Selecting for Parasite Resistant Katahdins

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People frequently ask us for suggestions on improving the parasite resistance of their flocks and how to identify resistant animals, and with good reason. Intestinal worms, or parasites, are one of the most limiting factors to profitable production for shepherds raising sheep on pasture. Lambs and late gestation and lactating ewes are the most susceptible. The main parasite of concern in much of the U.S. is Haemonchus contortus or barber pole worm.

The most obvious cost to producers is through deaths of lambs and ewes. But 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 coat quality and diminished milk production resulting in unthrifty lambs or pre-weaning mortality.

Multiple research studies have shown that some sheep are more resistant to parasite infections than others. Identifying and selecting animals, especially sires, who have genetic resistance to parasites, and the ability to transmit that resistance to their offspring, is an important option for improving productivity and performance of pasture-raised sheep by reducing worm loads.

The basic principles of selection are relatively simple and straightforward: 1) Identify resistant animals; 2) Cull the most susceptible animals; 3) Select more resistant replacements; and 4) Buy parasite resistant rams. Too often people are surprised and disappointed when an animal isn’t as parasite resistant as they hoped. In this article we will give more details on the selection process and considerations for improving accuracy and predictability.

Methods of Selection

We have found fecal egg counts (FEC) to be the best technique to identify resistant animals in our flocks and it’s the technique we use. FECs use a special McMasters chambered slide to estimate the number of eggs being shed in a gram of feces (epg). It is the most accurate method, but is labor intensive and/or expensive. If you want to generate FEC estimated breeding values (EBVs) through the National Sheep Improvement Program (NSIP), you will need FEC data. Several approved parasitology labs are available to analyze samples for producers ( ) or you can learn to do FECs on your own flock ( ).

Are there other selection methods? Yes, and they work great as a supplement to FECs, but they are much less accurate as the sole means of selection. If FECs are not an option, using several of these methods together and keeping good records will increase their accuracy.

  • FAMACHA – estimates the level of anemia (a sign of infection with Haemonchus contortus), by evaluating the color of the mucosa of the lower eyelid. It’s relatively easy, but can select for resilience, or the ability to tolerate heavier parasite loads. It does not necessarily lead to lower levels of pasture contamination, and may continue to expose susceptible animals to parasite larvae.
  • Body condition scoring – easy, but has not proven effective in identifying parasite burdens in our flocks. Even with good production records, using body condition score may select against the most productive or hard-working animals.
  • Deworming history – easy, but the least accurate, very subjective and only possible if selective deworming is used rather than a whole-flock deworming program.
  • Presence of symptoms such as bottle jaw, lethargy, weight loss, failure to thrive – easy but not exclusive to parasitism and not an accurate measure when used alone.


Before you invest in the equipment necessary to do your own FECs, it’s important to understand that FECs are very time consuming. While FECs are the best tool to identify animals with more or less susceptibility to parasites, they are of limited use otherwise. They can be used before and after deworming to estimate how well an anthelmintic (dewormer) is working. They can also be used to estimate the level of pasture contamination. FECs are not an effective tool for making deworming decisions, except possibly animals with very high counts to avoid further pasture contamination. It’s possible that an animal with low FEC numbers can have a very high parasite load if they are recently infected. The immature worms are feeding but not producing eggs yet. FECs are not an effective tool for identifying animals “sick” with parasite infection; you can’t determine health from a FEC alone.

Identify Resistant Animals

  • The heritability of parasite resistance in sheep is estimated to range from 20-30% (by comparison, maternal productivity traits have a heritability of less than 10%). It’s important to realize that the remaining 70-80% of the differences seen between animals is the result of environment or management. To make progress in selection, you need to objectively identify those animals that possess true genetic resistance regardless of environmental influences.
  • Resistant animals can only be identified by comparing animals in a common environment in the presence of a parasite challenge.
  • It is important that all animals being compared are a common age and/or stage of production, managed the same, offered the same nutrition, and are evaluated as a group on the same day.
  • The range in FECs can be huge – from 0 epg to 30,000 epg or more in the same group of lambs. It’s this wide range that makes selection possible.
  • In order to ensure an adequate challenge, the average FEC of all the lambs in the group needs to be at least 500 epg, and 1000 epg is better.
  • The number of lambs being sampled needs to be representative (not all the best or worst lambs) and the count needs to be high enough to see differences. A minimum of 10 lambs per sire is necessary for comparison, and 12 or more lambs is better.
  • The accuracy of FECs in identifying levels of resistance is improved with more than one FEC analyses. This is typically done the first time a parasite challenge is observed, and again 4-6 weeks later.


