The criteria for treatment of calves were increasing A. marginale
rickettsemia and 30% reduction of baseline PCV, determined
individually before inoculation. Rickettsemia was assessed through
Giemsa-stained peripheral blood smears and the percentage of
parasitized erythrocytes was determined by dividing the number
of infected erythrocytes in 40 fields by the total number of
erythrocytes in 40 fields at 100× oil immersion (IICA, 1987).