The effect of strategic anthelmintic treatment on internal parasites in communally grazed sheep in a semi-arid area as reflected in the faecal nematode egg count.

Communally grazed sheep were dosed at 4-, 12-, 24- or 48-week intervals for 1 year. Dosing every 4 weeks proved to be the most effective (p < 0.05), as reflected in a lower worm egg count compared to the 12-, 24- or 48-week intervals. Since most nematode life cycles lie between 3 and 6 weeks, the treatment has to be given during this critical period if maximum economic advantage is to be gained from deworming. However, treating communally grazed sheep every 12 weeks was found to keep worm egg numbers relatively low and may be advantageous in providing seasonal control, especially in semi-arid environments. Dosing communally grazed sheep once or twice a year under the same conditions is not recommended because reinfection appeared to result in similar faecal egg counts to those from the untreated animals (p > 0.05).