Worming isn’t working like you think It Is
By the time a parasite issue becomes obvious in a horse, it has usually been there for some time.
Weight begins to slip without a clear reason, a horse does not feel quite right in its work, or it simply fails to pick up as expected despite feeding and management remaining consistent. None of it is particularly dramatic, but it is often enough to suggest that something is not quite adding up.
At Willinga Park Equine Vet Hospital, internal medicine clinician Dr Shaunna McTernan says these are the types of cases that tend to present once the season begins to shift.
“Parasites are one of those things that can sit in the background and go unnoticed for some time” she says. “By the time you see an impact on the horse, the burden has usually been building for a while.”
A large part of that comes down to resistance.
Parasites, particularly small strongyles, have adapted over time to many of the commonly used worming products. In practical terms, this means a horse can be treated regularly and still carry a meaningful parasite burden, with very little to indicate a problem until there is a visible change in condition, performance or general health.
What makes this more difficult is that parasite-related issues rarely present in isolation, and are often masked by the normal transitions that occur at this time of year.
Autumn brings a shift in pasture quality and grazing behaviour, with moisture returning after the drier summer months and feed becoming less consistent in both availability and nutritional value. Horses may also be coming out of more consistent work or adjusting to a different level of workload, which means small changes in condition or behaviour are often expected and easily explained.
“We’ll often see a horse later in winter that’s dropped condition despite feeding being increased,” Shaunna says. “When you start to investigate it properly, parasite burden is part of that picture more often than people expect.”
Traditional worming programs do not always account for that.
Set schedules and rotational products treat every horse the same way, regardless of whether they are carrying a high parasite load or very little at all. Over time, that approach not only becomes less effective, but also contributes to increasing levels of resistance within parasite populations.
The shift in modern parasite management is towards a more targeted approach, where treatment is based on evidence rather than assumption.
Faecal egg counts provide a practical way to assess parasite burden, allowing those horses that are carrying higher loads to be identified and treated appropriately, while avoiding unnecessary dosing in horses that are naturally low shedders. It is a more precise way of managing parasites, and one that more closely reflects what is being seen clinically.
“The aim isn’t to eliminate parasites completely, which isn’t realistic,” Shaunna says. “It’s about managing the load so it doesn’t start to impact the horse.”
For most horses, parasite burden will never become a major issue. However, in the cases where it does, it rarely develops suddenly, and is far more likely to build gradually in the background while everything else appears to be normal.
As with many areas of equine health, the greatest advantage often comes from identifying what is changing early, while there is still time to respond with clarity rather than urgency.