Effect of Eperythrozoon ovis on the lysis of sheep erythrocytes in the complement fixation test

When erythrocytes from sheep experimentally infected with Eperythrozoon ovis were used in the titration of reagents for a standardised complement fixation test, increased amounts of both haemolysin and complement were required for erythrocyte lysis compared with preinfection titrations. The haemolysin requirement increased by up to 125% at 55 days post-infection and complement requirement increased by up to 40% at 40 days post-infection. These changes appeared to correlate with the development of a macrocytic anaemia in affected sheep rather than E. ovis parasitaemia. The results emphasise the need to carefully monitor the haematological parameters of sheep used as sources of erythrocytes for the complement fixation test.