Oxyuris – a pain in the backside!

Until very recently, the worm Oxyuris equi, better known as the “pinworm”, was believed to almost be a disease of the past. However it is now becoming increasingly recognised by vets as a cause of illness in horses inBritaintoday. What is most surprising is that the disease is occurring not only in horses with a sketchy worming history, as might be expected, but also in those horses that have had an excellent control programme for the worms we are more commonly concerned with i.e. the small redworm and the tapeworm.

Oxyuris is a roundworm that colonises the rectum (the very last part of the intestine before the anus) of an affected horse. The worms are white-grey in colour. The male worms are small but the females may reach 4 inches long and have a long tail tapering to a point, hence the name “pinworm”. The larvae feed on the mucous lining in the intestine and on maturing, the adult females move to the anal area where they lay eggs covered with a sticky fluid. A common first suspicion by an owner that their horse may be infested by Oxyuris is that they will see the end of a worm momentarily protrude from their horse’s back passage and then quickly disappear back into the rectum out of view. What they have just witnessed in fact is the adult female laying eggs.

Image around the anus

The presence of the adult female worms laying their eggs and particularly the presence of the eggs themselves stuck in a gelatinous mass around the anus, causes severe skin irritation. As a consequence, affected horses will rub their tails repeatedly. Some horses may be nervous and stop eating. The severe itching makes the horse rub its tail and rump so that the tail hairs break off. Rubbing, biting and scratching can open up the skin to infections. In effect, the signs very closely resemble sweet itch and for this reason some cases could be mistakenly thought to be sweet itch, especially if the affected animal has suffered from fly bite hypersensitivity in the past. To further confuse matters, Oxyuris eggs can be transferred to other parts of its body, often the mane and poll, with the horse biting and licking its hindquarters and by owners using contaminated grooming equipment. This further mimics the presenting signs of sweet itch.

How can we test for the presence of Oxyuris?

There are two fairly straightforward tests that your vet can perform which are largely successful in confirming the presence of Oxyuris in most affected horses. The first is examination of the droppings for the presence of Oxyuris eggs (this requires special preparation of a sample of droppings and then examination of the sample under a high powered microscope). The second method is simply applying sellotape to the skin around the back passage and then viewing the tape under a microscope to identify presence of eggs.

How is Oxyuris spread?
An adult female can produce huge numbers of eggs (up to 6,000 eggs a day) and horses become infested when they ingest eggs in the process of grooming an affected horse or by swallowing eggs that have dropped on to pasture, feed or drinking water.

Why is Oxyuris apparently becoming more widespread?

There is some concern that Oxyuris has developed a resistance to the wormers commonly available on the market but it is also possible that the modern wormers are so effectively absorbed from the fore gut that any drug reaching the hind gut is not present in sufficient quantities to kill Oxyuris. Also, the concentration reached by the worming agent in the mucous, on which Oxyuris feeds, are presumably lower than in the blood and may not be effective.

Treatment and Prevention

The treatment of Oxyuris is best carried out at three levels; treating worms present in the intestine, treating eggs present on the skin and treating the environment to prevent re-infection or spread to other horses.

Wormspresent in the gut can be removed by careful overdosing with some of the current wormers available so that not all the wormer is absorbed in the foregut and a sufficient dose arrives in the rectum to kill the worms.

The eggs adhering to the skin are removed by thorough daily cleaning of the tail, anus and hindquarters (and any other affected areas) with warm dilute disinfectant followed by painting on an injectable wormer (to kill the oxyuris larvae) mixed with propylene glycol (to cause the mixture to adhere to the skin and deliver the dose of wormer for a sufficient period of time).

Removal of worm eggs and larvae from the environment is achieved by first of all removing all bedding from the stable of the infected horse followed by power washing the floor and walls. After this, a heavy duty disinfectant is applied to the walls and floor and should be allowed to lie for an hour before re-hosing the stable. New bedding should then be introduced and kept to a minimum.

Poo picking should be performed on a twice weekly basis- poo picking is of course an essential measure for the control of any worms on a yard, whether Oxyuris or others. Obviously any animals sharing a paddock with an affected horse or having previously spent time in close contact, so that spread may have occurred, should be closely monitored for the appearance of signs and your Vet contacted should there be any suspicion of spread so that that horse can be treated as well.

As mentioned earlier, grooming of affected horses with contaminated brushes allows spread of Oxyuris, not only over the entire body of the affected horse, but also, when brushes are shared between horses, to other previously unaffected horses. Grooming should therefore be carried out with a scrupulously clean brush, beginning at the head and mane and working backwards, finishing with the tail and hindquarters. The brush should then be steeped in strong disinfectant and if possible steam cleaned to ensure it has been decontaminated of any Oxyuris eggs before it is used again. Grooming equipment must not be shared at all between horses when Oxyuris is suspected of being present.

Case Study
Oscar, a 14 year old Welsh Section B, last October started rubbing his tail, the hindquarters and the anus itself. Within 2 days he had developed a painful sore on the left side of his anus and Dawn called her Vet. The Vet enquired as to Oscar’s worming history and since he was on a regular targeted worming programme for small redworm, discounted Oxyuris as a possible cause of the illness.

Even though Oscar had no history of suffering from sweet itch before, the Vet felt that since the weather was still warm and flies still active, on the presenting signs, sweet itch was the most likely explanation. Treatment involved use of a fly rug, keeping Oscar in the stable when biting flies were most active (dawn and dusk) and giving Oscar anti inflammatories for the itching and antibiotics on account of the sores. Oscar failed to respond- in fact, the rubbing around his hind end worsened so that the skin became excoriated and bleeding and an abscess formed near the tail head. The itching also spread to his mane and forelock. At a subsequent visit samples of faeces were submitted to the lab along with impression samples taken with sellotape from the skin around the anus. Huge numbers of Oxyuris eggs were found and treatment started immediately: worming, washing the skin with an injectable worming product and power hosing and disinfection of Oscar’s stable after it had been thoroughly mucked out. Oscar, who by this time had become quite depressed, started to improve and within 3 days of treatment being started had stopped itching altogether.

The hair he had lost quickly grew back over the following weeks; now that the rubbing and biting had stopped and he has since gone on to make a full recovery, suffering no further problems. Oscar’s field companions have been carefully monitored and none of them have displayed any signs of Oxyuris infestation.


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