Franklin Vets Blog

Assessing worm burdens in your horses

December 18, 2019

The weather this spring is exactly what worms love to thrive and we’re seeing lots of poo samples being dropped at the clinic from owners looking for advice on whether to worm and if so, with what. Egg counts remain the most useful tool for this but there are a few limitations as follows:

Egg counts are a measure of egg production NOT how many worms there arehorse grazing

A horse’s immunity has a big part to play in how many eggs each adult worm produces. Adult horses with good immunity to worms and in good health will be able to suppress egg production in their worm burdens. However, horses with poor worm immunity or that are young, old, or sick do not control egg production very well so can have high egg burden in their faeces despite relatively low numbers of intestinal worms.

Egg counts tell us what your horse is doing, not what is happening on the pasture

When eggs are passed in the droppings, they sit in the paddock until conditions stimulate hatching of the larvae, which then move away from the dung pile ready to be eaten during grazing. This process is surprisingly quick in warm, wet seasons.

The longer a high egg output goes undetected, the more contaminated the grazing will be, so high counts will need pasture hygiene and management to reduce the risk of future high burdens.

Egg counts are good indicators of some worm burdens but don’t tell us the whole story

The most common intestinal parasite group in the modern domestic horse is the small strongyles, which we usually see on an egg count. However, eggs are only produced by adult worms and there is no reliable measure in NZ to detect immature worms. Also, other worms like tapeworms and large roundworms do not reliably produce eggs in faecal samples. Therefore, even if egg counts are low over a prolonged period, we still recommend biannual drenching to catch the undetectable species and phases.

Checking egg counts after treatment needs to be done at 2 weeks post drench to determine if the drench has worked

An active drench will reduce the egg count to zero in this time and we can presume the drench ingredient has good activity. However, where high burdens were initially seen, we may check this result frequently throughout the advertised length of activity of the drench. This is to make sure emerging drench resistance isn’t contributing to poor worm control.

Despite the limitations mentioned above, faecal egg counts remain the most useful way to monitor worm burdens in your horses and should be a vital part of your worm management regime. During the Spring and Autumn when worm development on the paddock is most active it is especially useful to monitor how things are going and allow you to stamp on increasing counts early before the paddock becomes highly contaminated.

Dr Katie Kindleysides BVSc CertAVP (Equine Dentistry)