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The interrelationships between the host organisms, their pathogens and environmental factors are shown in Fig. 7. In an unpolluted environment with only the normal fluctuations in ambient conditions, there will be a natural balance between H, P and E, leading to sporadic outbreaks of disease. However, a reduction in the quality of E will lead to a marked increase in the frequency and severity of D, mainly by reducing the resistance of the host organisms to diseases. Also, an increase in the population density of H will increase the risk of disease outbreaks, as shown later in Table 4, as will an increase in the virulence of P. This chapter describes some of the effects of reduced water quality on the susceptibility of fish to disease.

It is possible that adverse environmental conditions may decrease the ability of organisms to maintain an effective immunological response system, so that an increased susceptibility to different diseases might be expected to occur. This certainly occurs in aquatic organisms, particularly fish, where acute and/or chronic pollution of surface waters can cause a reduction in the level of unspecific immunity to disease. For example, a significant decrease in the concentration of total proteins, globulins and lysozymes in the blood plasma of carp can occur after a long-term exposure to sublethal zinc concentrations. A decrease in the number of leucocytes and significant changes in their differential count are typical effects caused by a number of pollutants (e.g. phenols, metals, pesticides etc.); a characteristic decrease in the percentage of lymphocytes and an associated increase in granulocytes can occur. Such a decrease in the number of small lymphocytes which are active in the increase and transfer of globulins, is followed by a decrease in antibody production and thus a decrease in resistance to disease.

Any marked change in surface water quality is reflected both directly (as has been already mentioned) and indirectly in the structure of the fish population. Indirect effects can occur from damage to the food web which consists of lower organisms in the aquatic environment. There is a wide range in the susceptibility of individual species of aquatic organisms to different pollutants. In most cases, the lower aquatic organisms (i.e. components of the zooplankton and zoobenthos) contain the more susceptible species. Thus, at low concentrations of pollutants (e.g. metals, pesticides, surfactants etc.), damage and mortality of sensitive food organisms can occur. In consequence, although fish are not affected primarily, they suffer from secondary effects; the reduction and/or complete absence of natural food leads to a poorer condition of the fish and this may be accompanied by a decrease of antibody production. In this way, the disease resistance of fish may be decreased.

Fig. 7

Fig. 7: Interactions between host, pathogen and environment, and the outbreak of diseases (after Wood, 1974; Bohl, 1989)

However, it must be stressed that not every outbreak of fish disease is due to pollution; other factors, such as overcrowding or the rapid increase in the number or virulence of disease organisms may be the primary cause. Nevertheless, the severity of the outbreak may be increased if accompanied by a reduction in water quality. The following fish diseases have been shown to have a close connection with reduced water quality.

6.1 Viral diseases

The presence or introduction of specific viral agents into ponds, reservoirs and streams is necessary to cause outbreaks of infectious pancreatic necrosis in salmonids, viral haemorrhagic septicaemia in rainbow trout, spring viraemia of carp, infectious swim-bladder inflammation, ulcerative dermal necrosis in salmonids, pox and other viral diseases. Where water quality is reduced, a more complicated course of the disease together with a higher mortality of fish are usually found. In particular, a reduced water quality is an important stress factor in viral haemorrhagic septicaemia. A low dissolved oxygen content and extreme changes in the pH of the water, together with an increased accumulation of metabolites, are other important factors associated with outbreaks of infectious haemopoetic necrosis in salmonids.

Table 4: Environmental factors which are harmful to warm and coldwater fish and increase their susceptibility to certain diseases (from Wedemeyer and McLeay, 1981)

DiseaseEnvironmental stress factors predisposing to disease
(Aeromonas salmonicida)
Low oxygen (≈ 4 mg l-1); crowding; handling in the presence of A. salmonicida; handling for up to a month prior to an expected epizootic
Bacterial gill disease
(Myxobacteria spp.)
Crowding; unfavourable environmental conditions such as chronic low oxygen (4 mg l-1); elevated ammonia (0.02 mg l-1 unionized); particulate matter in water
(Flexibacter columnaris)
Crowding or handling during warm (15°C) water periods if carrier fish are present in the water supply; temperature increase to about 30°C, if the pathogen is present, even if not crowded or handled
Kidney disease
(Renibacterium salmoninarum)
Water hardness less than about 100 mg l-1 (as CaCo3); diets containing corn gluten or of less than about 30% moisture
Hemorrhagic septicemia
(Aeromonas and Pseudomonas)
Pre-existing protozoan infestations such as Costia, Trichodina; inadequate cleaning leading to increased bacterial load in water; particulate matter in water; handling; low oxygen; chronic sublethal exposure to heavy metals, pesticides or polychlorinated biphenyls (PCBs); for carp, handling after overwintering at low temperatures
(Vibrio anguillarum)
Handling; dissolved oxygen lower than about 6 mg l-1, especially at water temperatures of 10–15°C; brackish water, of 10–15 per mille salinity
Parasite infestations
(Costia, Trichodina, Hexamita)
Overcrowding of fry and fingerlings; low oxygen excessive size variation among fish in ponds
Spring viremia of carp and tail rotHandling after overwintering at low temperatures. Crowding; improper temperatures; nutritional imbalances; chronic sublethal exposure to PCBs; or to suspended solids at 200–300 mg l-1
Coagulated yolk of eggsRough handling; malachite green containing and fry more than 0.08% zinc, gas supersaturation of 103% or more; mineral deficiency in incubation water
“Hauling loss”
(delayed mortality)
Hauling, stocking, handling in soft water (less than 100 mg l-1 total hardness); mineral additions not used; CO2 above 20 mg l-1
Blue sac disease of eggsCrowding; accumulation of nitrogenous metabolic wastes due to inadequate flow patterns

6.2 Bacterial diseases

As with viral diseases, the presence of specific bacterial agents is necessary to cause an infection. Many of these agents can survive naturally in the environment (e.g. Aeromonas punctata, Aeromonas salmonicida) and/or in the digestive tract of clinically healthy fish; with an increase in their virulence and/or a weakening of the host organism (e.g. due to a polluted aquatic environment) these agents can act as causative factors in the outbreak of a bacterial disease.

