March 14, 2019

4 horses in pasture

1) Age

Typically, a mature horse should be dewormed twice per year: once in the spring and once in the late fall. Young horses (under three years of age) are often higher shedders and require four deworming treatments per year.

2) ​Climate

In northern climates, egg shedding tends to be low in winter, regardless of whether the horses have been dewormed. Starting in early spring and continuing through the summer months, egg shedding increases.

3) ​Time of Year

Fall deworming should be performed with a product containing praziquantel (Quest Plus or Eqvalan Gold) to kill bots and tapeworms. Spring deworming can be done with fenbendazole, pyrantel, or the moxidectin/ivermectin drug classes. Switching yearly between drug classes can help to reduce resistance.

4) ​Biosecurity

It is completely reasonable to ask for new arrivals to the farm to have a fecal-float or deworming treatment prior to being turned out with your regular herd.

5) Stocking density

The worm burden of a pasture increases as the stocking density increases. Higher stocking densities also increase the difficulty of clearing manure from the paddocks on a daily basis (as we generally recommend). In situations with high-stocking densities, horses may require more frequent deworming to manage the parasite populations. This is not ideal since higher  deworming frequency is associated with the rise in drug-resistant parasites, but it may be necessary to protect the health of your horse.

6) ​Pasture Management

Removal of manure from the paddocks at least once every two to three days (especially in the warmer months) can really help to reduce the worm burden on your herd. This approach is far more effective — and much less likely to cause drug resistance — than frequent deworming.

7) ​Test individual animals

By performing fecal egg counts on all the horses on the farm prior to administering a deworming treatment, you can identify which are “high shedders” and which are “low shedders.” A high-shedding horse will contaminate your paddock at a higher rate than the average horse. Once the high shedders are identified, you can target them with more frequent deworming treatments (3 to 4 times per year instead of the average of 2 times per year). This strategy will significantly reduce the volume of eggs that are released into the environment.

Key Takeaway

The goal of herd parasite management is NOT to eradicate parasites from your farm. Not only is this impossible, but efforts to do so will accelerate the development of resistance to the deworming drugs that we have available.

Rather, our goal is to reduce the shedding of eggs to an acceptable level — thereby lowering the risk of clinical disease due to intestinal parasitism.


Allossery Equine Veterinary Services
11499 York Durham Line
Mount Albert, Ontario
L0G 1M0

(289) 338-2068