National Farm and Facility Level Biosecurity User Guide for the Equine Sector
Annex 11: Additional guidance on parasite control programs

The frequent use of products (anthelmintics) to control parasites in horses has resulted in increased drug resistance and changes in the parasite populations affecting horses. While large strongyles used to be a significant threat to horses, small strongyles have become widespread and pose greater risk to horses, particularly when significant burdens are found.Footnote 33

Anthelmintic resistance occurs when worms with genetics that provide resistance pass these genes to subsequent generations of worms. As the number of anthelmintic-resistant worms increase, treatments begin to fail.

New strategies are needed to address the changing threats; frequent parasite treatments should be replaced with timed treatments targeting the specific parasites with effective products.

Goals of a parasite control program may include

  1. Minimizing the risk of parasitic disease;
  2. Controlling the shedding of parasite eggs and development of infective larvae;
  3. Minimizing the use of drugs, maintaining effective drugs and preventing or delaying further development of anthelmintic resistance.

ReferenceFootnote 34

Fecal egg count reduction test (FECRT)

Use the FECRT to determine if strongyles and/or roundworms are resistant to a certain anthelmintic. Collect fecal samples prior to deworming and 14 days after the deworming treatment. The percent reduction in eggs can provide an indication of the absence or presence of drug resistance. Use fecal egg count reduction tests at the level of a herd and not the individual horse as they can be quite variable when repeated for a single horse and when performed between horses. For detailed guidance on conducting FECRT, consult your veterinarian and review the Ontario Veterinary Medical Association Proceedings 2014 and American Association of Equine Practitioners Parasite Control Guidelines (2016).

Horses that graze together can demonstrate huge differences in their levels of strongyle egg shedding.

  • Within a group of mature horses, strongyle egg counts are highly concentrated in certain horses;
  • 20-30% of adult horses usually shed approximately 80% of the eggs.
Worming program considerations
Considerations for deworming programs for adult horses: Considerations for foals, weanling, yearlings
  • focus treatments on small strongyles;
  • use one or two yearly anthelmintic treatments to target large strongyles, tapeworms, bots, and spirurid nematodes;
  • use additional treatments to target horses with a high strongyle contamination potential;
  • focus anthelmintic treatments during seasons of peak transmission (usually spring and fall when pasture refugia is at its highest);
  • evaluate the efficacy of the dewormers used on each farm at least every three years using the fecal egg count reduction test.
  • it is not recommended to target treatments (selective therapy) based on fecal egg counts in this age group;
  • treat foals during the first year of life with a series of anthelmintic treatments to treat ascarids, strongyles, and tapeworms;
  • perform fecal egg count reduction tests yearly to evaluate the efficacy of anthelmintics against strongyles and ascarids;
  • turn recently weaned foals out onto the "cleanest" pastures with the lowest parasite burdens;
  • treat yearlings and two-year-olds as "high" shedders.

General points to consider:

  • Ensure correct dosing of horses and do not under-dose horses and foals; use weight tapes or scales to determine body weights.
  • Small and large strongyles, and tapeworms are acquired on pasture. Roundworms and pinworms can be acquired in confinement as well as on pasture.
  • Have a fecal egg count done on all new arrivals and deworm appropriately before any turnout or sharing a stall.
  • Concentrate drug treatments when the local climate favors parasite transmission.
  • Design a parasite control program that considers the farm's management practices and region of the country. Consider the following:
    • Stocking density: Many horses and/or many different owners may make it more difficult and labor intensive to treat each horse as an individual.
    • Heavy stocking rates resulting in a consistently high level of parasite exposure can challenge even the best deworming program.
    • Time horses spend on pasture: Limited access or the absence of grass often contributes to low fecal egg counts.
    • Age of horses on the farm: Are there foals/weanlings/yearlings and/or mature adults? Treat youngsters as high shedders.
    • Is this an "open" herd: Institute a biosecurity program for all new arrivals that includes a fecal egg count and larvicidal deworming prior to turn-out with resident horses. What is the farm's ability or willingness to "clean up" the environment using non-chemical means such as pasture rotation, cross-grazing with other species, manure removal and composting?
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