Organic pollution of water, followed by a decreased content of dissolved oxygen, creates a favourable environment for the growth of bacteria. A direct relationship between the organic pollution of surface waters and outbreaks of furunculosis is well established, so that this disease may at times serve as a positive indicator of poor water quality; the causative agent, Aeromonas salmonicida, can survive for a maximum of one week in tap water, 12 weeks in stream water and as long as 6 months in organically polluted mud. Organic pollution of the aquatic environment is also an important factor in columnaris infection. Vibriosis occurs most frequently in brackish water, although in inland waters it can be found in localities receiving inputs of salt. Organic and even physical (e.g. inert suspended solids) pollution of water can be important factors in inducing flexibacteriosis in the gills of salmonids, by damaging the delicate gill respiratory epithelium.

6.3 Fungal diseases

A direct relationship between branchiomycosis and organic pollution of water is well known in fish culture practice. Usually, the disease is endemic in ponds and reservoirs; cyprinid fish species, whitefish, pike, but also wels and rainbow trout can be affected. The outbreak and duration of the disease depend on ambient environmental factors, the most important of which is water temperature. The disease occurs most frequently when the water temperature is above 20°C (with an optimum of 26°C) and is accompanied by organic pollution and associated fluctuations in the dissolved oxygen concentrations. Mechanical (i.e. physical) and/or chemical damage of the protective mucus layer of the skin, fins and gills are prerequisites for the disease outbreak. Such damage is also a precondition for the secondary development of saprolegnia; fungal spores develop to form greyish-whitish woolly growths on the damaged surfaces, particularly in weakened fish.

6.4 Fish parasites

The degree of pathogenic activity exerted by ecto-and endoparasites living on the body surface and/or in internal organs of fish, can be influenced by water pollution (KHAN and THULIN, 1991). Contaminating substances such as pesticides may have a harmful effect on the parasites but fish weakened by parasite infestation may be more sensitive to the toxic effects of substances in the water.

For a number of fish protozooses there is a conditional dependence on organic and other pollution of the aquatic environment; for example, such a reduction in water quality can be followed by a gill invasion with Cryptobia branchialis. Reduced pH values of the water (e.g. to 5–6), together with unsuitable breeding conditions, can contribute to an outbreak of ichthyobodosis. Poor hygienic conditions in ponds and reservoirs carry a potential danger for myxosporeoses outbreaks; low dissolved oxygen concentrations associated with low light conditions are favourable for chilodonellosis. Thermal pollution can lead to lethal outbreaks of ichthyophthiriosis. Domestic sewage discharged into surface waters can be a source of high populations of trichodines. Phenol and polychloropinen can cause fish to become more sensitive to Ichthyophthirius multifiliis; an increased sensitivity of carp to this parasite has also been found in connection with sublethal concentrations of cadmium.

As for the commonly found helminthoses, the relationship between a low oxygen concentration in water and the complicated course of dactylogyroses is well known. The oxygen content of the water is also an important factor affecting the growth and abundance of Gyrodactylus sp. populations; for example, a decrease in oxygen concentration of 50% caused a three to four-fold increase in their reproduction rate. This effect is probably caused by the weakening of the host organisms under these conditions rather than the direct effect of an oxygen deficiency on the parasites. Several Finnish authors (Koskivaara et al., 1991; Koskivaara and Valtonen, 1992) have found a high prevalence of monogenea, particularly the species Dactylogyrus similis, D. fallax, Gyrodactylus gasterostei, G. carasii and G. vimbi, in fish from lakes polluted by papermill effluents. However, this type of pollution also caused a decreased infestation of gill parasites in the fish.

Contamination of the aquatic environment can even affect the prevalence and intensity of the infestation of fish with multicellular endoparasites. In such cases, pollutants can act either on the intermediate host or directly on the fish organism, and can also affect the associated defence mechanisms and immune responses. In heavily polluted water bodies, there is a strong relationship between a high prevalence of parasites and the condition of the fish. A poor state of fish health is the result of enhanced effects of the parasites on fish harmed by the direct effects of pollution, rather than of the primary effect of the parasites themselves. However, it may be difficult to assign an increase in fish sensitivity to a primary cause, i.e. to the pollutant or the parasite. Nevertheless, it is clear that such an association can exist; carp fingerlings infested with tapeworm Bothriocephalus gowkongensis were found to be more sensitive to DDT (Perevozchenko and Davydov, 1974), and Pascoe and Cram (1977) found a higher sensitivity of stickleback to cadmium when infested with the tapeworm Schistocephalus solidus.

6.5 Conclusion

Substances which contaminate the aquatic environment can be harmful not only by their direct effects on the organisms there. It is well established that some diseases and developmental abnormalities may occur more frequently in fish living in a polluted environment. However, there is only a limited amount of information on this association, which is mainly related to experiences with farmed or cultured fish. Apart from the examples given above, other similar information is available, some of which is given in Table 4. The limited number of environmental stressors involved - low dissolved oxygen, extremes of temperature and pH, and ammonia - are probably due to the siting of fish farms on relatively unpolluted waters. There is now a real need to study the interrelationships between the pollution of surface waters by a wide range of chemicals and diseases in natural fish populations, and the processes involved. This represents an important but at present under-developed field of scientific research and fisheries management.

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