Parasitology
Avermectin resistance survey emphasises need for best practice
Dr Frank Jackson and David Bartley
Figure 1: Results from the 2004 Avermectin Resistance Study. |
In recent years there have been reports of multiple anthelmintic resistance; i.e. resistance against the benzimidazoles, imidazothiazoles/tetrahydropyrimidines and avermectins in sheep. This is disturbing, particularly the development of avermectin resistance, since the avermectins and milbemycins play a crucial role in the control of both roundworms and sheep scab.
In order to establish whether there was evidence of avermectin resistant Teladorsagia populations in the flocks of local Foundation members, parasitologists at Moredun conducted a small survey in 2004. Fifty members agreed to take part in the survey and were sent kits so that they could take samples from their lambing ewes, both before and after they had treated them with an avermectin. Returned samples were analysed using the faecal egg count reduction test which measures the reduction in faecal egg count resulting from treatment. 37 farmers returned two samples for the study, however only 16 of these had ewe egg counts that were sufficiently high (averaging over 150 eggs per gramme of faeces) to permit the test to be conducted using the World Association for Veterinary Parasitology guidelines. Normally the avermectins are highly effective and will reduce egg counts by more than 95%; usually by more than 99%. Reductions of less than 95% are taken as indicative of resistance; this situation was seen on 5 of the 16 farms where efficacies ranged from 66-92%; a prevalence of over 31% (Figure 1).
Although we need to be cautious in how we interpret the findings from such a limited survey, what is inescapable is that the mechanisms for avermectin resistance appear to be relatively common in local
Teladorsagia populations. This emphasises the need for farmers to heed the ACME message promoted by Moredun and which forms the central core of the recent SCOPS (sustainable control of parasites in sheep) recommendations.
Farmers need to consult with their veterinarian and;
Adopt an effective quarantine strategy, treating all sheep arriving on the property with a sequentially administered combination of anthelmintics a macrocyclic lactone and one other broad spectrum drug. Producers need to
Check that the drugs they are using are still effective.
Monitor their sheep to decide when to treat and what to treat against.
Ensure that they follow best practice advice on how to store, handle and administer anthelmintics.
ACME is not a cure for resistance but does provide an important way of limiting the spread of anthelmintic resistance and of reducing the selection of resistance on farm.
This research is funded by SEERAD.