Is a group of severe and commonly fatal affections caused by peculiar minute protozoan parasites belonging to the genus Piroplasma.

Texas fever is an acute febrile disease of cattle resulting from the presence in the blood of the Pyrosoma bigeminum. Parasites are extremely minute, 2 to 4µ X 1.5 to 2 µ, are rounded at one end, pointed at the other, and associated individuals always have the points directed at one another. As a rule, are enclosed within the red corpuscles. In this disease the common cattle tick, Rhipicephalus annularis (Boöphilus bovis), is the means by which it is spread. This parasite is peculiar in that it passes into the eggs of the tick and infects the embryos.

Montana spotted fever is a peculiar acute febrile disease found in a certain part of Montana, and supposed to be caused by a blood parasite resembling the pyrosoma of Texas fever. Is thought that it is transmitted by a woodtick, Dermacentor andersoni, that has first obtained blood from a variety of squirrel.

Leishmania donovani are peculiar parasites discovered in the splenic pulp in patients in India suffering from Dumdum fever. These bodies are extremely minute, ovoid organisms, enclosed singly or in groups in the splenic cells, or enclosed in amorphous masses of albuminous substance apart from the cells. They are each provided with a macronucleus, whose long diameter corresponds to that of the body itself, and a micronucleus, much smaller, more elongate and slender, placed perpendicularly to it. Similar bodies have been found in the spleen in tropical splenomegaly, known as "kala-azar," in "Delhi sore," and in patients in Panama.

Was at first thought that these organisms belonged either to the trypanosomes or to the piroplasma, but at present they are classed by themselves.