164 



THE TELOSPORASIDA AND THE COCCIDLA PROPER 



stages of many species are unknown, the 

 structure of the oocyst is most commonly 

 used. The feeling is sometimes expressed 

 that the oocysts have so few structures 

 that not many species can be distinguished 

 morphologically, but conservative calcu- 

 lation shows that at least 2, 654, 208 mor- 

 phologically different oocysts are possible 

 in the genus Eimeria alone (Levine, 1961). 



A second group of criteria is the loca- 

 tion of the endogenous stages in the host. 

 This has been discussed above. Host 

 specificity is a third criterion. This var- 

 ies with the protozoan genus and to some 

 extent with the species. In general, the 

 host range of Isospora and Tyzzeria spe- 

 cies is relatively broad. Several mem- 

 bers of the same host order may be in- 

 fected by the same species of these genera. 

 For example, Isospora bigemina occurs in 

 the dog, cat, ferret and mink, while 

 Tyzzeria anseris has been found not only 

 in the domestic goose and several other 

 members of the genus Anser, but also in 

 the Canada goose and Atlantic brant (both 

 Branla) and whistling swan (Olor). On the 

 other hand, the host range of Eimeria spe- 

 cies is relatively narrow. A single species 

 rarely infects more than one host genus 

 unless the latter are closely related. 



Cross-immunity studies are also used 

 in differentiating the coccidia of a partic- 

 ular host species from each other. Infec- 

 tion of an animal with one species of coc- 

 cidium produces immunity against that 

 species but not against other species which 

 occur in the same host. 



Diagnosis : Coccidiosis can be diag- 

 nosed by finding the coccidia on micro- 

 scopic examination. There are several 

 pitfalls in diagnosis. Each species of do- 

 mestic and laboratory animal has several 

 species of coccidia, some of which are 

 pathogenic and some of which are not. 

 Since an expert is often needed to differ- 

 entiate between some of the species, the 

 mere presence of oocysts in the feces, 

 even in the presence of disease signs, is 

 not necessarily proof that the signs are 

 due to coccidia and not to some other 

 agent. 



Following recovery from a coccidial 

 infection, an animal is relatively immune 

 to reinfection with the same species. This 

 immunity is not so solid that the animal 

 cannot be reinfected at all, but it does 

 mean that the resultant infection will be 

 low-grade (except possibly under conditions 

 of stress) and will not harm the host. Such 

 low-grade infections are extremely common, 

 i.e., the animals have coccidiasis rather 

 than coccidiosis. Hence, the presence in 

 the feces of oocysts of even highly patho- 

 genic species of coccidia does not neces- 

 sarily mean that the animal has clinical 

 coccidiosis. 



On the other hand, coccidia may cause 

 severe symptoms and even death early in 

 their life cycle before any oocysts have 

 been produced. This occurs commonly, 

 for example, with E. tenella of the chicken 

 and E. ziirnii of the ox. Consequently, 

 failure to find oocysts in the feces in a 

 diarrheal disease does not necessarily 

 mean that the disease is not coccidiosis. 



The only sure way to diagnose coccid- 

 iosis, then, is by finding lesions containing 

 coccidia at necropsy. Scrapings of the le- 

 sions should be mixed on a slide with a 

 little physiological salt solution and exam- 

 ined microscopically. It is not enough to 

 look for oocysts, but schizonts, mero- 

 zoites, gametes and gametocytes inside 

 the host cells must be sought for and rec- 

 ognized. 



Some species of coccidia can be iden- 

 tified from their unsporulated oocysts, 

 but study of the sporulated oocysts is often 

 desirable. Oocysts can be sporulated by 

 mixing the feces with several volumes of 

 2. 5% potassium bichromate solution, plac- 

 ing the mixture in a thin layer in a Petri 

 dish and allowing it to stand for 1 day to 

 2 weeks or more, depending on the spe- 

 cies. The potassium bichromate prevents 

 bacterial growth which might kill the pro- 

 tozoa, and the thin layer is necessary so 

 that oxygen can reach the oocysts. 



Treatment . The first compound 

 found effective against coccidia was sulfur, 

 which was introduced by Herrick and 



