Sheep GEMS News Brief 12 – March 2025
In the last Sheep GEMS news brief, we talked about the impact of parasites on production, and it was eluded that genetic selection can be a very promising means to manage worm parasites. Continuing with the Sheep GEMS theme of breeding for climatic resilience and robustness across environments, parasite resistant sheep express improved fitness. Determining fecal egg counts (FEC) at the time of weaning and post-weaning is a measure of parasite resistance already included in the National Sheep Improvement Program (NSIP). Collaboration with the GEMS innovation flocks and 3 USDA ARS facilities with Katahdin sheep will allow us to examine other measurements such as changes in body weight, condition, and FAMACHA© scores for resilience or fitness when parasitic worms present health issues in a flock.
Genetic selection can improve either resistance or resilience traits in sheep. Parasite resistance (different from dewormer resistance which refers to the worm parasite’s ability to resist and survive deworming treatment) is a sheep’s immune response to minimize an infection to worm parasites. Thus, FEC of parasite resistant sheep remain low when their contemporaries may be high. If barber pole worm is the main parasite, a resistant sheep will most often not become anemic or lose weight due to a worm infection. A less resistant or susceptible animal will have a high FEC and may become anemic or lose body weight and condition. Resilience to parasites, on the other hand, is another immune response by the animal that allows greater tolerance to the worm infection. Worms grow to the adult stage in the stomach or intestine and pass eggs in the feces, but animals do not become anemic or lose weight. Thus, FEC may be high, but FAMACHA© scores (1 = healthy; 5 = severely anemic) should remain low. Resilience can be improved with good nutrition and low stress. If both FEC and FAMACHA© scores are high, the animal is neither resistant or resilient.
Parasite resistance and resilience are lowly to moderately heritable, being influenced by the environment and genetics. For example, if lambing occurs during colder months and lambs are weaned onto pasture with minimal exposure to worm parasites, the infection intensity will be very low and FEC may be below the level of detection (typically 50 eggs per gram). In this case it is difficult to assess genetic resistance because it is not clear whether lambs were resistant to parasites or not exposed to parasites. On the other hand, if the infection intensity or stress is too high, even resistant lambs can become overwhelmed and FEC become very high in all lambs, thus FEC comparisons may be less meaningful. Most flocks submitting FEC data to NSIP fall in the first scenario of having too low of a FEC mean. Producers can assess the level of infection/exposure within a group by first measuring a pooled (representing feces from at least 10 animals including thinner sheep) FEC or scoring anemia levels by use of FAMACHA©. If the pooled FEC is above 500 eggs per gram or there are many animals with FAMACHA© scores above 2, then it would be worthwhile to collect and submit individual fecal samples for FEC. However, if animals appear very healthy (low FEC or not anemic), then producers may opt to wait a week or more and assess again.
For further information contact Joan Burke (joan.burke@usda.gov). For more information on parasites, see www.wormx.info.
Acknowledgements. We thank the many U.S. sheep associations, including the Eastern Alliance for Production Katahdins, the Katahdin Hair Sheep International, the National Sheep Improvement Program, and Katahdin sheep producers, for their contributions to this research. This work is supported by the Organic Agriculture Research and Extension Initiative (grant no. 2016-51300-25723/project accession no. 1010329), and by the Agriculture and Food Research Initiative Competitive Grant (grant no. 2022-67015-36073/project accession no. 1027785), from the USDA National Institute of Food and Agriculture. 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.
