure 6–19)

2. D. aristichthys, parasitizing the gill of Bighead.

3. D. hypophthalmichthys, parasitizing gill filaments of Silver carp.

4. D. vastator, parasitizing the gill filaments of Common carp, Crucian carp and pet fish.

Fig. 6–18 D. lamellatus.

Symptoms and pathological changes: the infestation of parasites makes the morbid fish secrete more mucus with local or entire gill pale, operculum open, dyspnea, evident dropsy of gills (more distinct on Bighead). The morbid fish swims slowly and shows anaemia. The number of monocyte and coenocytic leukocyte increases.

Epidemic situation: it is one of the common diseases, prevalent in late spring and early summer. The optimum temperature of the parasitism is about 20–25°C. Such a disease mainly endangers Silver carp, Bighead and Grass carp.

Control:

1. Before stocking, bathe fingerlings with potassium permanganate at a concentration of 20 ppm for 15–30 minutes to kill the Dactylogyrus parasitizing fingerlings.

2. Spread 90% crystal dipterex to the pond to make the pond water at a concentration of 0.2–0.3 ppm at the water temperature of 20–30°C.

3. Treat the pond with 2.5% dipterex powder to make the pond water at a concentration of 1–2 ppm.

4. Sprinkle the pond with the mixture of dipterex and sodium carbonate (ratio 1:0.6) to make the pond water at a concentration of 0.1–0.24ppm.

(6) Sinergasilusis

Pathogen: Some species of female parasites belonging to the genus Sinergasilus which parasitize fish gill and cause disease (Fig. 6–19). Three common species of the Sinergasilus that parasitize the cultivated fishes are as follows:

1. S. polycolpus, parasitizing the interior side of the gill filament tips of Silver carp and Bighead, and gill rakers of Silver carp.

2. S. undulatus, parasitizing the interior side of gill filament tips of Common carp and Crucian carp.

3. S. major, parasitizing the interior side of the gill filament tips of Grass carp, Black carp, Cat fish, Trout and freshwater salmon.

The adult female: the body is slim in cylindric shape, consisting of three parts: head, thorax and abdomen. The whole body is obviously segmented and the head appears slightly triangle-shaped. It has five pairs of appendages among which the second pair of antenna has been specialized into slim big hook which is capable to hook gill filament tissues. The width of the segments equals the length for the first four thoracic segments or the width is somewhat longer than the length. The fifth thoracic segment is comparatively smaller. The sixth thoracic segment is a narrow genital segment on which a pair of ovisacs are seen, hanging down during the breeding season. Five pairs of swimmeret on the thorax are in biramous type. There are three segments on the abdomen with a short pseudosegment between the first and the second and between the second and the third segment respectively. And a pair of caudal furca exist on the tail end.

The male: it is in a shape of cyclopida, leading a free life. It is many times smaller than the female. A pair of maxilliped exists on the head to embrace the female in mating.

Fig. 6--19 Sinergasilus major, S. polycolpus and S. undulatus

1–2 S. major and its second antenna

3–4 S. polycolpus and its second antenna

5–6 S. undulatus and its second antenna

Symptoms and pathological changes: S. major is more harmful than the other two. It usually invades Grass carp which are over two years old, occasionally parasitizes gill filaments of rather big underyearling of Grass carp. The female parasite clutches the gill with its big hook of the second antenna to destroy the gill tissues, making so many wounds that the local inflammation and festering for gill filament, occur and even the tips of gill filament curves and deforms. What's more, as the Sinergasilus eats food, its mouth secretes a kind of enzyme to dissolve the tissues of the host, breaking the branchial epidermis, loosing the cells, damaging the nearby capillaries, and causing anemia to some part of gill tissues. Diseased fish often jumps in water by uneasiness that affects the appetite and health. Complications of other disease may accelerate the death (Fig. 6--20).

Fig. 6--20 Gill of two-year-old Grass carp invaded by Sinergasilus major

Epidemic situation: it is widely spreaded in China. The reproduction period is from April to November every year along Yangtze River valley. The main epidemic season is from May to September. S. major mainly harms Grass carp which are over two years old, and S. polycolpus basically endangers Silver carp and Bighead which are over two years old. Serious case can cause mortality.

Control

1. Prevent the occurance by rotary farming system, because the pathogen has a strict selectivity to certain hosts.

2. Before stocking. Bathe fingerlings with mixture of copper sulphate and ferrous sulphate (ratio 5:2) at a concentration of 7 ppm for 30 minutes.

3. It is effective to spray the pond with the mixture of copper sulphate and ferrous sulphate (5:2) to bring the pond water to a concentration of 0.7 ppm.

4. Treat the pond with mixture of 2.5% dipterex powder and ferrous sulphate (ratio 1.2 : 0.2) to keep the pond water at a concentration of 1.4 ppm.

