How do I know if my horse has worms? The common signs are weight loss, diarrhoea, colic and tail rubbing - but to find out which worms, what wormer to use and when, you need to do a faecal egg count (FEC). Unnecessary treatments can be avoided by regular FECs because they allow you to plan a strategic worming program. This saves money on un-necessary worming treatments, helps prevent worms becoming resistant and lets you develop the best control program for your horses and your property. In terms of management priorities - a balanced diet, regular dental, vaccination and farriery - establishing an effective worm control program for your pastures and horses is part of being an owner. Now let’s have a peep into the lifestyles of horse worms – because there-in lies the key to controlling them.

Large strongyles wreaked havoc as a leading cause of colic and death in horses mid last century. Now however, the availability of anthelmintics with high efficacy has resulted in a marked decline in the prevalence around the world of S. vulgaris, equinus and edentata.

Lung worms are rarely found in horses except those that share paddocks with infected donkeys. Because they live in the lungs, searching for eggs in the manure is futile and your veterinarian may need to collect a sample of fluid from the lungs for diagnosis.

Pinworms reside at the very end of the intestinal tract, laying eggs around the rectum that cause intense itching and a rubbed tail is often the hallmark of pinworms (Oxyuris equi). Eggs resist desiccation, may become airborne in dust, and remain viable in stables for long periods. Transmission then occurs via contaminated feedstuffs. Diagnosis is by detection of operculated eggs, slightly flattened on one side, on transparent adhesive tape that has been pressed against the perianal skin and then placed on a microscope slide for examination or by the chance observation of an adult worm in the manure. Pinworms are yet to show resistance to drugs and most wormers will treat the infection. Because the eggs are laid on the skin, they aren’t often found in a FEC.

Threadworms (Onchocerca cervicalis) make your horse’s neck their home. The adults rarely cause issues but the baby worms can cause intense itching as they burrow down to the lowest part of the chest and abdomen where they hope to be picked up by a fly and transported to a new horse. The lesions can resemble a few other conditions, including sarcoids, cancer, fungal skin invasions and proud flesh. Your veterinarian may take a biopsy for a definitive diagnosis and treatment.

Botflies lay eggs on the leg and chest hair which hatch when the horse licks them. The larvae penetrate the gums and tongue en-route to their home in the stomach -  occasionally they get lost in their travels and end up in the brain, heart and lungs.

They reach maturity 10-12 months after infestation, pass out into the manure and pupate to become adult botflies. Removing eggs from the hair and strategic worming with the correct drug at the right time of year provide control.

Roundworms are quite gross - reaching 20-40cm in length and living in the intestines they produce 0.5 to 2 million eggs a day!! Infections can involve hundreds of worms – up to litre or more. As they travel through the liver and lungs before setting up home in the intestine, symptoms of infection include poor growth, weight loss, ill thrift, dull coat, diarrhoea, colic and respiratory symptoms - fever, coughing, a nasal discharge and worsening of lung infections. Gut movements are reduced and impactions and rupture of the intestine are not rare in horses with roundworms.

Small strongyles A more widespread, ever-present and persistent problem is the small redworm (cyathostomes or small strongyles). Mainly a problem in horses under 5yo, cyathostomes can cause rapid or chronic weight loss, anaemia and ventral oedema (fluid swelling under the belly) due to bleeding in the gut, ill-thrift and diarrhoea due to haemorrhagic inflammation, ulceration and nodule formation of the intestinal wall. The nodules are caused by hibernating larvae. A feature that is important for the control and management of small red worms is their ability to hibernate for years in the wall of the intestine, creating a reservoir of infection. There they wait, stock-piled, until the resident intestinal adults have vacated the gut – making way for the next generation. The synchronised en-masse emergence from the nodules and subsequent breaking out into the intestine of 1000s of worms can provoke severe symptoms that mimic salmonella and colitis and these should be ruled out by your vet.

Tapeworms Tapeworms set up home at the junction of the small and large intestine (the ileocaeco-colonic valve) where they affect gut motility and cause weight loss, emaciation and colic. Severe infection and clusters of worms, which can be up to 8cm long, can lead to small and large intestinal impactions, intersussceptions, twists and telescoping of sections of bowel, torsion and perforation of the caecum. The life-cycle of the equine tapeworm involves a mite that lives in grass and is most numerous in summer. These mites ingest the tapeworm eggs and the horse accidentally swallows the mites while grazing.

Dr Jennifer Stewart
Equine Clinical Nutrition

DISCLAIMER All content provided in this article is for general use and information only. It does not constitute advice or a veterinary opinion. It is not intended as specific medical advice or opinion and should not be relied on in place of consultation with your veterinarian.