The point prevalence of enteric parasite infection in dairy cattle in Thailand of 20% (Chaichanapunpol and Vitoorakul, 1996) was used to compute the number of heifers and heifer calves to be included in the study. Sample size was calculated using EpiInfo 6.04 (Center for Disease Control and Prevention, 1997). With a 20% expected prevalence, a sample size of 373 heifers and heifer calves would be enough to estimate prevalence within 5% of the actual prevalence, with a 5% probability of Type-1 error. We did not include a design effect in this calculation — but we have on average only ≈5 animals sampled per farm.