Autumn Equine Internal Parasite Control

The aim of a parasite control programme is not to eradicate or kill all worms but rather to prevent clinical disease, reduce contamination of pasture, and preserve the effectiveness of the anthelmintics which are currently available. There are essentially four drug classes available to use, with varying levels of resistance evident.

Table 1: Description of available drug classes of equine dewormers and their effectivity

*Faecal Egg Count Reduction Test

**Reference James Gibbons, Head of Microbiology, Irish Equine Centre

There is a growing population of drug-resistant parasites with widespread resistance in small strongyle populations to Fenbendazoles, and Pyrantels, while resistance in ascarid (roundworm) populations to Ivermectin and Moxidectin is common. So we simply must enact effective strategies to control parasites without stimulating more parasites to become resistant or evolve to become resistant.

A parasite free zone is not achievable. Co-existence of both hosts (equines) and parasites is normal and acceptable in all pastured animals. The existing levels of resistance cannot be undone but we can prevent resistance from becoming more common by a combination good pasture management and husbandry practices and an anthelmintic control plan which is enacts both strategic treatments (given to all stock) and selective treatments based on faecal egg count results.

Before designing a control plan it is critical to appraise the risk of parasitic infection on the farm (age, stocking density, history, degree of stock movement, quarantine practices – see table 3 below); the pasture management controls in place; which worms are present; which wormers work or if there is resistance present; and which horses are shedding the most eggs and therefore contaminating the environment.

Parasite control programmes where owners and vets make decisions based on each farm’s individual situation are most effective. Dr Hubertus Hertzberg PhD, a researcher at Switzerland’s University of Zurich Vetsuisse Faculty Institute of Parasitology and head of parasite monitoring at Health Balance (a private holistic animal management and veterinary practice) has been overseeing research where over half of Swiss horses are enrolled either entirely or partly in a parasite programme demonstrating that faecal egg counts are generally low and most adult horses do very well with a single annual deworming treatment with no animals presenting clinical problems under the strategy. Working with your vet to determine individual treatment protocols based on customised needs provides best protection for equines in your care. With new veterinary medicine regulations impending and the likelihood of prescriptions required for anthelmintics it is important to recognise that if a full picture approach is taken in consultation with the vet the use of anthelmintics on many farms may possibly be reduced with a saving to offset the expense of engaging the advice and long term benefits for the welfare of all equines.

Good Pasture Management and Husbandry Practices

1. Clean paddocks of dung – labour intensive without access to machinery, but effective as prevents eggs from hatching on pasture. Eggs and larvae of strongyles can survive on pasture over the winter period so cleaning paddocks before closing off for the winter is also good practice.
2. Compost manure – high temperature, above 40 degrees Celsius (102 degrees Fahrenheit), kills eggs and larvae. Although intact faecal balls may retain enough humidity to enable some larval stages survive for some weeks.
3. Rotational Grazing – give pasture time to rest by rotating grazing. This works best in hot dry periods. Under Irish conditions the resting/rotating of pasture as the sole pasture control remedy is unlikely to be successful.
4. Use mixed grazing with other species – cattle/cows; sheep etc. aren’t susceptible to equine strongyles and will eat where horses have defecated so they act as a natural hoover of equine parasite eggs with the added benefit of a more even sward.

Faecal Egg Counts

A key message is to check parasite burdens before treatment. Faecal egg counts show how many strongyle eggs individual horses shed but it is important to differentiate that egg count does not necessarily mirror intensity of infection but rather that the horse is a high shedder and more readily contaminating the environment with eggs. These horses are treated to expose the herd to fewer eggs and larvae on pasture. Often only 15-20% of adults shed about 80% of eggs at pasture and horses usually remain consistent in shedding rates throughout their lifetimes. They also aren’t necessarily the weakest or thinnest, oldest or youngest in the herd.

Table 2: Shedding Intensity aligned to Faecal Egg Count

Faecal egg counts cannot differentiate between large and small strongyles, do not accurately reflect adult worm burdens, cannot detect immature worms, won’t detect pinworm eggs, and provide variable tapeworm detection.

Treatment Plans

Because horses can live healthy lives with controlled small parasite burdens, it can be better to treat them less. While that might seem counterintuitive, science is revealing this prevents reducing the parasite load to only the parasites that resist anthelmintics. By allowing the light shedders to continue shedding, the parasites maintain generations of worms that aren’t developing resistance because they haven’t been exposed to the drugs as much—a population known as refugia.

Using FECs can identify trends on a farm over time and can determine treatment efficacy. By checking FEC a few weeks after giving anthelmintics, known as running a fecal egg count reduction test, the percentage of survivors, as well as how long it takes for eggs to start showing up in the feces again can be determined —both of which indicate levels of resistance.

The advice is to undertake Faecal Egg Count testing four times a year i.e. Jan; Apr; Jul and October, perhaps reducing to three times once a farm demonstrates stability. All horses receive a blanket/ stragetic anthelmintic treatment at the end of the grazing season (autumn). Strategic treatments are provided against large strongyles, encysted small strongyles and tapeworms. While selective treatment can be reserved based on FEC results for control of egg shedding by small strongyles and in adult horses only. Treatment for tapeworm and encysted small redworm may also become selective with the availability of new serum/ saliva ELISA tests.

Adult and young horses on low risk farms should receive treatment in November, after checking FEC in October, of Moxidectin and Praziquantel (i.e. Equest Pramox) to target small strongyles including encysted stages, large strongyles and tapeworm.

It is really important that young horses up to three years of age are otherwise on individual worming schedules according to their age and veterinary recommendations. Identify the adult ‘high shedders’ using faecal egg counts and give them additional deworming treatments targeted to their needs. Evaluate the efficacy of dewormers used every three years by measuring faecal egg counts before and 10-14 days after treatment (Faecal Egg Count Reduction Test).

Moderate or high risk farms should devise a suitable plan in consultation with their vet and on the basis of FEC results.

Ensure to administer appropriate dosage of all anthelmintics used based on accurate assessments of weight to ensure under or over-dosing is not done.

Also it is important to keep records and follow the trends. Monitor the efficacy of the control programme. It will save money in the long term. It will take a bit of time to adapt to a new approach of individualised treatment protocols. But, the aim is to prevent serious and irreversible situation of blanket parasite resistance where parasites no longer respond to treatment or our existing dewormers no longer work. Science is telling us we need to do things differently to manage parasites for the long term.

Table 3: Factors indicating risk level of parasite infestation (Reference: James Gibbons Head of Microbiology, Irish Equine Centre