Babesia major in Britain: blood-induced infections in splenectomized and intact calves

A strain of Babesia major, originally isolated from field collections of Haemaphysalis punctata in Kent, England was maintained in splenectomized calves by the intravenous inoculation of infected blood. Rapid passage from carrier calves, that had recovered from a tick-induced infection, resulted in a marked increase in virulence; 4 out of 6 calves of the second passage underwent fatal infections and the others suffered severe reactions.Five splenectomized and 5 intact calves of the same breed and age were infected with the same number of infected erythrocytes (RBC). The intact calves reacted mildly with maximum parasite counts ranging from < $\text{1}\text{1000}$ RBC to $\text{5}\text{1000}$ RBC; haemoglobin levels and packed cell volume values, however, fell sharply but recovered swiftly. The group of splenectomized calves exhibited one fatal case, 2 severe reactions and 2 mild infections; maximum parasitaemias varied from $\text{7}\text{1000}$ RBC to $\text{322}\text{1000}$ RBC. Packed cell volumes and haemoglobin concentrations declined to low levels and took several weeks to return to normal.It is concluded that B. major should be regarded as a potential pathogen of British cattle.