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January 2014
Blueprint for selecting parasite resistant sheep:
a shepherd's perspective



By Kathy Bielek
Misty Oaks Farm
Wooster, Ohio

The concept of selecting parasite resistant sheep is simple: consistently select the animals with the best resistance. Putting the concept into practice, however, can be more challenging. This paper will share some of the lessons learned over the last 10 years through several North Central Sustainable Agriculture Research and Education (NC SARE) grants and on-farm experience. A list of the SARE grants and the breeders involved is included at the end of this blueprint.


Gastrointestinal nematodes (GIN) are one of the most limiting factors to profitable production for shepherds raising sheep on pasture. GIN are often referred to as worms, internal parasites or just parasites, but the principal GIN of concern in Ohio and much of the country is Haemonchus contortus or barber pole worm.

The most obvious GIN cost to producers is through deaths of lambs and ewes. Probably more loss occurs in lambs because of anemia, reduced appetite, slower growth, delays in reaching market weight, and less immunity to other diseases. In ewes, a parasite burden can result in decreased fertility, poor wool quality and lower milk production resulting in unthrifty lambs or pre-weaning mortality. GIN have developed resistance to available dewormers in most flocks in temperate and semi-tropical regions of the U.S. and other sheep producing regions of the world. Identifying and selecting animals, especially sires, who have genetic resistance to GIN, and the ability to transmit that resistance to their offspring, is one option for reducing economic losses related to parasitism.

Resistance to parasites is an immune response. Hair sheep breeds seem to develop immunity earlier (3-4 months in our flocks) compared to most wool sheep (5-7 months). Some sheep appear to have an innate or early resistance. This was reported by one of our advisors, Dr. Charles Parker, at the Ohio Agricultural Research and Development Center in Wooster Ohio in the 1980s. Innate resistance is what the lamb is born with, while acquired resistance is the result of the immune response being stimulated after being exposed to parasites. Lambs with innate resistance appear to have a lower FEC at an early age, before their acquired resistance has developed. These traits appear to be genetically related but separate traits. It’s uncertain whether all sheep breeds express innate resistance, but it has been observed in some Katahdins, other hair sheep and Florida Natives.

Another component to resistance is the ability of some ewes to resist parasites following lambing and during lactation (the periparturient period). Most of the research has focused on identifying GIN resistance in lambs. Our on-farm research has also shown significant differences in fecal egg counts (FEC) among ewes during the periparturient period in our Katahdin flocks. Preliminary analysis of our data suggests that, although favorably correlated, resistance in ewes may

be different from resistance as a lamb. In our flocks, periparturient resistance appears to be heritable from dam to daughter. Selecting ewes that are more resistant will decrease the numbers of parasite eggs shed on a pasture during lactation, which translates to reduced pasture contamination for the lambs. Parasite resistant ewes should also produce more milk and do a better job of maintaining condition during lactation than a ewe with a high parasite load.


Resistance is not complete; even resistant sheep will have some parasites, but they will have lower levels of parasites and lower fecal egg counts (FEC) than non-resistant animals when exposed to similar levels of worm larvae on pastures. Resilient sheep, on the other hand, can tolerate higher numbers of parasites without showing anemia or significant production loss. They can have relatively high FEC (and continue to contaminate pastures) without showing signs of anemia.

There are wide variations in the level of resistance among sheep in an unselected flock. Roughly 20% of the sheep in a flock are responsible for depositing 80% of the parasite eggs on a pasture. In general, hair breeds of sheep have more individuals with resistance to parasites than wool breeds. However, regardless of the breed, there are individual animals in every flock that will be more susceptible and more resistant to parasites. Identifying these individuals allows us to make selection and culling decisions.


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