Unfortunately, there is no easy answer to your question. Parasite control is becoming more difficult as resistance to dewormers increases. Also, resistance can be both farm- and horse-specific. It is very good that you had a fecal exam done on both of your horses prior to deworming. Work closely with your veterinarian to determine which dewormers are effective for your mare's strongyles and which may no longer work. To determine if a dewormer works effectively, another fecal exam is taken 10 to 14 days after deworming to see if there is at least a 90% reduction in eggs passed in the feces. She may need to be monitored with fecal exams several times during the year. In addition, closely evaluate your management practices to reduce exposure to parasites. Consider factors such as how manure is handled on the farm (composted, harrowed if left in fields, etc.); how the horses are fed (in feed pans, troughs, or possibly on the ground); how the pasture is managed (over- or under-grazed); whether new horses should have a fecal test prior to residing on the farm; and the management of other horses on the property.

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