Liver coccidiosis in some marine fish species from Scottish waters
The Coccidia Goussia clupearum (Thelohan, 1894) Labbe, 1896 was found infecting the liver of four different fish species from Scottish water; herring Clupea harengus, mackerel Scomber scombrus, Norway pout Trisophterus esmarkii and poorcod Trisopterus minutus. Sprat Sprattus sprattus was not infected. The stages of development corresponding to gamogony and sporogony of this parasite are described by light and electron microscopy on livers of naturally infected fish. Macroscopical lesions due to coccidiosis were not found. A condition named as 'red liver' was found in some infected Norway pout. However, there is no evidence of its being a symptom of liver coccidiosis alone but may be related to environmental factors. Histopathological studies of infected livers revealed an effective host reaction in herring, mackerel and Norway pout with engulfing of parasitic stages by host phagocytes, lymphocytes and eosinophils. Occurrence of 'yellow bodies' was also sometimes observed in infected livers of herring and mackerel. Prevalence and intensity of Goussia clupearum infections was followed during the period from October '88 to January '91. There is evidence of a significant relationship between prevalence and season of the year and of a higher prevalence in adult fish compared with juveniles, although the difference was not statistically significant. No clear indication of a statistically significant decrease in condition factor with increasing intensity of liver coccidiosis was found. Experimental infections of uninfected herring with oocysts of G. clupearum collected from infected mackerel were unsuccessful. It is suspected that the life cycle of this coccidian involves euphausiids as intermediate hosts. This conclusion was drawn after studies of the feeding habits of the fish species examined during this work and their relative prevalences of G. clupearum.