"Why can't I use a pour-on on my horses?" you may wonder.
Not many scientific studies have been conducted into how well pour-on wormers work for horses. This is most likely because the orally administered pastes we give our horses have a high efficacy and are usually easy to administer for most horse owners, and so there has been no need for the development of a pour-on version. In fact, there are no commercially avaliable pour-ons for horses in Australia.
A small number of studies have been conducted, either investigating the efficacy of pour on ivermectin, or comparing pour-on ivermectin to orally administered ivermectin paste. Overall, pour-ons have been shown to be comparable to pastes for the removal of worms from horses. But if we go into it in more detail there are a few differences which may suggest why pour-ons have not been commercially developed.
The first difference between pour-ons and pastes is the time it takes to remove worm eggs from the faeces. In one study it took 4 days for eggs to cease being shed after orally administrated ivermectin compared to 8 days for pour-on ivermectin. In another study by the same group, it took 3 weeks post pour-on for eggs to stop being shed. Secondly, the apparent dose for pour-on ivermectin to be effective is far higher than orally administered. The recommended dose for ivermectin via a paste is 0.2mg/kg of body weight. Studies using pour-ons have compared this oral dose to both 0.5mg/kg and 1mg/kg invermectin poured-on. The efficacy of the 0.5mg/kg dose is questionable, however the 1mg/kg pour-on dose was comparable to the oral dose – it should be noted that this dose is 5x that of the recommended oral dose.
Some other studies also investigated how much drug and for how long it persisted in the horses’ blood and faeces. Orally administered ivermectin had a peak blood concentration that was 14 times higher than the poured-on ivermectin in one study. Throughout other studies this lower blood concentration of ivermectin following being poured-on (compared to oral) was observed. As the drug must be absorbed slowly through the skin, it also meant that the drug persisted for longer in the faeces than following oral administration. While persisting for longer, it persisted at a low concentration that could be subtherapeutic. Extended exposure of worms to subtherapeutic doses of drugs is a leading cause of drug resistance.
Overall, it was concluded throughout the studies that pour-on treatments with ivermectin is useful for animals that are difficult to handle (wild horses/young horses), however apart from ease of use, pour-on treatments have no advantages over oral administered pastes, and may drive the development of drug resistance faster than pastes.
If you wanted to use a pour-on, talk to your vet. While no negative side effects were seen in the horses in the studies, commercially available pour-ons have not been developed for horses, nor tested for safety.
References:
Francisco, I., Sánchez, J. A., Cortiñas, F. J., Francisco, R., Mochales, E., Arias, M., ... & Sánchez‐Andrade, R. (2009). Clinical trial of efficacy of ivermectin pour‐on against gastrointestinal parasitic nematodes in silvopasturing horses. Equine veterinary journal, 41(7), 713-715.
Francisco, I., Sánchez, J. A., Cortiñas, F. J., Francisco, R., Suárez, J., Cazapal, C., ... & Paz-Silva, A. (2011). Efficacy of ivermectin pour-on against nematodes infecting foals on pasture: coprological and biochemical analysis. Journal of Equine Veterinary Science, 31(9), 530-535.
Gokbulut, C., Cirak, V. Y., Senlik, B., Aksit, D., Durmaz, M., & McKellar, Q. A. (2010). Comparative plasma disposition, bioavailability and efficacy of ivermectin following oral and pour-on administrations in horses. Veterinary parasitology, 170(1-2), 120-126.
Gokbulut, C., Ozuicli, M., Aksit, D., Aksoz, E., Korkut, O., Yalcinkaya, M., & Cirak, V. Y. (2016). Comparative plasma and milk dispositions, faecal excretion and efficacy of per os ivermectin and pour‐on eprinomectin in horses. Journal of veterinary pharmacology and therapeutics, 39(6), 584-591.