Age, not Infection Dose, Determines the Outcome of Isospora suis Infections in Suckling Piglets

Data from 13 trials involving 124 suckling piglets experimentally infected with Isospora suis were evaluated for the effects of infection dose and age on the clinical and parasitological outcome of infection in four different models, infections with 1,000 oocysts on the 1(st) day of life (d.o.l.) (model 1; 9 piglets/3 litters), 1,000 oocysts on the 4(th) d.o.l. (model 2; 25 piglets/11 litters), 1,500 oocysts on the 4th d.o.l. (model 3; 40 piglets/20 litters) and 10,000 oocysts on the 4th d.o.l. (model 4; 50 animals/10 litters). Weights were determined on the day of birth and in weekly intervals. Faecal consistency and quantitative oocysts excretion were evaluated for 2 weeks starting 4 days after infection (d.p.i.). The weight gain depression was most noticeable in model 2 (infection on the 1(st) d.o.l.), where animals only gained 2.08x their birth weight until the 22(nd) d.o.l., compared to 2.31-2.52x in the other groups. This correlated with the occurrence of watery diarrhoea which was found in 37% of the samples in the acute phase (4-11 d.p.i.) in model 2 but only in 12-20% of the samples in the other models. Median oocyst excretion peaked earlier in the models with higher infection doses but reached the highest values in model 2 (early infection). As in previous studies, this cross-sectional analysis of a larger number of animals confirms the influence of age on the outcome of isosporosis in suckling piglets, stressing the need to control the infection at an early life phase.