Formalin for external protozoan parasites: A report on the prevention and control of Costia necatrix

Abstract

The smallest and most destructive of the ectoparasitic protozoans infecting salmon and trout, Costia necatrix , has unfortunately been relegated to virtual obscurity during the past few years. Few references to this parasite can be found in the recent literature and, where such things are discussed, one seldom hears a mention of Costia necatrix . This apparent lack of interest in Costia certainly does not result from the infrequency of its appearances nor from any lack of pathogenicity on its part when it does occur. Preserved specimens of ailing trout an dsalmon submitted to the Seattle Pathology Laboratory for diagnosis have yielded very surprising indications concerning the frequency, intensity, and geographic distribution of Costiasis - surprising in that a parasite should be so widespread, so pathogenic, and yet so seldom mentioned. The answer undoubtedly lies in the fact that Costia , being small in size and usually sedentary in its habits, is being overlooked during parasitic examinations. Although Costia is not at all difficult to recognize, even well-trained workers unfamiliar with its appearance almost invariably pass it by.