Considerations for mature horses:

Deworming programs for adult horses should be designed with the following principles in mind:

• Evaluate the efficacy of the dewormers used on each farm at least every three years using the FECRT (Fecal Egg Count Reduction Test).
•  A basic foundation of anthelmintic treatments should be considered to all horses. This should consist of one or two yearly treatments to target large strongyles, tapeworms, bots, and spirurid nematodes responsible for causing summer sores (Habronema spp. and Draschia spp.). In most cases, one or two yearly treatments will achieve this goal.
• All further treatments should be targeting 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).

Considerations for foals, weanling, yearlings 

• Targeted treatments (selective therapy) based on FEC (Fecal Egg Count) is not recommended in this age group. Instead the following considerations should be made.
• During the first year of life foals should receive a minimum of four anthelmintic treatments. First deworming should be carried out at about 2-3 months of age, and a benzimidazole drug is recommended to ensure efficacy against ascarids. Second deworming is recommended just before weaning (approximately six months of age). An extra treatment can be justified before weaning if the time period between the two treatments exceeds 3 months. At weaning FEC are recommended to determine whether worm burdens are primarily strongyles or ascarids, to facilitate the right choice of drug class. Third and fourth treatments should be considered at about 9 and 12 months of age, respectively, and treatment should primarily be targeting strongyles. Tapeworm treatment should be included on one of these latter treatment occasions.
• Perform FECRT yearly to evaluate the efficacy of anthelmintics against strongyles and ascarids.
• Strongyloides westeri is rarely a cause of diarrhea in young foals thanks to the advent of the benzimidazoles and macrocyclic lactones. It is widely used to deworm mares just prior to foaling to prevent the lactogenic transmission of this parasite. However, if mares were recently treated with a macrocyclic lactone that same spring, there is little justification for this.
• Recently weaned foals should be turned out onto the “cleanest” pastures with the lowest parasite burdens.
• Yearlings and two-year olds should continue to be treated as “high” shedders, and receive about three to four yearly treatments with efficacious drugs.

General points to consider  

• Do not under-dose horses and foals; use weight tapes or scales to determine body weights.
• Cyathostomins, large strongyles, and tapeworms are acquired on pasture. Ascarids and pinworms can be acquired in confinement as well as on pasture.
•  Use properly performed FECs to determine shedding status and drug efficacy of new arrivals before turnout in common pastures.
•  Consider using tapeworm serology (ELISA) submitted on at least 20% of resident herd members to determine exposure potential for tapeworms.
•  Concentrate drug treatments when the local climate favors parasite transmission.
•  Decrease treatments when climate conditions are adverse (hot summer / freezing winter) for larval survival and / or transmission.
• Design a parasite control program that considers the farm’s management practices and region of the country. 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 FECs.
• 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 FEC 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?

Developed by the AAEP Parasite Control Subcommittee of the AAEP Infectious Disease Committee May 15, 2013