(7) Lernaesis (anchor worm)

Pathogen: some species of genus Lernaea. The most important three species in China are as follows:

1. Lernaea polymorpha, parasitizing Silver carp, Bighead and Wuchang fish.

2. L. cyprinacea, parasitizing Common carp, Crucian carp, Silver carp and Bighead.

3. L. Ctenopharyngodontis, parasitizing Grass carp.

The appearance of the female parasite is slim, like a needle. The body measures 6--12.4mm in length, consisting of head, thorax and abdomen. There is no distinctive demarcation between the three sections. On the head, there is a pair of dorsal horns and a pair of abdominal horns. They are like anchors that enable the parasite to fix itself into the host musculature. So it is also named “anchor worm”. The shapes of the cephalic horns differ in species. Thoracic region is long like a tube. The anterior part is narrow but it gets broad towards the posterior end. There is a pair of genital pores on the abdomen of the terminal part. In the reproduction season, there is a pair of long ovisacs measuring 2--3mm in length, hanging down from the genital pore. The thoracic region of the female parasite has five pairs of swimming legs. After obtaining abundant nutrition, the thorax is expanding and extending. These make the five pairs of swimmeret degenerate and split apart. (Fig. 6--21). The male parasite is small, resembling cyclopida, living a free life, but it occasionally lives on fish for a short time.

	1. Ventral horn
	2. Head lobe
	3. Dorsal horn
	4. First swimming leg
	5. Second swimming leg
	6. Third swimming leg
	7. Fourth swimming leg.
	8. Fifth swimming leg
	9. The front part of genital segment is protuberant.
	10. Ovulation pore
	11. Caudal furca
	12. Egg sac

Fig. 6--21 Structure of female Lernaea

After living on fish, the female parasite develops into three phases: “baby parasite”, “matured parasite” and “old parasite”. The baby parasite looks like fine white hair, without ovisac. The matured parasite has a transparent body. The moving of intestine inside the body is visible, and there is a pair of green ovisacs near the genital pore. Touching with your hand, the parasite will erect. The body of old parasite is rather turbid and soft, and carries many protozoans that live a free life such as Epistylis, showing an old sight. Such parasite will die soon and fall off the fish.

Symptoms and pathological changes: in an initial stage, the sick fish shows uneasy, poor appetite, thin body and slow movement. The surrounding parts where Lernaea penetrates are inflammated and swollen, and tissues are necrosing. The wounds are often invaded by Saprolegnia. 4–5 individual Lernaea on a juvenile fish of 6--9cm can cause death. 1–2 parasites on a young fish are even able to retard the growth or deform the host.

Epidemic situation: It is widely spread in Guangdong, Guangxi Fujian Provinces. The incidence is high and its intensity is great. Prevalent season is long. The disease is one of the major diseases in these places. The epidemic season along the Yangtzs River Valley is from April to October (when water temperature is 15–33°C) that's the right season for Lernaea to reproduce, especially in Autumn. Control:

1. Bathe the fingerling attacked by Lernaea with potassium permanganate solution at a concentration of 10–20 ppm for 1.5--2 hours before stocking.

2. Spray the pond with 90% crystal dipterex to make the pond water at a concentration of 0.3–0.5 ppm for 3–4 times at an interval of 3–4 days.

3. Treatment with potassium permanganate: for the diseased Grass carp, bathe the fish for 1.5–2 hours with solution at a concentration of 20 ppm at water temperature of 15–20°C, or 10 ppm/21–30°C. And as for the diseased Silver carp and Bighead, the procedure is as follows: immersion in potassium permanganate solution at a concentration of 33 ppm below 10°C; 20 ppm 10–20°C; 12.5 ppm between 20–30°C; 10 ppm over 30°C. The baby and matured Lernaea can be eliminated by using above-mentioned solution after one hour.

4. Immunity: since the Lernaea has a strict selectivity to certain hosts, we can prevent the disease by adopting the method of culturing in rotation or immunizing fingerlings. Thus, the immunized fish will gain a resistance over one year, and stocking such fish into large water area will not cause Lernaelasis.

5. Change the water quality suddenly, e.g. apply 400kg of fermented cow dung or pig manure, or 100–150kg of distiller's dregs/mu/1m water depth.

3) Control of other diseases and fish enemies

Undersirable water temperature, water quality, mechanical lesions, insuffieient feeding, chemical substances may cause diseases, even mortality, such as gas bubble disease, horse-running disease and deformality (Fig. 6--22).

Fig. 6--22 Gas bubble disease, horse-running disease and deformality

In the fry nurturing stage, we often find some fish diseases caused by lower plant. e.g. Filamentous green algae (Fig. 6--23) and the disease caused by animals. e.g. water centipede, amd fish killers Kirkaldyia deyrollei. Frog is also a dangerous predator of fry and fingerlings.

Fig. 6--23 Filamentous green algae

Fig. 6--24 Water centipede and fish killers

References

Science of the culture of Freshwater fish species in China
Huang Chiyian, Tang Silian “Fish pathology” 4, 1983.
Zhou Wengong, Chen Hinfu “Handbook of common fish disease control” 10, 1981.
“Handbook of fish disease control”
“Handbook of fish disease investigation”
“Pond fish culture in China” 7, 1983.