Objective: To determine the association between Isospora suis infection and weight gain in pigs up to 8 weeks of age.

Materials and methods: A prospective cohort study was conducted on a convenience sample of three swine farms in Ontario. Fecal samples were collected from randomly selected piglets from each of 72 litters and examined for oocysts using centrifugal floatation. Piglet weight was recorded six times, during the first, second, third, fourth, fifth, and eighth weeks of life. If one or more suckling piglets from
a litter were shedding I suis oocysts, then the litter was classified as infected. A linear mixed model was used to examine the effect of infection on weight gain.

Results: Isospora suis infection during suckling was associated with lower weights of pigs at the end of the nursery stage (average 62 days of age). Pigs from infected litters were 1.4 kg lighter than pigs from non-infected litters (95% CI, 1.1-1.8 kg; P < .001).

Implications: Infection with I suis during the suckling period can have an impact on postweaning performance. Thus, calculations of the economic impact of coccidiosis need to include the effect of slower growth rates in the first few weeks after weaning, as well as a reduction in suckling pig performance.