Identifying ewes resistant to gastrointestinal parasitic worms during gestation and lactation

Project Overview

Project Type: On-Farm Research
Funds awarded in 2009: $14,866.00
Projected End Date: 12/31/2009
Region: Southern
State: Arkansas
Principal Investigator:
Dr. Joan Burke
USDA, Agricultural Research Service

Annual Reports


  • Animals: sheep


  • Animal Production: parasite control, livestock breeding, preventive practices
  • Education and Training: on-farm/ranch research
  • Production Systems: holistic management
  • Sustainable Communities: sustainability measures


    Selecting sheep resistant to gastrointestinal nematodes will diminish parasite challenges in pastures, leading to reduced infections. The relationship between fecal egg counts (FEC) in the ewe during lambing and her offspring was examined. There was a positive relationship between FEC of lambs at 90 days of age and dams at 30 days post-lambing, and FEC of lambs at 120 days of age and dams at 60 days post-lambing. There was a negative relationship between FEC of dams at 60 days post-lambing and weight of offspring at 60 days of age. These relationships will help develop selection strategies for producers.


    Sheep are an ideal species for small farms, especially those interested in organic or forage based production. However, an alarming increase in resistance of parasites to chemical dewormers has occurred. Long-term sustainability depends on developing alternative parasite control. Selecting sheep with resistance to parasites has the potential to produce animals that are less susceptible to parasites and to diminish the parasite challenge in their environment.

    The key management issue for raising small ruminants in the Southeastern US is the barberpole worm (Haemonchus contortus), a nematode inhabiting the stomach (abomasum). This parasite causes extensive losses due to anemia, leading to death and decreased weight gain.

    There are two approaches that producers can use to control parasites: management practices to minimize level of infestation and selection of sheep that are genetically resistant to worms. Management practices have been explored (SSARE grants LS05-177, LS04-164, LS02-143; However, practicality of these practices on small farms common in southeastern US is not always feasible. Using sheep that are genetically resistant to parasites is an option that can be used by both smaller and larger operations.

    There are two periods when sheep are more susceptible to parasites; as young lambs (6 weeks to 8 months) before immune resistance develops and as ewes during late pregnancy and early lactation (peri-parturient period).

    Producers in this study raise Katahdin sheep and have worked with the National Sheep Improvement Program (NSIP;; to develop an EPD (expected progeny difference) to identify parasite resistance in lambs. Heritability of parasite resistance is moderate in the lamb, but it is not known if selection for parasite resistance in the lamb is genetically related to parasite resistance in the ewe. There is little known on parasite resistance in the peri-parturient ewe.

    The lactating ewe, during this time of nutritional stress, is the major source of worm infestation on pastures. In this study, we propose to look at resistance in the lamb and in the ewe and begin determining if progress can be made in identifying genetics that are highly resistant to the parasites during the peri-parturient period. If resistance in a ewe is correlated to that of offspring, animal selection can be made at an early age and at a more logical time of sample collection (it is more difficult to collect from ewes at a time when they should be bonding with offspring).

    Also, if producers can identify sheep that are resistant to nematodes both as a lamb and as a peri-parturient ewe, they will be able to limit infestation of the pastures, reduce mortality and performance losses and reduce their reliance on dewormers. Increasing the resistance of the sheep will also work to prolong the effectiveness of dewormers that have not succumbed to worm resistance. Sheep that are genetically resistant to parasites, will also remove the major barrier to producing organic lamb meat in warm, moist climates and increase environmental and financial sustainability.

    Project objectives:

    Objective 1) Measure worm FEC of ewes during late gestation and lactation. Fecal samples will be collected directly from rectum from ewes every 14 days, starting approximately two weeks before lambing is due to begin until approximately four weeks after a ewe has lambed (only samples that approximate Days -14, 0, 14, and 28, where Day 0 = day of lambing, will be used for FEC analysis). A pooled fecal sample from several ewes will be cultured to determine population of gastrointestinal nematodes. Flocks without a predominant Haemonchus contortus worm species may not be included in data analyses. However, it is expected that H. contortus will be predominant worm. Cultures will be conducted at ARS, Booneville and FEC will be determined at Louisiana State University (Dr. James Miller’s lab; Drs. Burke and Miller have an extensive collaborative history) using a modified McMaster’s technique.

    Objective 2) Measure FEC of offspring at first significant exposure to worm parasites. Fecal samples will be collected from lambs for FEC as described above at approximately 60, 90, and 120 days of age. Offspring from ewes described above will be used. A pooled fecal sample from lambs will be cultured to determine population of nematodes.

    Objective 3) Examine genetic resistance of ewes during the peri-parturient rise with that of their offspring. Genetic analyses will be conducted to determine whether resistance to parasitic worms in the lactating ewe is heritable and correlated with resistance to nematodes as lambs. Correlations will be determined between FEC from each time point from ewe and offspring. A regression analysis will be conducted between ewe FEC and offspring FEC (FEC will be log transformed).

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.