Factors affecting accuracy of FEC

  • FECs show the worm level of an animal ON THAT DAY. It does not tell you whether the immune system will kick in and bring that level down to near zero or whether the count will double or triple in the next few days.
  • FEC levels vary within the same animal over time as their immune system develops. Exposure to parasites is necessary to stimulate an immune response. The timing and intensity of this response varies in each animal and is reflected in their FEC. Immune responses cause fluctuations in the worm count, so where a lamb is in this response can affect how resistant it appears on the day of collection.
  • When selecting for parasite resistant animals, FECs done on younger lambs seem to be more predictive. Lambs that are older and heavier when first challenged by parasites tend to have a more developed immune system and lower FEC. They may appear more resistant than they actually are.
  • Parasite resistance is not absolute and even resistant animals can be overcome with a severe challenge or when stressed. This is especially true of the most vulnerable groups: lambs and lactating ewes, especially when ewe lambs are bred to lamb at one year old.
  • Even the least resistant sheep will usually have periods of low FEC.
    • Without adequate exposure to parasites, most sheep will have low FECs
    • Mature animals not under stress (i.e., mature rams and dry ewes) will also have lower FECs and FAMAHCA scores.
    • Most animals, including lambs, tend to have low FECs during the winter when parasites are dormant or hypobiotic.


Cull susceptible animals

The first step for many people is to identify and cull the animals under common management that consistently have the highest parasite challenge based on your selection criteria (high FEC, high FAMACHA, frequent deworming or poor body condition). If you participate in NSIP and have FEC EBVs, you might consider culling animals with positive FEC EBV values or that are above the average for your flock. Sometimes, though, these animals are your best for other production traits so careful consideration is required. If retained, you may want to mate them to your most resistant animals to improve the parasite resistance of their offspring. While culling susceptible animals will help with parasite management, it’s not enough by itself to improve the parasite resistance of a flock.

Select more resistant replacement animals

At the same time, you should be identifying the most resistant animals. These are the animals with the lowest parasite load and the lowest FEC EBVs. Keep their offspring as replacements, assuming they are acceptable on other traits of interest. Also keep as replacements animals whose sire and dam performed better than the flock average as lambs. If you have identified animals with poor parasite resistance but excellent production traits, you may want to consider breeding them to more resistant rams and keeping their offspring to improve parasite resistance in the flock over time while not losing other important production traits.

It’s important to realize that mating a ram with low FECs to a ewe with low FECs doesn’t always result in a low FEC lamb, but it increases the chance that it will. Parasite resistance is not a single gene trait; multiple genes are involved. Therefore, the more parasite resistance genes an animal inherits from each parent, the more resistant and the more predictable the offspring will be. Think of it like a combination lock, the more combinations a lamb inherits from each of its parents, the more likely the trait for parasite resistance will be “unlocked” or expressed.

Purchase parasite resistant rams

The next level for improving resistance is to identify and purchase rams that have documented resistance to parasites. A ram contributes 50% of his genetics to his lambs. A ram with below average FEC EBVs, or below average FECs as a lamb is expected to sire offspring whose group average FEC is lower. That doesn’t mean that every single lamb will be low, but the average for the group should be.

The easiest and most accurate way to identify a ram with above average parasite resistance is to find one with negative FEC EBVs. The lower or more negative the FEC EBV is (-100 is the best), the more resistant the ram is expected to be. The accuracy value is especially important when considering FEC EBVs. It is possible for an animal to have FEC EBVs without ever having an actual FEC submitted, if relatives with FEC EBVs are in the system. These animals will have lower accuracies, since no actual data was submitted. The best accuracies are seen in rams with high numbers of progeny that have had FEC data submitted. The higher the accuracy value, the more reliable the FEC EBV will be. It never hurts to ask the breeder if FECs were submitted, or even to see the raw data.

It’s possible to identify parasite resistant animals without FEC EBVs, but it can be challenging. First, identify a farm that keeps accurate FEC records. Then select a ram whose FEC as a lamb was well below his group average, assuming there was an adequate parasite challenge. A single FEC with an adequate challenge is the minimum. A second FEC conducted 4-6 weeks later on the same group will give better accuracy. We would not recommend trying to select a parasite resistant animal, especially a ram, without FEC data at the very least.

Remember that the ram provides 50% of the genetics; your ewes provide the rest. If you have a serious parasite problem because your ewes have poor parasite resistance it can take a few generations using parasite resistant rams to completely remedy the issue.

Words of Caution

It’s very important to avoid single trait selection. Frequently when you select hard for one trait, it will be at the expense of other important production traits. Set acceptable minimums for other traits while improving flock resistance.

Pasture management can be challenging. Having a large number of parasite larvae on the pasture early in the summer is great for providing a challenge. But it can quickly become dangerous, even for resistant animals, later in the year. Even if there are no animal deaths, production will be decreased.

Putting it all together

Making changes to your flock using the principles and methods described above can result in relatively quick and positive changes. Culling high shedding animals will have the immediate effect of lowering the number of eggs deposited on the pastures; using sires with low FEC EBVs or documented low FEC as lambs will result in lower average FEC in their offspring; and selecting replacement ewes from these matings will improve overall flock resistance.


For information on laboratories providing FEC:

For information on doing your own FECs:

Fecal Egg Counting Primer Dr. Dahlia O’Brien

How to do Modified McMaster FEC Procedure by Dr. Anne Zajac

For more information on using FECs to select resistant animals.

Selecting Worm Resistant Animals (2020) Dr. Dahlia O’Brien

Selection for Parasite Resistance (2017) Dr. Joan Burke

By EAPK Communications Committee

Kathy Bielek and Roxanne Newton





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