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October 2017

Selection for Parasite Resistance

Dr. Joan Burke
USDA-ARS, Booneville, Arkansas


Genetic resistance to gastrointestinal parasites is one of the most promising means to control worms in a flock/herd.

As a visitor to this website, you are already aware that gastrointestinal parasites, particularly barber pole worms (Haemonchus contortus), are a primary health issue of sheep and goats, and are associated with reduced weight gain and wool growth, diarrhea, anemia, and sometimes death. Some of these death losses can be considered natural selection against parasites, and in wild populations of small ruminants are considered essential in maintaining a healthy herd.

In domestic populations, animal welfare is of high importance, and it is desirable to select against parasite susceptible animals before we see poor performance and death losses. Fortunately, the ability of an animal to resist parasite infection is  moderately heritable, and resistant parents will most likely have resistant offspring. Resistance to parasite infection is noted by low fecal egg counts [FEC]. Parasite tolerance is when animals become infected, but continue to grow well and do not become as anemic; FEC may be high, but FAMACHA© scores will be low.

The easiest way to measure parasite resistance is to collect fresh fecal samples and determine FECs within a population. For this collection, all animals should have been dewormed at the time of weaning, and samples collected from all lambs or kids 4 to 6 weeks later. This should be repeated in 4 to 6 weeks. Alternatively, those wishing to minimize deworming or who have no effective dewormers, can skip deworming altogether. If animals must be selectively dewormed, do not include these samples for analyses.  Fecal egg counting can be done at home or samples can be sent to a parasitology lab.

For best selection, the mean FEC of the group of animals should be 500 epg or greater. The National Sheep Improvement Program ( allows producers to submit FEC data, regardless of breed, but Katahdin and Polypay have significant data that has improved accuracy. FEC data along with body weights are entered into a software program and submitted to NSIP, which is then sent to Australia Sheep Genetics for data analyses. This genetic analysis sorts out environmental and genetic effects so that the genetic contribution can be estimated. Generated

breeding values (EBV) estimate the inherited genetic potential from the sire and dam.


Factors that influence FEC of animals include birth type (singles will be lower than multiple births); dam age (offspring from ewe lambs and older ewes will be more susceptible than from dams 2-5 years of age); and body  condition of dam or offspring (those in better condition will be more tolerant). Days post-lambing will also have an

effect. The FEC of dams can be collected at lambing and post-lambing, which is termed the peri-parturient rise, and is also heritable. The greatest FEC of dams are typically seen 20 to 30 days post-lambing, which if high, can contribute substantially to infection of pastures for offspring.

The FEC of lambs will often be highest around the time of weaning due to stress and pasture exposure. For Katahdins, FECs are usually highest around 120 days of age and begin to decline thereafter. Fortunately, there appear to be few antagonisms between selection for parasite resistance and growth or maternal traits. This means that we can still meet goals in selecting for growth or maternal traits within the flock or herd while selecting for resistance.

As an alternative to individual selection, there are breeds that are more resistant than others. St. Croix, Barbados Blackbelly, and Louisiana and Florida Native are considered resistant breeds. Katahdin may be resistant, but there are individuals within the breed that are susceptible, and others with good tolerance. Goat breeds within the U.S. are not considered resistant, but it is generally agreed that Spanish and Kiko are more tolerant than Boers. Crossbreeding resistant breeds to less resistant or susceptible breeds will offer some resistance to the offspring through heterosis, perhaps meeting other flock/herd goals.

Genetic resistance to gastrointestinal parasites is one of the most promising means to control worms in a flock/herd. Selection and use of resistant sires using EBVs leads to lower FEC and FAMACHA© scores. Thus, both registered and commercial flocks and herds can benefit by using resistant genetics, seeing a reduction in the use of dewormer and death losses.

The St. Croix is a resistant breed.

You can view Dr. Burke's webinar on Selection for Parasite Resistance at

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