Keeping on top of worms in young horses requires a plan – and here it is

Faecal worm egg counts are part of the strategy to maintain good worm control in young horses.
Faecal worm egg counts are part of the strategy to maintain good worm control in young horses.

Worm control in young horses requires special attention, with owners needing to consider issues such as stocking density, pasture management and previous disease history, as well as the age of the youngster.

Good worm control is essential for maintaining health in young horses, and while information on the best worming strategies for foals and yearlings remains largely anecdotal, veterinarian Wendy Talbot offers some suggestions to owners.

Talbot, with animal health company Zoetis, says foals and yearlings are usually more susceptible to worms than adults because they have had little chance to develop any tolerance.

They are more vulnerable to parasite-related diseases and tend to have higher egg shedding, which increases the risk of infection.

The main parasitic culprits in Britain for foals less than six months of age are large roundworms.

In older foals and weanlings, small and large redworms, tapeworms (and pinworms) are the main considerations.

Yearlings may also have a second wave of large roundworm infection at 8-10 months of age.

Any control strategy will need to take into account the individual circumstances such as stocking density, pasture management and previous disease history.

Guidelines for parasite control in foals suggest treatments for roundworm at 2-3 months and 5-6 months of age. At weaning (around six months of age), it is advisable to perform a faecal worm egg count to determine if treatment for redworm is also needed.

At 9 and 12 months of age treatment for redworms and encysted small redworm are advised. A tapeworm test or treatment should be included with one of these doses.

For yearlings, two methods have recently been proposed for worm control: the first involves using faecal worm egg counts at more frequent intervals than for adults to guide dosing for redworms in the grazing season.

Ideally they should be conducted every 6-12 weeks depending on the worming product last used and individual circumstances.

The second method suggests three baseline treatments targeting redworms in spring, summer and late autumn, with egg counts in between to identify and treat any still shedding high numbers of eggs, up to a maximum of six treatments per year.

Both strategies include a treatment for encysted small redworm combined with testing or treating for tapeworm in late autumn.

Talbot says it is important for owners to discuss their worm control plan with their vet or suitably qualified person who will be able to devise the best strategy for their individual circumstances.

The key points are:

  • Clean pasture is key to keeping foals and young stock healthy. Regular removal of droppings is crucial to a successful worm control plan.
  • All foals are considered to be susceptible and at risk of acquiring large roundworm infection. Some degree of anthelmintic intervention should be considered and in most circumstances administration at 2-3 months of age and again at 5-6 months of age is advisable.
  • Older foals are primarily at risk of strongyle infection; macrocyclic lactone anthelmintics (moxidectin or ivermectin) have the highest expected efficacy.
  • Worm egg counts are likely to be higher in yearlings compared to adults and interpretation of them requires good knowledge of all the circumstances including management, clinical history, previous worm control and test results. Treatment of encysted small redworm is advised for all youngsters (more than 6 months of age) in late autumn.
  • Faecal worm egg counts are advisable in late winter/start of spring to identify high shedders. These horses may require treating again for encysted small redworm, particularly after a mild winter.
  • New arrivals should be dosed with a wormer effective against all stages of small redworm, roundworm, tapeworm and bots and then quarantined for three days.
  • A yearly faecal egg count reduction test is advised to check efficacy against strongyles and large roundworms for all class of drugs used on the premises.

1. Matthews
2. Nielsen (2017) Endoparasite Control. 9th ECEIM congress proceedings, 63-64
3. AAEP (2016) Parasite Control Guidelines
4. Nielsen (2016) Evidence-based considerations for control of Parascaris spp. infections in horses. Equine vet. Educ. 28 (4) 224-231

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