SUMMARYA strain of the cattle piroplasm Babesia divergens isolated from a fatal human infection was propagated in the Mongolian gerbil through 150 semi-continuous intraperitoneal passages. The infection was normally fatal; death, accompanied by profuse haemoglobinuria and debilitation, occurred as early as 44 h after intraperitoneal inoculation of heavily parasitized blood with precipitous drops in red blood cell and platelet counts. The average maximum parasitaemia achieved increased on continuous passage reaching 80% by the 150th stage. Twenty-four hours after infection erythrophagocytosis and splenic congestion were apparent by light and electron-microscopical examination and by 48 h hepatic necrosis, renal tubular damage with haemoglobin cast accumulation and ischaemic necrosis of ileal mucosa had developed. Gerbils were highly susceptible to small numbers of parasites when the inoculum was either fresh parasitized blood in high dilution or erythrocytes concentrated from animals showing minimal parasitaemia. Animals inoculated with parasites preserved in dimethyl sulphoxide at low temperatures usually developed fatal infections. However, occasionally animals suffered at most a low grade parasitaemia subsequent to recovery with parasite elimination. These animals were immune to further challenge, and no chronic infections developed. A. field strain of B. divergens isolated locally from a case of bovine redwater behaved similarly to the human strain on continuous passage in gerbils.

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