Due to reclusive nature of shrimp and the difficulty of observing dead shrimp in ponds, little is known about the importance of disease and parasites in pond culture. However, heavy mortalities from disease occur frequently in more intensive types of shrimp culture, such as hatcheries, raceways and tanks. Since unexplained mortalities do occur in ponds, a close watch should be kept for signs of disease or parasites.
Unfortunately, dead or diseased shrimp are not easily observed in a pond. Frequently, a farmer does not learn that his shrimp have died until he harvests a pond and finds there are only a few shrimp left. For this reason, it is important that the shrimp in a pond be sampled regularly, at least weekly, and examined for disease.
Diseases of shrimp are just beginning to be studied seriously. Consequently, the few examples listed below are probably just the start of a long list. For instance, it is suspected that virus diseases are much more important than is now known. Similarly, the importance of fungus and bacterial disease has only been realized during the last few years, and many more pathogenic forms will probably be identified in the future.
Black gills in shrimp can be caused by several things.
Accumulation of debris in gills. This is usually associated with poor pond bottom conditions. It is not known if this condition can cause death. If shrimp with dirty gills are placed in clean water the gills become clean.
Fungus. Fusarium sp. This disease is epizootic and can cause mass mortalities. In infected shrimp, the gills have a very dark, deep black colour. Sindermann (1974) reports that Hatai (1974) found Nystatin and Azalomycin F were effective in treating this disease.
Bacteria. In the initial stages of this disease, the gills turn orange-yellow or light brown. Eventually, the gills turn darker until they are black. Losses of shrimp from this disease are not as large as in some others. Treatment of infected shrimp by bathing in a 2 to 3 ppm concentration of furazolidone for two to four nights is an effective treatment (Shigueno, 1975).
This disease is caused by bacteria. It is characterized by black eroded parts on the exoskeleton. Progressive destruction of the exoskeleton provides places for the entry of secondary infections which may cause death. Epizootics have occurred under crowded conditions, with mortalities caused by destruction of the gills. A mixture of malachite green (0.5 to 1 ppm) and formalin (20 to 75 ppm) in water, reduced losses from this disease considerably (Sindermann, 1974). When incorporated in food, the following were found to be effective treatments: terramycin (0.5–1 ppm); sulfisozole, nifurstyreic acid, and chloramphenicol (Shigueno, 1975; Sindermann, 1974).
In shrimp suffering from muscle necrosis, there are white patches in the tail, or the whole tail is white. This condition is usually associated with stress behaviour such as swimming at the surface or jumping out of the water. It is caused by a combination of high temperature and low dissolved oxygen. The white colouration is caused by degenerative tissue. If the environmental conditions are improved, some of the shrimp will survive. Otherwise, massive mortalities take place (Rigdon and Baxter, 1970).
This disease is caused by microsporidian parasites in the muscle tissue or reproductive organs. It is characterized by a white colouration of the infected area. Sometimes, there is a blue-black colour on the back and sides of the shrimp. The disease usually starts at the telson or uropods and works its way forward. There may be some orange or reddish colour due to deterioration of tissues. Infected individuals can be weakened or killed, especially by stress. The percentage of shrimp infected is usually not great (Sindermann, 1974).
This disease is caused by fungus. Portions of the exoskeleton turn white.
Infected shrimp initially show reduced activity. As the disease progresses, the base of the antennae, the base of the oviduct and seminal duct, the hepatic carina on the carapace, and the posterior and lateral edges of the tail shell become blackened or whitened. Frequently, black or white spots are present on the sides of the tail just above each swimming leg. This disease causes mass mortalities. Repeated oral doses of varied concentrations of sulfisozole, nifurstyreic acid, and chloramphenicol, were effective in treating infected shrimp (Shigueno, 1974).
This disease is characterized mainly by abnormal behaviour. The shrimp are uneasy, jumping out of the water, then laying on their sides; body muscles may become milky white in colour; there is often a pronounced flexure at the third abdominal segment. It can cause mass mortalities (Sindermann, 1974).
The shrimp become slow moving and disoriented; their is a flexure of the tail at the third segment; the tail has an opaque white colour; their is a red discolouration of the pleopods and pereiopods. This disease causes mass mortalities. In tanks outbreaks of the disease have usually followed handling of the shrimp. Treatment with terramycin, added to food at a minimum rate of 360 mg/kg of body weight per day, resulted in improved survival of infected shrimp (Sindermann, 1974).
This disease has no visible signs. Stress, such as exposure to insecticides and crowding has been found to encourage development of this disease (Sindermann, 1974).
