George Lager, PhD, professor of Geosciences at the University of Louisville, earned his PhD in geological sciences at the University of British Columbia in 1976. He and his wife, Marjorie, live on a small farm near Corydon, Ind., where, in addition to wild mustangs, they raise an assortment of animals, including donkeys and dairy goats.
Anthelmintic resistance is recognized globally as a serious problem in the control of equine internal parasites. Fecal egg count reduction tests indicate that on many farms, certain drug classes are no longer effective, or egg reappearance periods have shortened significantly. Drug-resistant parasite emergence is related primarily to anthelmintic overuse and the lack of "evidence-based" deworming strategies.

When we started our small farm in south-central Indiana, we decided to base our parasite control program on a rigorous management plan, which involved frequently removing all fecal matter from pastures (about 5 acres), corrals, and stalls. We do not deworm until fecal egg counts (FECs) reach a predetermined target value. This is a labor-intensive approach, but it's more sustainable than traditional protocols because it limits anthelmintic use and delays drug resistance. In addition, horses carry a small worm burden that helps build immunity to more serious parasite infections.

George Lager

The author, shown here with mustangs Nevada and Mesteño, addressed parasite resistance early when he established his farm.

Beginning with the adoption of our first BLM mustang, we collected all feces manually from pastures three times/week and from corrals and from stalls twice daily. We tracked FECs of two mustangs at one- to three-month intervals over a period of about two years (2-year-old colt at adoption on Aug. 3, 2007, named Nevada) and about one year (1-year-old colt at adoption on March 11, 2008, named Mesteño).

We dewormed both mustangs with ivermectin-praziquantel at the time of adoption and quarantined them for one month before release to pasture. Within one to two months of adoption, we used either pyrantel/piperazine or piperazine to treat for any remaining strongyles and/or large roundworms (Parascaris equorum). The Corydon Animal Hospital determined strongyle and large roundworm FECs, or we made the counts in a small laboratory at our farm using the McMaster egg counting technique.

The pastures, which we developed during our first year at the farm, had not been grazed by equines within the last 40 years. For the first two years, these two horses were the only ones on the farm. They usually had 24/7 access to the pastures, unless precipitation was significantly above or below normal for extended periods. In these cases we took them off pasture until the grazing conditions improved. Whenever possible, pastures were cross-grazed with a small goat herd to control the equine parasites that might inhabit the taller grasses and nongrassy vegetation. We dewormed the horses when strongyle egg counts reached 200 eggs per gram (EPG), a relatively low target count that does not increase the occurrence of colic or result in serious pasture contamination.

The target egg count was reached without intervening treatments after about 21 and about 12 months for Nevada and Mesteño, respectively. Because there are only two horses involved and no control group for comparisons, this result is of more educational than scientific value. Therefore, no conclusive statements can be made about our plan's effect on parasite burdens. Nevertheless, the length of time between treatments is unusual for horses in this age group. Five months after deworming, FECs for both horses are less than 100 EPG.

Other than Nevada's initial FEC at the time of adoption, no roundworm eggs were observed for either horse. The hygienic conditions on the farm and the development of new, "virgin" pastures might explain the absence of roundworm eggs, which can persist for years in soils under temperature extremes. There is some anecdotal evidence that mustangs have developed good immunity to parasites. In the wild, herds migrate over large areas, naturally limiting parasite transmission. Unfortunately, without a control group and a larger sample population, it's difficult to assess whether the low egg output is related to genetic makeup, management practices, or a combination of factors, including local weather variability.

In spite of the limited data set, this grass-roots approach shows what an owner on a newly developed farm can do to address parasite drug resistance. The only requirements are time and a small investment in basic lab equipment to determine FECs. Field studies with a good scientific design would be required to determine if our practices would produce similar results on other, more established farms with "domestic" horses. The raw data and a plot of FECs versus time can be obtained at

Originally published in the February 2010 issue.