FISH PATHOLOGY 



By W. T. Foster 

 Easton, Pennsylvania 



Early in the summer of 1920 the writer was called to the State 

 fish hatchery at Hackettstown, N. J., for the purpose of carrying on 

 some experiments in fish pathology, with special reference to the 

 bacterial diseases of fish. But a comparatively small amount of work 

 has been done along these lines, especially in the United States. The 

 only investigator who has done much work along the line of fish dis- 

 eases in general is Hofer of Germany, whose book, "Das Handbuch 

 der Fisch Krankheiten," has not yet been translated into English. 



Most diseases among fish are caused by plant and animal micro 

 and macro organisms. Comparatively little is known concerning the 

 diseases caused by the microscopic vegetable organisms or bacteria. 

 Bacteria are microscopic plants comprising a subdivision of the fungi. 

 The two more important diseases with which we have come in con- 

 tact at the hatchery, and which are more or less universal, are those 

 caused on the one hand by a microscopic animal and on the other by 

 a microscopic plant. The first mentioned disease is' caused by an 

 inf usorian of the protozoan group known as the Ichthyophthirius mul- 

 tifilis. It is comparatively easy to detect this organism by the pres- 

 ence of small, grayish, pimple-like protuberances on the bodies of the 

 fish. By scraping off one of these and placing it under the micro- 

 scope the actively moving organism can be seen, thus substantiating 

 the naked eye diagnosis. 



A specific cure for this disease is known, and, therefore, should 

 not receive our serious attention. It is a matter of keeping our eyes 

 open, and once the disease is detected in its incipiency, the thera- 

 peutic measure is simple. This disease is fully discussed elsewhere. 

 With the bacterial diseases the case is quite different. A number of 

 diseases of this type have been fully described ; the organisms causing 

 them have been isolated and classified, but the eradication of these 

 diseases when once they appear creates a difficult problem which is 

 not easy to solve. 



The one bacterial organism with which we come in contact per- 

 haps more than any other and which creates more varied morbid 

 conditions in fish, is that described by Alarsh, formerly of the U. S. 

 Bureau of Fisheries, and known as the "Bacterium truttae." At the 

 hatchery we have isolated what we believe to be the same organism, 

 from the lieart's blood and from local lesions. The disease as we 



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