Resistance to pyrantel pamoate has been identified in horses receiving pyrantel tartrate daily as part of a worm control program.
Cyathostomes (small redworms) resistant to anthelmintics (dewormers) present a growing problem throughout the world. Of the three main groups of drugs that are used to control roundworms in horses, only the macrocyclic lactones have not yet encountered problems of resistance. Benzimidazole resistance is widespread. Resistance to pyrantel is becoming more common, especially in the south-eastern United States.
Pyrantel pamoate* is used as a single treatment for adult roundworms and fourth-stage larvae in the gut at a dose rate of 6.6mg/kg. It is also effective at double the dose rate against the horse tapeworm Anoplocephala perfoliata.
Pyrantel is unique among the main deworming compounds, being also available (in the USA) for use as a daily preventative. Pyrantel tartrate is fed at a daily dose of 2.64mg/kg. As the drug is present in the gut all the time, the infective larvae are exposed as soon as the horse picks them up from the pasture. Pyrantel paralyzes the larvae and they are removed from the digestive tract by the normal peristaltic movements. The larvae are expelled before they are able to cause any damage or become encysted in the intestinal wall.
However, concerns have been raised that the constant presence of pyrantel in the gut favors the development of resistant worms. This may contribute to the growing resistance problem.
A recent report in the Journal of the American Veterinary Medical Association describes a farm on which a worm control program based on daily doses of pyrantel, broke down due to the development of resistance.
Dr Emily L Brazik and Dr Jan T Luquire of the Carolina Coastal Equine Veterinary Service, found that horses on a particular farm had a high average fecal worm egg count (FWEC) despite having been maintained on a preventive daily regime of pyrantel tartrate. Further investigation, in association with Dianne Little, research assistant in the gastro-intestinal physiology laboratory at North Carolina State University College of Veterinary Medicine, showed that many of the horses carried cyathostomes that were also resistant to pyrantel pamoate at the full treatment dose of 6.6mg/kg.
The initial problem came to light when, as part of the routine health assessment, FWECs were carried out on all of the horses on the farm. Feces from sixteen horses of mixed breeds and ages was tested. The average count was 478 eggs per gram (epg), although there was a wide range from 0epg to 4075epg. Three individual horses accounted for 85% of the worm egg production. This level of worm egg production would not have been expected had the prophylactic treatment with pyrantel tartrate been effective.
The clinicians went on to carry out a fecal worm egg count reduction test (FWECRT) to determine if the full dose of pyrantel pamoate was still effective.
Nine horses with FWECs greater than 100 epg on the first test were treated with pyrantel pamoate at 6.6mg/kg , and a second fecal sample was tested two weeks later. The treatment was only fully effective in two horses. The rest showed only a slight decrease, or even an increase, in worm egg production.
The authors recommend that:
Before using pyrantel tartrate as a daily preventative measure, a FWECRT should be performed to check that the cyathostomes on the farm are senstive to pyrantel pamoate
FWECs should be used twice yearly to check that pyrantel tartrate is still effective. The problem on this farm was only detected because of the routine fecal testing. If the testing had not been carried out the cyathostome burden in the horses could have built up unchecked, eventually resulting in signs of disease.
A control program should be based on selectively treating individual horses when their fecal worm egg production reaches 200 epg . Often, as on this farm, only a few horses are responsible for most of the worm eggs released onto the pasture. By treating only those with significant worm burdens it is possible to limit the use of anthelmintics (while still maintaining safe control of large strongyles, tapeworms (and ascarids in young horses)). This should help preserve the efficacy of the only remaining drug group that has not yet had resistance reported.
Also, important as ever, pasture hygiene reduces the exposure of the horse to cyathsostomes - therefore lower FWEC, and fewer treatments necessary. Removal of feces from pasture, composting feces from stalls before spreading on pasture, resting pastures, cross-grazing, chain-harrowing when dry can all help reduce the need for dosing.
*Pyrantel pamoate is also known as pyrantel embonate (in Europe)
For more details see: Pyrantel pamoate resistance in horses receiving daily administration of pyrantel tartrate.
Emily L Brazik, Jan T Luquire, Dianne Little.
JAVMA (2006) 228 , 101 - 103