Abstract:
Surveys to determine the prevalence and degree of resistance of Haemonchus spp. of sheep and
goats to the available anthelmintics in South Africa indicate that small ruminant production is entering
a crisis situation .
Three surveys employing the faecal egg count reduction (FECR) test to determine resistance were
conducted in some of the main sheep-producing areas in the summer rainfall region of South Africa,
where H. contortus is the principal worm species in sheep.
After analyzing the data recorded in the surveys by six different methods, including the RESO test at
two different levels of confidence, the results obtained in the least stringent one (geometric mean
reduction of the worm egg counts of drenched, vs untreated group of sheep) are reported in this paper,
so that if any bias was obtained it would be in the favour of the anthelmintic.
In Mpumalanga and KwaZulu-Natal there was anthelmintic resistance in Haemonchus spp. on all the
52 farms surveyed. Sixteen percent of the strains of H. contortus were < 60 % susceptible to three of
the four anthelmintics tested , and 8 % of the strains were < 40 % susceptible to all four of the
anthelmintics. FECR tests of sheep in six localities in the Lebowa district of Northern Province indicated
that even in previously disadvantaged communities where anthelmintic treatment is less intensive,
anthelmintic resistance is developing, and is possibly at the level at which the situation on commercial
sheep and goat farms in South Africa was 25 years ago.
From the data it appears that the level of anthelmintic resistance of H. contortus in South Africa is
possibly the highest that has so far been recorded in the world and that strains of it are emerging that
may soon not be controllable by treatment with any of the existing anthelmintics. Farmers in the summer
rainfall region , if not the whole country, must be alerted to the immediate need for testing the
parasite burdens of their sheep for susceptibility to preparations in all four groups of anthelmintic
compounds currently available. Alternative methods of integrated worm control , including biological,
must be sought and implemented with urgency, to reduce further selection for resistance and to induce
reversion of the resistance that has already developed.
