Recent studies disagree on the value of pour-on preparations of ivermectin for deworming horses.
Topical applications are easy to administer and may reduce the risk of injury to the operator. Pour-on anthelmintics have become popular for treating cattle and have been shown to be effective. Could they be used in horses?
A study1, supervised by Professor Adolfo Paz-Silva at the University of Santiago de Compostela in Spain, and published in the Equine Veterinary Journal, examined the use of a pour-on anthelmintic in naturally infected Pura Raza Galega horses.
The researchers applied an ivermectin-containing pour-on product at a rate of 1mg/kg (This was double the recommended dose rate for cattle.)
They compared the faecal egg production in treated and untreated animals over a 21 week period.
The faeces were clear of strongyle eggs between 3 and 9 weeks after treatment. Strongyle eggs reappeared in the faeces 10 weeks after treatment. Topically-administered ivermectin suppressed faecal egg production of Parascaris equorum and Oxyuris equi for the whole of the study period.
The treatment group also showed changes in the whole blood cell count - there was a significant increase in red cells, and a reduction in white cell count. No adverse side effects were seen.
The researchers concluded that the pour-on ivermectin preparation was highly successful against gastrointestinal nematodes. They suggest that it appears to provide a useful treatment option for large groups of horses at pasture.
However, a less favourable picture is painted by Dr Cengiz Gökbulut and others in a report published in Veterinary Parasitology2.
Their study looked at the absorption of ivermectin after oral, topical or intravenous administration and how the mode of administration influenced the efficacy of the treatment.
Eighteen horses were divided into three treatment groups. One group was treated with equine oral ivermectin paste (at 0.2mg/kg); the second with bovine pour-on (at the recommended cattle dose rate of 0.5mg/kg) and the third group was treated with an injectable cattle preparation (given intravenously at 0.2mg/kg).
Compared with the oral paste, the pour-on preparation resulted in lower but more persistent plasma concentrations. It was also less effective at reducing the faecal strongyle worm egg count. The paste resulted in a reduction in egg count by over 95% for 10weeks. In contrast the pour-on preparation gave reductions ranging from 82% to 97%.
The authors of the report suggest that factors that may account for the lower bioavailability of the topical preparation compared with the oral paste include binding of ivermectin to the hair, degradation at the site of application, and biotransformation of the drug in the skin.
They warn that the poor plasma availability after topical application could result in sub-therapeutic levels of ivermectin which could encourage the development of ivermectin-resistant parasites.
For more details see:
1 Clinical trial of efficacy of ivermectin pour-on against gastrointestinal parasitic nematodes in silvopasturing horses.
I Francisco, JA Sánchez, FJ Cortiñas, R Francisco, E Mochales, M Arias, P Mula, JL Suárez, P Morrondo, P Diez-Baños, R Sánchez-Andrade, A Paz-Silva.
Equine Vet J (2009) 41, 713-715
2 Comparative plasma disposition bioavailability and efficacy of ivermectin following oral and pour-on administrations in horses.
C Gökbulut, VY Cirak, B Senlik, D Aksit, M Durmaz, QA McKellar
Vet Parasitol (2010) 170,:120-126