Quarantine Drenching

Worms are on all horse farms; they are on the pasture that the horses graze and also in the horse. This is why its common practice amongst horse owners to administer a wormer to horses at specific times of the year.

This practice of controlling worms with a paste has served horse owners well over the last 20 years. Wormers contain certain drugs, or “active ingredients”, that have been proven to be efficacious at killing internal parasites. However, more recently there is some evidence of the worms becoming resistant to certain active ingredients and therefore surviving in the horse after the paste has been administered. This means that worm eggs are continually deposited onto the pasture, which in turn increases the risk of the horse becoming re-infected. If the same active ingredient is administered again, then this leads to even more resistant worms surviving in the horse and the whole cycle begins again.

The selection for resistance usually takes many years to occur and slowly builds up on a farm over time. However, the quickest way to acquire resistant horse worms on your property is via a new horse or horses arriving via a truck or horse float.

A new horse arriving on your property may contain different types of worms that your farm does not currently have, for example, the pathogenic worm Parascaris equorum (ascarids) in young horses, pinworms (Oxyuris equi) or worms that are very resistant to the wormer that is currently effective on your property. These new animals can put your current resident horses at risk and may also cause your worm control programme to fail.

Any new horses arriving on your property must be treated in a way that will minimise the risk of resistant worms contaminating the pasture. Parasitologists and veterinarians recommend that the following procedures are undertaken to minimise the risk:

The use of two separate wormers at the same time ensures that the worms are exposed to multiple active ingredients from different chemical groups at the same time, which increases the number of parasites being killed compared to a single active wormer.

Using this procedure when bringing new horses onto your property will minimise the risk of importing in another farm’s parasite problems.

By Dr Tim Elliott Dip. Lab. Tech, B.Tech, PhD (Parasitology) Large Animal Scientist