A spontaneous epidemic of nephritis among young rabbits associated with a protozoan parasite has been observed in a certain breeding stock since 1918. It is regarded as a nest infection. The parasite is tentatively classed among the microsporidia and the stages encountered are regarded as vegetative which may perhaps pass through several generations in the same host. It is a parasite of the epithelial cell, provoking no immediate host reactions. These are supposed to follow injury such as destruction of the epithelium and denudation and plugging of the tubules. The localizations in the brain are also without cell reaction, except under special conditions, and the many cell foci present are attributed to coccidia and perhaps bacteria and other intestinal parasites. The kidneys are looked upon as the normal habitat and the brain parasites as aberrant and outside the normal cycle unless it can be shown that the spores discharged into the circulation may again multiply in some organ like the kidneys in direct communication with the exterior.

It is highly probable that many reported cases of nephritis among older rabbits used in experiments had their origin in an early invasion of the parasite described.

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