Wormer resistance

What is wormer resistance?

Anthelmintic resistance occurs when a proportion of the parasites picked up from a particular pasture are no longer affected by the chosen worming treatment. A number of factors can contribute to the development of resistance, including underdosing and the frequent, perhaps unnecessary, administration of wormers over time.

All worms in any given population (i.e. inside a horse) are each as genetically different from each other as humans are genetically different from one another. A very few of those worms will have a natural genetic variation that can enable them to survive a dose of anthelmintic, or wormer. This sort of variation exists within all populations of all organisms. This occurs partly because random mutations occur in the genome of an individual organism, and these mutations can be passed to offspring. Throughout the individuals’ lives, their genomes interact with their environments to cause variations in traits. The process of natural selection, as detailed by Charles Darwin’s theory of evolution, states that those living things that are the best adapted to survive in their environments will survive and be the most successful at reproduction, therefore having more offspring, which also have the genetic mutation. Eventually, through natural selection such mutations become the norm.

How does wormer resistance develop?

Using too low a dose of wormer is one of the key ways that helps those slightly, naturally resistant worms to survive. These then breed with each other to potentially produce a higher proportion of resistant worms and a - smaller population of susceptible worms within the population. Successive doses of the same wormer, or wormers from the same drug class, can lead to an established resistant population.

Frequent, unnecessary worming can also increase the potential for resistance, by actively selecting for worms that are resistant and killing out susceptible worms before they can reach sexual maturity and produce off spring.

Once resistance is present in a worm population, the health, welfare and performance of those horses infested with resistant worms will be compromised. It is also impossible to revert back to a susceptible population.

Reducing & preventing resistance

The key to reducing the likelihood of resistance developing starts with the identification of those horses which need to be treated. This can be achieved by testing individual horses, thereby identifying those animals with a significant worm burden, and then using the correct wormer to treat them - calculating the correct dose and time to worm.

Worm egg counts help to ensure that wormers are only given when they are needed and therefore may reduce the likelihood of resistance developing. This helps your individual horse and the equine community as a whole.

As part of this more targeted approach to worming, industry experts are calling for horse owners to make better use of Worm Egg Counts (WECs). A WEC is a microscopic examination of a dung sample from a horse to detect and count the number of roundworm eggs present. The egg count is expressed as eggs per gram (epg), and in most cases if the count is greater than 200epg then worming should be considered (some foals and horses, especially where there is no previous worming history, may still require treatment where the worm egg count is less than 200 epg; consult your vet for further advice). A WEC therefore helps to identify those horses that have an excessive worm burden and would benefit from a treatment. It also identifies the main species of worms with the exception of tapeworms, and immature and encysted worms.

It is known that approximately 80% of worms are carried by only 20% of horses. The regime of regular worming at fixed intervals, a system that has traditionally been popular on large yards, has provided an effective means of limiting worm burdens, but it is possible that many horses are being wormed unnecessarily, which encourages the development of resistance. With the consensus of expert opinion on the future of worming firmly behind the use of targeted programmes incorporating WECs, there has never been a better time to make worm egg counts a regular part of your worming programme.