There are numerous reports of the spread of resistance to dewormers. So it is encouraging to receive some good news for a change.
Research carried out by Dr Craig Reinemeyer and others at East Tennessee Clinical Research Inc., has revealed some positive findings.
In separate studies they examined the efficacy of dewormers against Oxyuris equi, and against macrocyclic lactone resistant Parascaris equorum.
Oxyuris equi (equine pinworm) rarely causes serious problems for the horse, but does cause irritation when the female lays the sticky eggs on the skin around the anus of the host. It has been assumed that most routine dewormers will control it. However, anecdotal reports that anthelmintic treatments were not as effective against Oxyuris equi as expected prompted Dr Reinemeyer’s study.
Dr Reinemeyer found that pyrantel pamoate (at 13.2mg/kg) was 91.2% effective, and ivermectin (200mcg/kg) 96.0% effective, against the adult worms. Both products were more than 99% effective against the fourth-stage O. equi larvae.
Unlike O. equi, Parascaris equorum does cause significant problems, especially in foals, in which heavy infestations can lead to intestinal obstruction or rupture.
Recent reports have recorded an increasing problem of reduced efficacy of macrocyclic lactone anthelmintics (such as ivermectin) against these parasites.
In a study of experimentally infected foals, the researchers found that pyrantel pamoate administered at 13.2mg/kg was effective against a macrocyclic lactone -resistant (ML-R) isolate of Parascaris equorum.
Mean ascarid egg counts were reduced by 98.8% two weeks after treatment - compared with a reduction of only 47% two weeks after ivermectin (200mcg/kg). Post mortem examination of intestinal contents showed that the paste formulation of pyrantel pamoate was 97.3% effective against the MLR P. equorum.
It is reassuring to know that these drugs were effective under the specific conditions of the study. But that’s not to say that they will always remain so, or that the results are representative of all worm populations. The advice to treat for worms only when necessary and to monitor the effectiveness of any treatment still stands.