Page 63 - The Veterinary Care of the Horse

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Figure 2.14 Small strongyles

Life cycle The adult worms live in the caecum and colon of the horse where they cause
ulceration and inflammation of the gut wall. They lay eggs in large numbers which are

passed out in the droppings. The larvae hatch and develop in the faeces, then migrate onto the
pasture. In warm, moist conditions, they can develop to the infective stage in as little as 1–3

weeks. During spring and summer, the number of larvae on the pasture increases rapidly. If
ingested by a horse, the larvae burrow into the wall of the large intestine. Here they develop

further and then emerge into the gut lumen to mature into egg-laying adult worms. This cycle
usually takes between 6 and 12 weeks. In the autumn, however, some of the ingested larvae

enter a state of arrested development and remain in the wall of the large intestine where they
encyst until late winter or early spring of the following year when they may emerge in large
numbers. Larvae that are not ingested can survive on the pasture during mild winters in the

UK.

Clinical signs In late autumn and early winter, large numbers of adult worms, together with
the burrowing larvae, can cause symptoms which include weight loss, loose droppings,
ventral oedema and colic (Figure 2.15). Emergence of large numbers of encysted larvae in

the late winter and early spring can cause loss of condition, severe watery diarrhoea, colic,
inappetance and death. Young animals are often more severely affected than adults, as horses