Recent research in America demonstrated that cyathostomes are becoming resistant to pyrantel. The investigation, led by Dr Ray Kaplan, also confirmed widespread resistance to the benzimidazole group of anthelmintics.
Cyathostomes (also known as small strongyles or small redworms) are the most important internal parasite of horses. They often cause sub-clinical impairment of intestinal function, which may lead to poor performance, weight loss or failure to thrive. They may cause overt disease with diarrhoea, severe weight loss and oedema.
Three main groups of anthelmintics (wormers) are used to control cyathostomes:
* benzimidazoles (including fenbendazole, oxfendazole, mebendazole),
* pyrantel group (tetrahydropyramidines), and
* avermectin/milbemycins (ivermectin and moxidectin).
All were effective when first introduced. However, resistance to the benzimidazole group has been widely reported, and resistance to pyrantel has been recorded in USA and Europe. So far, there have been no reports of resistance to ivermectin or moxidectin.
Dr Kaplan and his colleagues studied horses on ten farms in Georgia. The horses were allocated to one of four groups. Three groups were treated with fenbendazole, pyrantel or ivermectin. The other group was left untreated. Faecal samples were collected 1 day before, and 2, 4 and 6 weeks after treatment. The effect of the different anthelmintics on the faecal worm egg count (FWEC) was studied.
The efficacy of each anthelmintic was determined on the basis of the mean percentage reduction in FWEC two weeks after treatment. An anthelmintic was considered to be effective when it produced a reduction in FWEC of more than 90%. It was ineffective (ie the cyathostomes were resistant) if the FWEC did not reduce by more than 80%.
Fenbendazole resistance was widespread. Fenbendazole was not effective in horses on any of the farms examined. Pyrantel resistance was detected on 2 of the 10 farms and equivocal results were obtained on a third farm. Ivermectin was effective in horses from all ten farms.
In those horses in which the treatment was effective, the time taken for worm eggs to reappear in the faeces after treatment was noted. The egg reappearance period (ERP) was defined as the time for FWEC to return to 20% or more of the pre-treatment value.
Often the development of resistance is preceded by a reduction in ERP. For example, when the benzimidazoles were introduced in the 1960s, they had an ERP of between 6 and 8 weeks. Now the ERP is often only 4 weeks.
On 30% of farms in this study the ERP after ivermectin was 6 weeks. After ivermectin was introduced in the 1980`s several researchers reported an ERP of 9-10 weeks
This reduction in ERP may be the first sign that cyathostomes are developing resistance to ivermectin. But the sample size is too small to be sure. It is probably only at matter of time before cyathostomes start to develop resistance to the drug, warns Dr Kaplan. "The development of anthelmintic resistant cyathostomes has been facilitated by an extensive reliance on the use of drugs" he points out. "Strategies to reduce the development of resistance should be used where possible to prolong the useful life of current wormers."
To reduce pasture contamination and reduce selection for drug resistance Dr Kaplan suggests:
* properly timed treatments (depending on the ERP of the drug used )
* sound pasture management
* periodic testing of drug efficacy. In particular fenbendazole should not be used as a single dose anthelmintic without previously demonstrating its efficacy.
* giving the correct dose of anthelmintic
* rotating effective wormers on an annual basis (Rotating with each treatment does not appear to reduce the development of resistance and may actually increase it by exposing worms to more than one type of anthelmintic simultaneously)
* biological control may become available in the future (eg using a fungus to destroy developing worm larvae in the faeces.)
"Annual monitoring of anthelmintic efficacy should be part of all health management programs in horses." advised Dr Kaplan.
for more details see:
Prevalence and clinical implications of anthelmintic resistance in cyathostomes of horses.
Jaime L Tarigo-Martinie, Amy R Wyatt, Ray M Kaplan
JAVMA (2001) 218 (12) 1957-1960