Concern about cases of pinworm (Oxyuris equi) has been on the increase over the last few years. We think more targeted worming practices and a preference for ivermectin/moxidectin wormers which are less effective against this parasite have combined to cause this. While not a direct threat to horse health and unlikely to cause serious illness, pinworm can cause significant irritation for some horses, a literal pain in the bum! It's a pesky little parasite that can be tricky to get rid of.
The life cycle of the pinworm is different to the majority of other worms found in the horse’s gastrointestinal system in a number of ways:
- Pinworm eggs are ingested by the horse, usually from contaminated surfaces where an infected horse has rubbed in discomfort and deposited eggs. These hatch within the horse to live in the intestine. There is no intermediate host and no migration through any other organ in the body and the worms themselves are relatively harmless in the horse.
- The female pinworms go on a suicide mission to lay their eggs, venturing out of the rectum deposit them on the skin surrounding the anus area. The eggs are contained in a sticky, yellowish residue which can sometimes be seen under the tail of infected horses. It's thought to be this residue that contributes to the itching of infected horses.
- The life cycle of the pinworm is relatively long; worms can take up to five months to mature. Immature stages of the worm are less sensitive to wormers so may survive post treatment.
Effects in horses
Although pinworm isn't a major health threat like other worms, infection can cause irritation, sometimes so severe that horses will rub themselves raw around their rear, hind legs and tail. This can lead to skin infections, discomfort and depression. As the female pinworms travel to the anus to lay their eggs they can often be expelled in dung which can be unsightly, especially in company!
DETECTING PINWORM IN HORSES
As well as pinworm, moulting, mites, lice, sweetitch, roundworm, allergies, bacterial, yeast infections or other skin complaints can all cause itching. To determine if pinworm is the cause there are a number of things you can look for to get a positive ID.
Pinworm eggs are unlikely to show up on a faecal worm egg count as although the worms live in the intestinal tract, the eggs are laid externally. A Sellotape or adhesive tape test can instead be carried out to look for the presence of the parasite eggs. While this is not a definitive test it gives us one of the best chances of confirming infection.
The female worms can sometimes be seen in the dung of infected horses as they are passed out when they come to lay their eggs. They appear like white beansprouts with a distinct point to one end.
Look for a yellow sticky residue under the tail of the horse which the eggs are laid in.
TAKING AN ADHESIVE TAPE test
To test for pinworm take an adhesive tape impression from under the horse’s tail.
- Collect the sample in the morning, ideally before 9am. Pinworm are most active at night so this will give the best chance of detecting any activity that is present.
- Take a 4” length of clear sticky tape and press this firmly onto the skin around the anus (not the hair).
- Fold the tape in half, sticky side to sticky side.
- Pop the folded piece of tape into the sample bag ensuring this is clearly labelled.
- Post to Westgate Labs in the prepaid envelope with your completed paperwork.
- The sample will be put under the microscope in the laboratory to look for the presence of pinworm eggs.
- We'll email you your results.
Pinworms can be a tenacious problem to get rid of. If an infection is confirmed then pyrantel or a 5 day course of fenbendazole are our preferred choice of treatment. Ivermectin and moxidectin are also licenced against Oxyuris equi, but anecdotally we find them less effective. The worming treatment should be always be administered orally, following the instructions on the data sheet. This best targets the worms in the digestive tract where they live.
We have heard of some advice suggesting wormer is administered into the anus or applied to the skin around the anus area of the horse. This is not a recommended treatment regime as it will have very limited, if any effectiveness on the pinworm but is a very good way of putting additional chemical into the environment with eco-toxic consequences. The female pinworms that travel from the colon and through the rectum to lay their eggs outside the horse are on a suicide mission and will die anyway. The direction of travel of faeces will quickly sweep any wormer administered here with it which will do nothing to target the population in the horse's digestive tract.
Treatment should be combined with upping your field and stable hygiene to remove as many eggs from the environment as possible to prevent reinfection.
- Clean around the anus area once or twice daily to remove any eggs that have been laid.
- Apply petroleum jelly, coconut oil or similar to the area to help prevent eggs sticking and soothe the area.
- Clean and disinfect any areas where infected horses may rub, such as gate posts, fencing, stable doors or mangers, field shelter posts and walls every few days. Remember you can't disinfect a dirty surface so washing with an eco friendly detergent is advisable first before using a good veterinary disinfectant such as Virkon or Interkokask that is safe for animal use. It is advisable to do this every few days until you can be certain treatment has been effective.
- Pinworm tends not to infect a whole herd kept together so blanket treatment is rarely required. Like all parasites some horses tend to be more susceptible than others to pinworm infection - often the young or older will be more likely candidates. Supporting their immune system, addressing any underlying health problems and ensuring that they are happy in the herd will give them as much resilience as possible.
- Adhesive tape test again two weeks after treatment. If any eggs are present the treatment hasn't been fully effective. For persistent cases you may need to contact your vet who is able to prescribe anthelmintic treatment at shorter intervals than your SQP.