2 U. S. BUREAU OF FISHERIES. 



SYMPTOMS OF THE DISEASE. 



The first symptoms of the presence of Ichthyophthirius are the 

 queer actions of the fish and the appearance of small, whitish-gra}', 

 sharply defined pimples sticking out over the surface of the body. 

 When the fish are first infected they rub themselves continually 

 against the bottom and sides of the tank in an attempt to brush off 

 the young parasite. In a short time they will be seen to hang listlessly 

 in the water with drooping fins, or gasping at the surface due to the 

 impaired condition of the gills. All parts of the body, head, sides, 

 fins, and especially the mouth and gills, may be heavily infected by 

 these organisms. 



As the parasite grows it becomes visible as a raised light-colored 

 spot on the fish, and if these are numerous the adjacent ones may 

 touch each other and unite to form a large spot, giving the skin a 

 mottled appearance. As the disease spreads the raised spots often 

 occur by the hundreds and thousands, covering the entire external 

 surface of the fish. Fish that become heavily infected soon stop 

 eating, and in the last stages of the disease they are covered with a 

 heavy slime and red blotches or " scalded " areas. Fungus soon de- 

 velops on the raw parts of the fins and body, and the fish live but a 

 short time. 



DESCRIPTION OF THE PARASITE. 



The adult parasite may be identified by gently scraping off one of 

 the " spots •■ from the fish and carefully examining it under the 

 microscope. It may be necessary to separate the animal from the 

 tissue surrounding, in which it is generally found in active rotating 

 movement. The adult parasites range in size from one-half to 

 slightly less than 1 millimeter in diameter. The body is round or 

 elongate oval in shape when it is inactive, but will assume almost any 

 form when in motion. Covering the outer surfaces are numerous 

 short threadlike cilia arranged regularly in rows, which serve to 

 propel the animal through the water. The protoplasm of the body 

 contains numerous opaque granules, a large crescent-shaped nucleus. 

 a small oval opening or mouth, and many fine contractile vacuoles. 

 (See fig. 7.) In warm w'ater the animal soon surrounds itself with 

 a semitransparent membrane or cyst and there undergoes many suc- 

 cessive divisions, forming thousands of young parasites. 



LIFE HISTORY OF THE PARASITE. 



In order intelligently to combat the disease, a thorough knowledge 

 of the life history is necessary. The young parasite is a microscopic, 

 free-swimming form, which revolves rapidly through the water in 

 search of a host. Upon coming in contact with a fish it endeavors to 

 burrow into the epidermis, especially in the gills, fins, and unsealed 

 regions. At this time the fish is irritated the most and tries hardest 

 to brush it off. If the young parasite is unable to find a host, it will 

 die within a few days, as it seems incapable of taking food from the 

 water. Fish that have been previously infected are covered with a 

 layer of slime, which offers an unusually favorable medium for at- 

 tachment for the young parasites. When once embedded in the skin 

 of the fish, the parasite begins to grow rapidly with the nourishment 