This occurs during handling and harvesting on hot days. The body of cramped shrimp curves and becomes rigid. Mortality is high. The real cause of this condition is unknown, but mortality is reduced if shrimp are handled during cool weather (Liao, et al, 1977).
These are associated with poor water quality, such as high content of dissolved organic matter. These organisms occur on the outside of the shell and are removed with the shell when the shrimp molts. They are primarily a problem when growth is slow and the shrimp are not molting. The best remedy is to improve conditions of the pond.
Ciliate disease. This is caused by the protozoan Zoothamnium sp. It typically occurs on the gills. It can cause mortalities when dissolved oxygen is low. Treatment with a 25 ppm formalin dip has been effective in controlling this protozoan. Some forms have environmental tolerances that can be used for control. In one case, raising salinity to 20 ppt eliminated a low salinity form (Sindermann, 1974).
Filamentous bacteria. These bacteria occur on areas of the body surface which have many setules and on the gills. Treatment with potassium permanganate at 5 to 10 ppm for one hour is an effective treatment. However, reinfestation usually occurs within 5 to 10 days (Sindermann, 1974).
Blue-green algae. This usually occurs when growth is very slow. Once it gets started, growths of blue-green algae may cause feeding and movement to be reduced even further. The shrimp then become even less tolerant of an adverse environment.
Some important factors concerning the occurrence of disease were pointed out by Sindermann (1974). When considering the occurrence of disease and parasites in a shrimp pond, it must be remembered that water quality, nutrition, and pathogens are closely related. An outbreak of infectious disease may have been brought on by poor water quality or inadequate diet. Simply adding chemicals to control the disease is not enough. The chemical imbalances in the pond or the food must be changed if long-term success is to be achieved. The importance of this is borne out by the observation by researchers in Tahiti that attacks of black spot disease were noticed only on shrimp in tanks where the conditions were poor, or after too many handlings (Aquacop, 1977).
Sindermann also points out that mortalities and signs of stress in organisms in aquaculture must be investigated from the viewpoint of possible toxicants as well as infectious disease. For example, it has been suggested that there is a relation between the presence of chlorinated hydrocarbon contamination and the occurrence of virus disease of shrimp.
Generally, treatment with chemicals should be used only as a last resort in the control of disease. It is more useful to first locate the shrimp farm in areas not affected by pollution. Then follow good management practices to keep the pond environment good and the shrimp healthy and disease-resistant. Most of the compounds found effective in treating the various diseases of shrimp have not been cleared for use by health or food authorities. Little is known about most of them and their use in ponds can have harmful effects.
Some of the chemicals may be carcinogenic (malachite green for example) or they may cause other damage to people who handle them
Harmful residues may accumulate in the shrimp and cause illness to the people who eat them
The treatment may upset the chemical balance in the pond by affecting useful organisms like nitrifying bacteria
Food organisms in the pond may be killed (Sindermann, 1974).
Chemical treatment probably is most appropriate for controlling disease in broodstock or in intensive types of culture with feeding.
In addition to the chemotherapeutic agents given as treatment with the individual diseases, several compounds which have proven useful in fish culture have been tested for their toxicity to shrimp by Hanks (1976). The following table lists the concentrations of therapeutic chemicals tested by Hanks which produced 0, 50 or 100 percent mortality in the 96-hour period following a one-hour exposure of Penaeus californiensis.
|Hyamine (ppm)||Formalin/malachite green (ppm)||Copper sulfate (ppm)||“Cutrine” (ppm)||Potassium perman- ganate (ppm)||Methylene blue (ppm)|
|LC100||90||1 000/50||750||1 000||1 000||100|
Furanace is a relatively new chemotherapeutic that has potential for use in shrimp culture. It is an effective agent for a number of bacterial and fungal pathogens of fish and crustacea. It was found to be non-toxic to Macrobrachium rosenbergii at effective levels of treatment by Delves-Broughton (1974). He observed that the drug is absorbed rapidly by the prawns and that after treatment it is excreted rapidly. Furanace appears to be an effective agent against Vibrio, Cytophaga and Aeromonas bacteria. Pseudomonas and Gaffkya homari are resistant. Most species of the fungus Saprolegnia also can be effectively controlled by this compound. The compound can be administered by means of baths at either high concentration with a short exposure time or low concentration with a long exposure time. With the short bath a dose of 20 mg/1 for 20 minutes is near the upper level of safe tolerance for Macrobrachium. With long-term baths, it was felt that a level of 2.0 mg/1 was an adequate treatment.
Enomoto (personal communication) has found that Monofuran for fish (Dainihon Seiyaku Co.) is useful in the treatment of bacterial diseases of shrimp.