Parasite control remains complex and a constant challenge for horse owners and veterinarians alike. Treating all horses at fixed intervals all year around is no longer sustainable or the recommended approach. In fact, regular and intensive anthelmintic treatments have led to an increase in drug resistance levels.
To achieve good parasite control one must prevent contamination of the environment of a horse or horses 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, though 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
· 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 20% of horses in a population are responsible for 80% 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 relative 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 any 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.
Deworming programmes for adult horses should be designed with the following principles in mind:
Evaluate the effectiveness of the dewormers 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 foundation of anthelmintic treatments should be considered for all horses and include treatments to target encysted small redworms, tapeworms, and bots.
An autumn (Oct/Nov) and spring (Feb/Mar) dose aiming to kill encysted small redworms which are not readily detectable in the faeces
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 is not recommended in these age groups.
Instead, during the first year of life foals should receive a minimum of four anthelmintic treatments. First deworming should be carried out at about 2-3 months of age, and a benzimidazole drug is recommended to ensure efficacy against roundworm. Second deworming is recommended just before weaning (approximately six months of age). An extra treatment can be justified before weaning if the time period between the two treatments exceeds three months. At weaning faecal egg counts are recommended to determine whether worm burdens are mainly redworms or roundworms, to enable the right choice of anthelmintic. Third and fourth treatments should be considered around nine and 12 months of age and treatment should mainly be targeting 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 lowest parasite burdens. Yearlings and two-year-olds should continue to be treated as 'high' shedders, and receive about three to four yearly treatments.
Composting of manure is an effective practice in eradicating parasites. 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 it if 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 as many as six to nine months during colder weather.
Grazing infected pastures with cattle, sheep or goats assists in parasite control also as the larvae are host-specific and cannot infect these animals and are thus destroyed when they are ingested.
Do not under-dose horses and foals; use weight tapes or scales to determine body weights.
Consider using a blood test (ELISA) on at least 20% of horses to determine exposure potential for 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 deworming programme.
Faecal egg count and deworming 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 - and 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. There is no such thing as a 'one size fits all' programme.
Targeted control programmes are now considered the best means to treat parasites with a view to maintaining efficacy of current drugs for as long as possible. No new anthelmintic classes appear to be under development for use in horses and there is no evidence that anthelmintic sensitivity reverses.
It is important when purchasing anthelmintics to read the labelling carefully and check not only which parasites (and life cycle stages) the product is effective against, but also whether the product is suitable for use in your horses. Not all products are licensed for use in foals, pregnant, and lactating mares. The use of anthelmintics not licensed for use in horses carries substantial risk, and the possibility of acceleration in resistance.
Dr. Theo de Waal, Senior Lecturer and European Veterinary Specialist in Parasitology, is currently asking horse owners (thoroughbred and non-thoroughbred) to partake in a UCD Equine Wormer Survey. The deadline for completion is September 30th.
This study is aimed at obtaining information on the current common worm control practices on farm and is to be followed up with a more in-depth study on wormer resistance in Ireland. Horse owners can partake by following the link a https://www.surveymonkey.com/s/BPC95LK