The reappearance of eggs in faeces of horses after treatment with ivermectin.

The reappearance of nematode eggs in faeces after ivermectin treatment was studied in 104 horses on 10 farms during the stabling period. Faecal samples were taken at weekly intervals. Sampling was discontinued when the mean egg output per farm was > 10% of the pre-treatment egg output. This point was reached after 63 days, when the mean output of eggs had decreased to 70.3%. Before treatment, 95.9% of the cultured larvae were of the cyathostome type, the others belonged to Gyalocephalus capitatus, Strongylus vulgaris, S. edentatus, Oesophagodontus/Poteriostomum spp., Triodontophorus spp. and Trichostrongylus axei. After treatment, the cultures nearly always produced 100% cyathostome larvae, although occasionally low numbers (< 1%) of larvae of large strongyles (mainly S. edentatus, rarely S. vulgaris and Oesophagodontus spp.) and T. axei were seen.