A small proportion of laboratory mice, which appear to be normal, are in reality suffering from an obscure disease characterized by the presence of marked meningoencephalitic lesions which are often accompanied by protozoan-like parasites.

Both the lesions and the parasites closely resemble others previously reported in rabbits, which likewise fail to reveal themselves by pronounced clinical symptoms.

In the absence of direct experimental evidence it is suggested either that one species of parasite is capable of producing the lesions in both animals, or else that we have to do with two kinds of parasites which are closely related and, in the stages thus far observed, are indistinguishable one from the other.

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