Rotational deworming was the preferred method of parasite control in horses for decades. It has led to parasite resistance and a new treatment schedule based on fecal egg counts. The University of Kentucky’s Martin K. Nielsen, DVM, PhD, DipEVPC, explains how we got to the place we are today with equine parasitology and parasite resistance, and the approaches we can take to fight internal parasites in our horses.
While information on anthelmintic resistance in equine parasites has been available for a long time, the equine industry has been very slow to acknowledge or respond to it. Numerous studies have documented the increasing prevalence of resistance to the different commercially available drug classes. Not surprisingly, parasitologists have strongly recommended reducing treatment intensity by moving away from the traditional calendar-based treatment protocols, which are based upon anthelmintic treatment of all horses at regular intervals year-round.
Current recommendations involve regular analysis of fecal samples for the presence of parasite eggs before decisions on treatment are made. Several questionnaire-type surveys have revealed that despite these recommendations, horse owners in many countries still rely on frequent, regular anthelmintic treatments without any consideration of the parasite species that may be involved and the efficacy of anthelmintic drugs used against those parasites.
Several possible reasons may account for this approach:
- Parasitologists lack effective channels of communication to convey their message;
- The anthelmintic resistance has not yet reached a level that represents a threat to equine health;
- The calendar-based approach has been much easier to follow than the more complicated treatment strategies based on testing fecal samples; and
- In many countries, cheap anthelmintic drugs have been available over the counter, so horse owners have not had to retain the services of a veterinarian, thereby lessening the expense.
Singly or collectively, the foregoing factors have made it very difficult to change old habits when it comes to parasite management on horse farms.
However significant these obstacles, major changes in parasite management on farms have been observed in recent years. Awareness of anthelmintic resistance among equine veterinarians and their clients appears to be increasing. As a result, testing for parasite eggs in fecal samples is becoming more and more common. Several reasons have been put forward to account for this change.
Several European countries have now implemented prescription-only restrictions on anthelmintic drug formulations, and these restrictions have led to a much greater degree of veterinary involvement in the treatment decision process. Under prescription-only conditions, veterinarians are expected to perform parasite surveillance and prescribe appropriate anthelmintics. As a result, the intensity of treatment has decreased considerably. Even in countries without this legislation, substantial changes have been observed. Many veterinary practitioners in the USA have adopted fecal testing, and several laboratories now offer egg counts and advice on a larger scale.
Apart from legislation in Europe, the most important factor that has promoted change is likely the influence of the Internet. In the past decade, several equine news media have established efficient portals for dissemination of knowledge to horse owners. Online broadcast of news, blogs, webinars, questions and answers, etc., has helped make new information more accessible and the average horse owner more aware of equine health issues than he or she was a decade or two ago.
As a result, veterinarians and horse owners worldwide are now realizing the problems resulting from following the traditional calendar-based deworming protocols for controlling strongyles and ascarids, which is generating many queries and challenges for parasitologists. Included are questions on how to interpret current diagnostic tests, the need for new diagnostic techniques, assessment of the impact of parasitism on equine health, equine performance under different deworming regimens, understanding modes of drug resistance, etc. The challenges confronting parasitologists, veterinarians, and members of the horse industry are many, and the need for research in equine parasitology is greater than ever.
By Martin K. Nielsen, DVM, PhD, DipEVPC