Spring Worming for Horses

In this article, Caroline George BVMS MRCVS discusses spring worming, the importance of worm control, the Intelligent Worming Programme and correct testing methods.

Following the winter, our thoughts swiftly turn to appropriate spring worming strategies for our horses. A variety of endoparasites (worms) infect horses in the UK, the majority of which are carried with minimal harm to the adult horse when managed correctly.

However, certain horse worms which in low numbers are unlikely to cause harm, can lead to problems when a larger burden is established. When the population of worms carried by a horse becomes greater than a certain threshold this leads to clinical signs which can have devastating consequences such as colic or lead to a more chronic ill thrift.

Spring Worming – Timing

Spring is an important time for horse worm control as the breeding cycle is dormant during the colder winter months. The increase in temperature in the spring leads to recrudescence of larvae and the recommencement of the worm breeding cycle.

Therefore, it is recommended to start your spring worming programme by testing the faeces to confirm the presence of eggs. Testing all horses at the stable yard should happen every 8-12 weeks (2-3 months). The results from faeces testing will recommend treatment, if any.

Types of worms

The two most significant spring worms we manage are redworms (cyathostomes) and tapeworms (anoplocephala).

Redworm (cyathostomes)

There are two types of redworm found in the horse; small (2.5cm long) and large (up to 5cm long) strongyles. The small strongyles are most common and responsible for the most significant number of worms (burden) in the modern equine, whereas the large ones are often less in number while causing the most significant harm.

Encysted small redworm (larvae) hibernate during the winter and emerge from the gut wall in the spring, therefore it is recommended these are targetted in the winter months to reduce burden (see testing below).

The threshold for a significant redworm burden in horses is 200 eggs per gram, and a burden above this level will need targeted treatment.

The Faecal Worm Egg Count test kits can be used to detect burden levels.

Tapeworm (anoplocephala)

There are three types of tapeworm, Anoplocephala perfoliata, Anoplocephala manga and Anoplocephaloides mamillana. They bury into the horse’s gut wall and live off food that the horse ingests.

Tapeworm also begins to cycle in spring, a blood test is available to monitor antibodies to tapeworms, and there’s also a commercially available test for tapeworm burden, using a sample of your horses’ saliva.

The EquiSal Tapeworm Saliva Testing Kit for Horses is available to purchase from Viovet.

Roundworms (ascarids) and young horses

Roundworms, or Ascarids, are most common in horses under the age of 4. They grow up to 40cm in length and can cause devastating effects if left untreated. Although common in young horses, older horses can become infected when grazed with heavily infested young stock.

Ascarids can cause poor health, poor growth, constipation or diarrhoea. It is essential youngstock are on a carefully planned and maintained worming programme.

Roundworms can also be detected using a Faecal Worm Egg Count test.

Intelligent Worming Programme

With a limited number of treatments for burdens of worms in horses wherever possible, an “intelligent” targeted worming (IWP) approach should be taken to avoid increased resistance to parasite control.

It is critical to remove horses’ droppings from paddocks and fields frequently to reduce the risk of ongoing exposure and recontamination. Worming horses unnecessarily kills only the worms sensitive to the product used, any resistant worms will survive. If this practice continues, the number of horse worms sensitive to the product decreases leaving a burden in the horse.

It is critical to remove horses’ droppings from paddocks and fields frequently to reduce the risk of ongoing exposure and recontamination.

Evidence has shown that only 20% of horses carry a significant burden requiring treatment. An IWP involves testing the droppings of your horses and only treating the horses with significant burdens followed by retesting to confirm effective treatment using an Egg Worm Count Kit.

Testing

Tests should be carried out in the spring, summer, and autumn every year. The presence of encysted redworm larvae can’t be detected via an egg count kit, therefore treatment should be administered using a product containing moxidectin or fenbendazole during early winter, after the first frosts.

Youngstock and Broodmares

Alongside careful consideration of ascarids, on burden management in youngstock and broodmares IWP is recommended for the majority of horses, but does require set-stocked (consistently the same horses) fields and regular poo picking. Whilst monitoring redworm burden in young stock and broodmares is valuable, their immune system is less able to cope with horse worms and, therefore, will require more regular analysis.

Analysis for spring worming (and worming for other periods) is ideally required on a fortnightly basis and with a specific worming programme drawn up. Treatment for ascarids using ivermectin or pyrantel and Strongyloides Westeri (Threadworm) using ivermectin should be considered.

Conclusion

Careful choice of a spring worming programme, to suit your situation and stock in conjunction with your veterinary surgeon, should be evaluated each year. This should be done to ensure your horses are managed effectively, with resistance avoided and monitored.