Oral Versus Pour-On Ivermectin
There are a few ways to administer dewormers to horses, depending on what product is being used:
- Giving a drug by nasogastric tube (stomach tube) used to be common, but is rarely done now because, while you can be sure the drug goes where you want it to go, it has to be done by a veterinarian and is not particularly pleasant for the horse.
- There are injectable versions of ivermectin, but they tend to cause significant pain and reaction at the site of injection, and are not generally recommended for horses.
- Most commonly, dewormers are given as oral pastes. These are easy to get and easy to administer - to some horses. Failure to get the whole dose into the horse is a problem (and some horses are remarkably good at avoiding what they don't want to swallow!). This can result in underdosing, which can reduce the effectiveness of deworming and likely contributes to the emergence of drug-resistant parasites.
Another approach used by some is topical or "pour-on" administration of dewormers. Pour-on formulations are only available for cattle, so use of them in horses is "off-label". The ease of administration is appealing, but an important question is how well do pour-ons work in horses?
A study published in a recent edition of Veterinary Parasitology (Gokbulut et al 2010) compared oral, intravenous and pour-on ivermectin in horses. (Ivermectin is a common horse dewormer, sold under a variety of brand names.) The researchers treated three groups of horses witheither ivermectin paste, bovine pour-on or bovine injectable formulations. They then looked at blood and hair/skin levels of ivermectin and fecal egg count reduction. Here are the highlights:
- Ivermectin concentrations were high on the skin and hair at the pour-on site, not surprisingly. Detectable concentrations were found on the skin at distant sites.
- Blood levels of ivermectin were lower in horses treated with the pour-on compared to the paste, but blood levels persisted longer with the pour-on, presumably because of longer, slower absorption. That's not necessarily a good thing, however. Prolonged exposure to low levels of an anthelmintic is a good recipe for resistance. What we want are high (therapeutic) levels for relatively short time to kill susceptible parasites and decrease the emergence of resistance.
- Oral ivermectin produced better reductions in fecal egg counts, meaning it did a better job eliminating parasites in the intestinal tract. Pour-on ivermectin did not work as well, but produced a longer effect.
What does this tell us?
Basically, it means that pour-on ivermectin is absorbed and can reduce intestinal parasite levels. It can also provide a more long-lasting effect than oral ivermectin. However, it also has the potential for inadequate levels in the intestinal tract and theoretically a greater likelihood of resistance.
Since we have pastes with demonstrated efficacy against intestinal parasites and which are licensed for use in horses ("on-label"), it's hard to justify the use of pour-ons in this species. Perhaps in situations where a particular horse is extremely resistant to oral deworming and there is no way to get the dewormer paste into the horse by any method (after trying many different tricks), topical treatment of that specific horse could be considered. Such treatment of a rare single horse here or there is probably of limited consequence in the broader context, but if people start treating all horses on the farm like this just because one is hard to dose orally, or because they think it's easier, resistance concerns increase greatly.
Photo credit: Lesley Ward (click for source)
