Factors in Host Susceptibility and Oocyst Infectivity in Eimeria acervulina Infections*

SYNOPSIS. Eimeria acervulina infections were studied by counting oocysts collected from individual chickens before they passed into droppings. More oocysts of E. acervulina were produced on day 4 post‐infection than at any other time. The reproductive potential per oocyst decreased as the infecting dose increased. The mean numbers of oocysts present in the intestine after 4 days were 6,590 per oocyst fed when the infecting dose was 10,000 oocyts; 6,180 when it was 50,000 oocysts; 4,150 when it was 100,000 oocysts; 444 when it was 1 million oocysts; and 71 when it was 10 million oocysts. Oocysts become infective within 13 hours sporulation at 30°C, reach a peak of infectivity before 27 hours and start to decline after 60 hours. Oocyst yield increased linearly with age, from a mean yield of 80.1 × 106 oocysts in the intestine of 13‐day‐old birds to a mean yield of 1223 × 106 oocysts in the intestine of 107‐day‐old birds. There was a seasonal effect upon oocyst yields, with higher yields in the winter during the year in which observations were made.