Farm Ireland

Saturday 16 December 2017

Parasite control requires a farm specific approach

Grass management and faecal surveillance are vital elements

Parasite control in horse requires careful planning
Parasite control in horse requires careful planning

Dr Wendy Conlon and Dr Theo de Waal

Parasite control remains a complex and constant challenge for horse owners and veterinarians alike. Treating all horses at fixed intervals all year around is no longer sustainable or recommended.

In fact, regular and intensive treatments have led to an increase in resistance levels to anthelmintic drugs.

To achieve good parasite control one must prevent contamination of the equine environment with high numbers of parasite eggs and larvae.

Treatments need to be timed to kill worms before they start to pass large numbers of eggs into the environment, although it is still desirable to maintain a proportion of non-resistant worms that escape selection by the wormer.

The goal of parasite control should never be to eradicate any given parasite completely.

This is impossible and an inevitable result is the accelerated development of parasite drug resistance.

The goals of parasite control are:

• To minimise the risk of parasite disease

Also Read

• To control parasite egg shedding

• To maintain effective anthelmintic control

• To avoid further development of anthelmintic resistance as much as possible

To achieve these goals it is important to know the degree of egg shedding of individual horses by performing periodic faecal egg counts. A sample of faeces is forwarded to a laboratory for assessment of the number of parasite eggs present.


Research has shown that only 20pc of horses in a population are responsible for 80pc of the worm eggs excreted into the environment.

Although the egg shedding status of a horse may change over time, it has been shown that adult horses may be relatively consistent in the number of eggs that they shed.

Horses with low worm-egg counts at one assessment are likely to have low worm-egg counts at subsequent assessment and vice versa.

Faecal-egg count surveillance is necessary to properly develop and monitor the parasite control programme.

However, it is also important to note that the number of eggs shed in faeces does not always correspond to the number of parasites in the intestine.

De-worming programmes for adult horses should be keep the following principles in mind.

Evaluate the effectiveness of the de-wormers used annually during the summer period using faecal-egg count reduction test.

A faecal sample is collected and analysed prior to deworming; the anthelmintic in question is administered; and a faecal sample is collected 14 days following treatment.

The percentage reduction in the number of eggs is calculated and used to make assumptions regarding the presence or absence of drug resistance.

A basic course of anthelmintic treatments should be considered for all horses including treatments to target encysted small redworms, tapeworms, and bots.

Autumn (Oct/Nov) and spring (Feb/Mar) doses aimed at killing encysted small redworms, not readily detectable in the faeces is recommended

Annual treatment for tapeworms at the end of the grazing season. All further treatments over the grazing season should target horses with high faecal egg counts.

Foals, weanlings, yearlings

Horses less than three years of age require special attention as they are more susceptible to parasite infection and are more at risk of developing disease. Roundworms are a major parasite of concern in foals and weanlings.

Targeted treatments - based on faecal egg counts - are not recommended in these age groups.

Instead, during the first year of life foals should receive a minimum of four anthelmintic treatments.

First de-worming should be carried out at about 2-3 months of age with a benzimidazole drug recommended to ensure efficacy against roundworm

Second de-worming is recommended just before weaning (approximately six months of age).

An extra treatment can be justified before weaning if the time period between treatments exceeds three months.

Faecal egg counts are recommended at weaning to determine whether worm burdens are mainly redworms or roundworms. This enables the owner to make the right choice of anthelmintic treatment.

Third and fourth treatments should be considered around nine and 12 months and treatment should mainly target redworms.

Tapeworm treatment should be included on one of these latter treatment occasions.

Recently weaned foals should be turned out on the 'cleanest' pasture available with low parasite burdens.

Yearlings and two-year-olds should continue to be treated as 'high' shedders, and receive about three to four yearly treatments.

Grassland Management

Composting of manure is an effective parasite control measure.

Non-composted manure should never be spread on pasture as this increases the level of parasite contamination.

Removing droppings from pasture is effective in preventing further infestation of horses grazing, if it is conducted at least once weekly.

Leaving pasture unoccupied for several months of the year may or may not reduce the risk of infection, depending on the time of the year.

Infective large redworm larvae can survive for only a few weeks in hot weather, but for up to six to nine months during colder weather. Grazing infected pastures with cattle, sheep or goats also assists in parasite control as the larvae cannot infect these animals and are destroyed when ingested.

Check List

• Do not under-dose horses and foals; use weight tapes or scales to determine weights.

• Consider using a blood test (ELISA) on at least 20pc of horses to determine exposure to tapeworms.

• Design a parasite programme that considers the farm's management practices in conjunction with your vet:

• Heavy stocking rates result in a consistently high level of parasite exposure and can challenge even the best de-worming programme.

• Faecal egg count and deworm for new arrivals before turn out with resident horses.

• Pasture rotation, mixed grazing with other animals, manure removal and composting manure are effective in control.

• If a horse is showing evidence of parasite disease during the times of the year when treatments are not recommended then this horse should still be treated.

• If the horse is showing symptoms of internal parasites then moxidectin would be the treatment of choice since it is important to kill encysted larvae in these animals.

• Each farm (with veterinary guidance) should develop its own programme tailored to the specific needs of the farm and each animal.

Targeted control programmes are now considered the best means to treat parasites and maintaining the efficacy of current drugs.

No new equine anthelmintics appear to be under development and there is no evidence that anthelmintic sensitivity reverses. It is important when purchasing anthelmintics to read the labelling carefully.

Dr. Theo de Waal is a senior lecturer in the School of Veterinary medicine at UCD. He is currently seeking horse owners to partake in a UCD equine wormer survey. The deadline for completion is Octber 30. Horse owners can partake by logging on to

Wendy Conlon works with the Farm Management & Rural Innovation department at Teagasc in Athenry, Co Galway.

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