Worm Egg Counts in horses – are they worth it?

The hibernation of the small roundworm is particularly dangerous to the horse as in the spring the larvae emerge in large numbers. This mass emergence can cause severe damage to the gut wall, which can lead to weight loss, diarrhoea, and colic, all with potentially fatal consequences.

Roundworms (Ascarids – Parascaris equorum) 

Infection with these worms is quite common worldwide with adult roundworms growing up to 50cm in length. The adult female worm produces exceptionally large numbers of eggs which may persist in the environment for several years. They are also sticky, which may contribute to spread by attaching to the udder and teats of the pregnant mare. 

Roundworms migrate from the gut causing damage to the lungs and liver. In large numbers they can block the gut, causing it to rupture which could lead to death. Within the lungs, they are coughed up, swallowed, and returned to the intestine. 

These worms are dangerous to foals and yearlings that become infected from the first month of life – heavy infection is common in foals and yearlings: Some infected foals pass millions of eggs in the faeces. Some foals and older horses eventually develop immunity to roundworms, and this worm is less commonly a threat to adults. 

Pinworms (Oxyuris equi)

Pinworms lay their eggs around the outside of the anus causing intense itching and irritation. Persistent scratching will result in hair loss and open sores, around the tail head which can become infected.

Threadworm (Strongyloides westeri). 

These are found in the small intestine and are mainly a problem in young foals. They may cause diarrhoea, weakness, and severe weight loss. They transfer to the foal via the mother’s milk. Often foals on stud farms are given a wormer within the first couple of weeks of life. 

Like roundworms (Ascarids), they migrate through the lungs. Horses and foals older than 6 months tend to be immune to threadworms. Other than worming, control is by the removal of dung and provision of dry bedding.

Tapeworms (Anoplocephala spp.)

Tapeworms can grow up to about 20cm in length and have a width of 1.5cm. They form into clusters at the junction between the small and large intestines (where they can cause digestive disturbances, loss of condition, colic, and fatal blockages). Horses become infected through eating the intermediate host – the orbitid or Harvest mite – found on grass and forage. The lifecycle of a tapeworm is six months.

Lungworms (Dictyocaulus arnfieldi)

Lungworms prevail in pastures shared with donkeys – the lungworm’s natural host. These worms cause persistent coughing in horses as respiratory problems develop. Donkeys can tolerate very large worm burdens without showing any clinical signs.

Bots (Gastrophilus)

Bot flies are the most common irritant to horses during the summer grazing season. They lay sticky yellow eggs on the horse’s coat which are ingested as the horse grooms itself or another horse