Ab Horse SEPT pages 1 to 39_Layout 1 20/08/2015 11:38 Page 34
HEALTH & WELFArE
understanding:
PINWorm
By Blaise Scott-Morris BVSc MRCVS Virbac Veterinary Advisor
I
t is frequently reported that Oxyuris equi, more commonly known as Pinworm, can be seen in horse dung even after treatment. There are regular postings on equine forums on the subject which are often inaccurate or illinformed, and there can be a tendency to blame wormer resistance for the problem. This article aims to describe the lifecycle of the worm, in order to help explain the treatment and management options available.
Lifecycle Adult worms live in the large intestine (caecum and colon). From there, the female worms migrate to the rectum when they are ready to lay their eggs. They poke their tails through the anus and deposit clumps of eggs in a gel like substance. This can sometimes be seen as yellow/white jelly like streaks around the anus (Figure 1). The eggs are not capable of infecting other horses immediately, but they mature quickly and can be
infective within 5 days. Horses are intensely itchy when the eggs are laid, rubbing their hind quarters on objects. This can cause eggs to fall off into the environment. The horse becomes infected by ingesting infective eggs from the environment, eg pasture, stables, bedding, food. Once digested, the larvae migrate to the large intestine and burrow into the intestinal walls and develop into adults. This process from egg to adult can take 5 months. The adult female worm lives around 6
Figure 2
Figure 1
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months and can lay up to 50,000 eggs in this time, presenting a significant source of environmental contamination. Pinworm cannot be detected routinely in faecal worm egg counts, as eggs are laid under the horse’s tail and not often passed out in faeces. One way to confirm the diagnosis is by doing a “stickytape” impression from under the tail and examining it under a microscope for eggs, however this will only detect 50% of infections. Pinworms are very distinctive and are recognisable by their rounded bodies that taper down to a point, very similar to bean sprouts. The eggs are also recognisable as they are oval shaped with one slightly straighter side and a “cap” at one end. Figure 2 shows an embryonated egg and the larva can be seen within the shell. Pinworm infection within the intestines rarely cause gastrointestinal clinical signs, and a horse may have a few eggs or