Bob Judd, DVM, DABVP (Equine Medicine), DABVP (Canine and Feline Practice)
If you are still deworming your horse every one to two months as we did 40 years ago, you are likely allowing resistant parasites to develop. A recent study showed that this resistance occurs fairly quickly.
Dr. Thomas Geurden, Head of EU Global Clinical Development, Statistics and Data Management at Zoetis, Brussels, performed a study on two draft horse farms that practiced intensive calendar-based deworming. Regardless of the need, the horses were dewormed four times a year with 2% Moxidectin, or six times a year with ivermectin.
To check for drug resistance, a stool analysis was checked for parasite eggs. The analysis showed that parasite eggs reappeared twice as fast on the farms studied due to resistance, as is normally the case.
Because of this resistance, scientists involved in the study checked fecal egg counts in all horses every two weeks from April to September for three years. Horses with high fecal egg counts received a different dewormer, and the rest of the horses were only dewormed twice a year.
This deworming program effectively decreased the total deworming needed by one-third, showing this alternative method of deworming was highly effective in controlling fecal egg counts at normal levels in both herds and slowed the drug resistance of the parasites up to 300%.
The horses that did require three treatments per year instead of two were generally younger horses with less immunity to fight off parasites. This study proved that horses did not need to be dewormed every 2-3 months and deworming less actually decreased resistance and was more effective.
Fecal egg counts in stool samples are routine for your veterinarian and the test is usually inexpensive.