Wednesday, April 21, 2010
Here is an under the tail view of a horse I encountered earlier this week. When I first looked under the tail, a large female pinworm (Oxyuris equi) was sticking its head out of the anus, looking for a nice place to deposit some eggs. By the time I had my camera in hand, she had already done the deed. The yellow creamy looking stuff is what she left behind (no pun intended).
We do not often seen pinworm infections because they are usually easily killed by most conventional dewormers. The interesting thing about this case was that this horse has just been treated with ivermectin two weeks prior. So either the worms were resistant to ivermectin or she became reinfected immediately after treatment. Resistance of pinworms to dewormers has become an emerging concern over the past several years. However, a study done just last year looking at just that, was unable to support any pattern of increasing resistance. That study showed that pyrantel was 91% effective and ivermectin was 96% effective at killing pinworms in horses.
The primary presenting complaint in horses with pinworm infections is tail itching or rubbing. The itching is caused by the eggs being deposited around the anus. There are other things that can cause tail rubbing as well, so an itchy tail does always mean there are pinworms present.
Pinworm eggs do not show up very well on regular fecal exams. If we suspect a pinworm infection we have to use a more creative approach to finding the eggs. One technique that really works well is to dab some scotch tape around the anus and then look at it (the tape that is) under a microscope, specifically looking for the eggs. The scotch tape on this horse showed an overwhelming large presence of eggs.
Treatment usually is fairly straightforward since most dewormers do a pretty good job at killing these pests. It is best to treat all the other horses at the same time, since if one horse has it, their pasture mates are also likely infected as well. Contamination of the environment, especially with heavy and long standing infections can be a concern as well. Occasionally, the more challenging cases, such as this one, require more creative means of treatment. It is best to talk to your veterinarian about the possibility of pinworms if your horse seems to be rubbing his/her tail excessively.