Changes in the haemolytic activity of serum complement during acute Babesia bovis infection in cattle

The haemolytic activity of serum complement was measured in cattle during acute Babesia bovis infection. The level fell significantly on day 5 post infection (p.i.) and was not detectable by day 10 p.i. After clinical recovery between days 15 and 20 p.i. haemolytic activity was again detected in serum and gradually returned to the pre-infection level. The components of both classical and alternative pathways were involved in complement depletion and factors such as protease secretion by the parasites, antigen-antibody reactions, and the release of haemoglobin were implicated as causes of the decline. The absence of complement activity during the acute phase of the disease may interfere with immune mechanisms at a critical time for the host.