Why Have There Been So Many Horse Parasites and Worms This Year?

Across yards, vets, labs and owners, we are reporting a noticeable surge in worms and parasites, even in horses on well managed programs with good grazing.

If you have seen higher fecal counts, more itchy tails, unexpected weight loss, or horses just not quite thriving this year, you are not alone.

One factor keeps coming up repeatedly: The weather.

Unusually mild winters, prolonged wet periods, warm, damp conditions and periods of heat and drought have created perfect breeding grounds for equine parasites. Instead of being knocked back by frost, worm eggs and larvae have survived longer, developed faster, and contaminated pasture more heavily than usual.

So while management practices and resistance play a role, this year’s parasite problems are largely being driven by environmental conditions that favour worms over horses.

Below, we explore how weather patterns directly influence parasite lifecycles, why this year has been particularly challenging, and what horse owners can do to reduce the impact.

Wetter, Milder Weather and Heat Stress

Weather doesn’t just affect parasites directly, it also changes how horses graze, which can dramatically increase worm intake.

While wet, mild conditions help larvae survive, periods of heat and dry ground force horses to graze grass much lower than normal. Worm larvae tend to concentrate in the lower few centimetres of the sward, especially around dung patches.

When grass is short:

• Horses are more likely to ingest larvae while grazing
• There is less clean grass available above contaminated areas
• Overgrazing increases overall parasite exposure

Heat stressed pasture can therefore act like a parasite hotspot, even on fields that are well managed.

At the same time, some parasites actually benefit from warmth. Certain species, including tapeworm, are known to thrive in warmer conditions. Heat supports the activity of oribatid mites. These are the tiny pasture mites that act as the intermediate host for tapeworm.

When warm temperatures are combined with low rainfall, conditions are ideal for both tapeworm survival and transmission.

This year we had a wet, mild Spring. The grass started growing earlier and the worms started breeding!

The Summer arrived with some long, hot, dry spells. Small redworm don’t like this weather but they are perfectly adapted to survive it. If conditions outside the host are unsuitable for them – they just slow down and stay indoors. They can encyst all year round. They don’t have to wait for cold weather.

With the hot weather the grass stopped growing and horses were grazing closer to the ground. The Tapeworm were having a great time!

When Harvest was about to commence the heavens opened and we went back to warm, damp, mild conditions – Parasite Paradise.

Here we are, in December. It takes a period of five or six days where temperatures are consistently below 6°c to slow down or stop worm activity. We are not seeing temperatures falling low enough to achieve this. Horse owners need to keep sticking to their worm control activities.

Here are some tips to help you:

Increased Stocking Density and Pasture Pressure

More horses on less land means higher pasture contamination.

• Overgrazed fields concentrate droppings and larvae
• Horses grazing close to the ground ingest more parasites
• Limited pasture rotation allows eggs and larvae to accumulate

Yards under pressure may struggle to manage parasite control consistently across all horses. Especially during periods of minimal regrowth of the grass.

Regular poo-picking is one of the most effective ways to control parasites, yet it’s often overlooked or inconsistently applied.

• Infrequent manure removal allows eggs to hatch and spread. Poo picking every two or three days is the optimal way to do this whilst still allowing your Dung Beetle population to thrive and help you
• Shared turnout increases cross contamination
• Harrowing can spread larvae rather than reduce them. In years gone by, harrowing in the Winter to open up any droppings to the frost was an effective control. However, in recent years we rarely have a long enough period of hard enough frost for this to happen. Harrowing is no longer an effective way to kill larvae.
• Cross graze with cattle or sheep. Horse parasites do not live in other species

The Role of Dung Beetles in Larvae Control

Dung beetles can play a surprisingly valuable role in natural parasite control. By breaking down and burying droppings quickly, dung beetles reduce the time worm eggs and larvae remain on the pasture surface, where horses are most likely to ingest them.

• Dung Beetles lower pasture contamination by dispersing and burying eggs

• They remove the breeding ground Most equine worm eggs hatch and develop in fresh dung. When dung beetles break it apart or bury it, the eggs and larvae are either destroyed or moved into conditions where they’re far less likely to survive.
• They reduce larval migration onto grass Worm larvae crawl out of dung onto surrounding grass, especially in wet or warm conditions. Dung beetles shorten the time dung remains intact on the surface, limiting how many larvae reach the sward.
• They expose larvae to lethal conditions by fragmenting droppings. Dung beetles expose eggs and larvae to drying, UV light, temperature extremes and predators, all of which reduce survival. Parasites rely on the dung remaining intact.
• They improve pasture structure Healthier soil and stronger grass growth encourage taller grazing, which keeps horses away from the larva rich zone close to the ground.

Supporting dung beetles won’t replace good parasite management, but as part of an integrated approach, they can significantly help reduce larval burden on grazing land.

What Else Can Horse Owners Do?

• Keep up with regular fecal egg counts and twice yearly saliva tests for tapeworm
• Use wormers (anthelmintics) only when needed and based on test results
• Weigh horses accurately before dosing
• Poo pick fields at least twice weekly
• Rotate grazing where possible
• Quarantine and test new arrivals
• Work with a vet or SQP to create a tailored worming plan
• Encourage Dung Beetle activity in your fields

Final Thoughts

The rise in horse parasites this year isn’t due to one mistake or one bad decision, it is the result of environmental conditions, changing management practices, and growing resistance to treatments. Understanding these factors is the first step toward better control.

With informed management and a targeted approach, it’s still possible to protect horses’ health and reduce parasite pressure, even in challenging years.

Get in touch with Worming Wisley where our Equine SQP can help with pasture management and an effective testing and care program based on your horses‘ needs.