FISHKRY BULLKTIN: VOL. 76. NO. 4 



neither instance were antibody titers determined. 

 In the latter instance, A. Uquefaviens is an 

 ubiquitous water bacterium, but only causes dis- 

 ease and mortalities in fish with lowered resis- 

 tance (Snieszko 1962). 



Reduction in antibody response to injected virus 

 was demonstrated by Perlmutter et al. (1973) in 

 blue gourami, Trkhogaster trichopteri/s, to result 

 from overcrowding. The authors postulated that 

 stressed fish released a pheromonelike im- 

 munosuppressive factor under crowded condi- 

 tions. It is reasonable to expect that other types of 

 environmental stresses could result in a similar 

 response. 



Among the invertebrates, indirect evidence for 

 reduction of disease resistance caused by conta- 

 minant exposure is available and has already been 

 discussed in previous sections on crustacean shell 

 disease and shrimp virus disease. Direct experi- 

 mental evidence however, is scarce. Fries and 

 Tripp (1976) exposed hard (hard-shell) clams to 

 phenol and found damage to gill and digestive 

 tract epithelia — tissues which are considered im- 

 portant components of internal defense 

 mechanisms. The authors suggested, but did not 

 demonstrate, that phenol-treated clams may be 

 more susceptible to microbial infections than 

 normal ones. In other studies with invertebrates, 

 Telford (1968, 1974) demonstrated that environ- 

 mental stress affected blood glucose levels in 

 Homarus americanus and crayfish, Cambarus 

 clarkii. 



POLLUTANT-PARASITE 

 INTERACTIONS 



Much has been said and much documentation 

 exists about the role of environmental stress in 

 induction, severity, and persistence of disease. 

 Some of the best information about stress and dis- 

 ease in fish comes from studies concerned with 

 aquaculture — where environmental factors such 

 as temperature, oxygen, water quality, salinity, 

 and diets clearly influence the course of disease 

 and the impact of disease on cultured populations. 



There is also a developing body of information, 

 from experimental work as well as from field ob- 

 servations and surveys, about the possible rela- 

 tionship of parasitism and pollution. The relation- 

 ship is not simple, and in essence involves a 

 double-edged phenomenon, in which pollutant 

 stress may result in an increase (or in some in- 

 stances decrease) in the prevalence of certain 



740 



parasites, or in which parasitization may decrease 

 host resistance to toxic pollutants. Subsidiary is- 

 sues quickly emerge however, such as the effects of 

 pollutants on intermediate or alternate hosts in 

 parasite life cycles, possible effects of pollutants on 

 free-living life cycle stages of parasites, and effects 

 of pollutants on host defenses against parasite in- 

 vasion. 



Thus far in this review, the role of microbial 

 infectious agents, principally viruses and bac- 

 teria, has been emphasized, but there is some li- 

 mited evidence that environmental pollution may 

 change the relationships among animal parasites 

 and their fish hosts (Esch et al. 1975). 



Looking first at the influence of parasites on 

 host susceptibility to contaminants, several recent 

 papers (principally from studies in freshwater) 

 offer significant insights. Boyce and Ydmada 

 (1977) found in laboratory experiments that sock- 

 eye salmon, Oncorhynchus nerka, smolts with 

 preexisting parasitization by the intestinal 

 pseudophyllidean cestode Eiihothrium salvelini 

 were more susceptible to zinc poisoning than un- 

 parasitized siblings. Similarly, Pascoe and Cram 

 (1977) found that survival times of the threespine 

 stickleback, Gasterosteiis aciileatus, exposed to 

 various concentrations of cadmium, were much 

 shortened if the fish were parasitized by the larval 

 cestode Schistocephalus solidiis. Perevozchenko 

 and Davydov (19741 found that juvenile carp 

 parasitized by the intestinal cestode Both rioceph- 

 alus goivkongensis were more susceptible to DDT 

 poisoning than were nonparasitized individuals. 

 These results are not surprising, since fish already 

 weakened by parasites would undoubtedly be less 

 able to tolerate other environmental stresses. The 

 nature and degree of parasitization offish clearly 

 must be considered in bioassays and in studies of 

 effects of contaminants on fish and shellfish 

 species. 



Looking next at the reverse viewpoint, the 

 influence of contaminants on parasite prevalence, 

 definitive information is less readily available for 

 marine species, but some information is available 

 for freshwater species. Thermal loading was as- 

 sociated with changes in the distribution and 

 abundance of two larval trematodes in mos- 

 quitofish (Aho et al. 1976). Similarly, thermal 

 loading from a nuclear power plant was directly 

 correlated with incidence of the ciliate Epistylis 

 sp. and the bacterium Aeromonas liquifaciens 

 ( = A. hydrophila) in six species of centrarchids in 

 South Carolina (Esch et al. 1976). Effects of ther- 



