Decoding Dewormers: Types, Resistance Concerns, and Use for Horses

varies with age due to acquired immunity. Young horses typically have a greater risk for parasite infection most commonly ascarids and small strongyles and therefore require more frequent deworming treatments. Adult horses those that are 5 years or older tend to develop higher immunity that stays relatively consistent throughout their adult life. This acquired immunity will control ascarid infections and while it wont prevent infection from small strongyles it will regulate the horses risk of disease and egg shedding. In most cases for mature adult horses low shedders often remain low moderate shedders remain moderate and high shedders remain high. However as a horse becomes geriatric over 20 years old it may start to shed a higher number of strongyle eggs. This may be associated with lower immunity. Therefore it is important to continue shedding rate evaluations at various ages in adult horses.
FECs should continue throughout horses life stages to ensure the dewormer drug classes remain effective on that horse and farm using an FECRT. Foals should not have FECs until they are at least 6 months of age.
Recommended Deworming Frequency for Adult Horses
The AAEP recommends that all adult horses be treated with dewormer at least one to two times a year. Even if a horses FEC reveals 0 EPG they should still receive at least one treatment because the test focuses on strongyle eggs and doesnt always reveal the presence of other parasites such as tapeworms and bots. Additionally an FEC cannot account for encysted small strongyles that are within the horses intestinal system. There is also a chance the test may be inaccurate if the manure sample was not fresh enough or if the horse was dewormed recently.
Deworming programs for adult horses are typically focused on controlling small strongyles. Therefore the AAEP recommends more frequent treatments for horses

with higher shedding rates. Targeting treatments on horses shedding the majority of the eggs can help reduce the farms overall small strongyle load and increase refugia.

Strongyle Egg Shedding Category	FEC Results (EPG)	Number of recommended deworming treatments per year
Low shedders	0 to 199	1-2
Moderate shedders	200 to 500	2-3
High shedders	>500	3-4

Note. Information in table gathered from the AAEP Internal Parasite Guidelines.

Treatments should be given during times of peak transmission typically spring and late fall for those in the northeastern US. Any additional treatments for moderate and high shedders should be given in between spring and fall. Peak transmission times can vary by region of the US. Your local veterinarian should be aware of recommendations in your region. Always consult with them when making your internal parasite management plan.
Recommended Deworming Frequency for Foals Weanlings Yearlings and TwoYearOlds
The AAEP has specific guidelines for foals weanlings and yearlings that focus on controlling ascarids and small strongyles. Foals should be dewormed at 23 months of age 46 months of age specifically just before weaning 9 months of age and 12 months of age. The first treatment should target ascarids with the AAEP advising benzimidazole as the drug class to use. After weaning an FEC should be conducted to evaluate small strongyle and ascarid parasite loads. The FEC results should inform the drug classes used for both the 9month and 12month treatments. Treatments should focus on targeting small strongyles and the 9month treatment should also include tapeworm control.