Sheep Industry News August 2023

Technology Proving Invaluable In Combating Parasites

JAKE THORNE Texas A&M AgriLife R eaders of the Sheep Industry News are likely all too familiar with the dreaded Haemonchus contortus. Rare is the shepherd who hasn’t lost sheep to Barber’s Pole worm or at least has dark memories of spending an afternoon drenching animals on a hot summer day. Making matters increasingly worse, the anthelmintics avail able to treat Haemonchus infections are becoming less and less effective, thanks to the buildup of resistance by these worms to a limited number of treatments approved for use in the United States. However, having even more dewormers at our disposal would only be a temporary fix because gastrointestinal nema todes are an ever-growing problem, and we can no longer rely on treatment alone to bail us out. If you own sheep, some form of genetic selection for increasing natural parasite resistance in your flocks is a must. Parasite resistance is sometimes a catch-all term used for animals or breeds that don’t seem to get wormy. In truth, there are many layers of complexity to what animals are truly resistant and how we can identify those individuals within or across breeds. In somewhat simplified terms, sheep with high levels of parasite resistance have immune systems that are more profi cient at recognizing the newly introduced worm pathogen for what it is and rapidly mounting an immune response effec tive against it. Research has shown the most effective immune response against GIN is a Th2-type, in respect to the T-helper lymphocytes production of IL-4, IL-5 and IL-13 cytokines (proteins used for cellular signaling) and the corresponding immune cells and antibodies that are recruited to the mucin layer along the inner lining of the abomasum – the actual site of parasite/host interaction. Generally, breeds of sheep that were developed closer to the equator and have adapted to a parasite-rich environment for centuries will mount a strong Th2-type response, whereas those from more seasonal areas do not. In some breeds – particularly animals that have not had previous exposure to Haemonchus – their immune systems treat the parasitic worm similarly to a bacteria or virus and induce a pro-inflammatory response (Th1 type), which is not nearly as effective at combat ing a parasitic infection. Through the University of Idaho (where I’m pursuing my Ph.D.) and Texas A&M AgriLife Research, I have had the

opportunity to work on better understanding the response of certain sheep to parasites, particularly animals from suscep tible breeds such as the Rambouillet and Dorper. True parasite resistance is maybe not the realistic goal in these breeds, but more so reducing their susceptibility to a level where they can reasonably defend themselves against a moderate parasite chal lenge. Here in San Angelo, Texas, we have flocks of about 200 ewes each of Rambouillet and Dorper enrolled in the National Sheep Improvement Program, and for the last eight years have submitted fecal egg counts on our lamb crops to gener ate a range of weaning and post-weaning egg count Estimated Breeding Values. These two flocks provide a great foundation for research as we try to further understand how the response of Dorper and Rambouillet to Haemonchus contortus com pares to other breeds. The first of our parasite research projects was in 2020, when our Rambouillet lamb crop was sired by rams that had diverging FEC EBVs – above and below breed average. We placed these lambs in a worm-free environment – feedlot – and actually gave them all a consistent amount of just Haemonchus contortus infective-stage larvae. We proceeded to collect weekly FECs for the next six weeks from the lambs and then compared their results back to their sire’s FEC EBV. As hypothesized, lambs from the low FEC sire had reduced FECs compared to the high EBV sire group across the entirety of the artificial challenge trial. While this might seem like a fairly straightforward result, it’s important to note that the sire’s EBVs had been developed using data collected from animals grazing on range, where there are a mix of para site species present and consumption of worms by each sheep cannot be determined. It was reassuring to know that FEC EBVs of the rams were predictive of the lambs’ FECs when challenged solely with Haemonchus contortus. In short, this provided somewhat of a validation that National Sheep Improvement Program EBVs are an effective strategy for reducing susceptibility of Ram bouillet to the most threatening GIN. Following this first study in Rambouillet, we next conducted an artificial challenge with Dorper lambs, as well as Barbados Blackbelly crossbred lambs that were expected to have a high level of Haemonchus contortus resistance. As expected, the Barbados cross lambs barely registered an infection, as they most likely were able to prevent the vast majority of the larvae from ever becoming established. However, in line with our previous findings, the FEC EBV of the Dorper lambs them

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