Sheep and goat farmers have been battling internal parasites for decades, and the problem isn’t going away. Throughout the Southeast and Northeast, Haemonchus contortus (barber pole worm) continues to top the list as the most serious internal parasite for small ruminants.

Dr. Scott Bowridge, associate professor, food animal science at West Virginia University, has studied parasitism in sheep and goats from both a genetic and immunology standpoint. He’s worked with small ruminants across the nation and realized the extent of the resistance issue when he moved to Maine from California, then continued to see serious issues as he worked in Virginia and West Virginia.

“We started working with Katahdin sheep,” said Bowridge, recalling his time in Maine. “Then at Virginia Tech I started working with St. Croix sheep, which originate from an island environment. It’s 85º every day, year-round, with high humidity – a perfect environment for Haemonchus to grow.”

Sheep in the study had been raised on an elevated floor barn, which allowed researchers to generate parasite-naïve sheep. “Those sheep had never been exposed to Haemonchus,” said Bowridge. “After weaning, we took some of the naïve St. Croix lambs and gave them a primary infection – the first time they experienced Haemonchus.”

After lambs were given 10,000 larvae, fecal egg counts (FEC) rose to 32,000 eggs/gram of feces. Egg levels dropped by the fifth week without dewormer treatment. “They cleared it by themselves,” said Bowridge. “Then I dewormed the lambs at week seven, rested them for two weeks, then gave them a challenge infection to challenge their immune response and develop a response against something they had been exposed to previously.”

Lambs didn’t develop a notable FEC, which means St. Croix sheep can clear the infection by themselves and maintain low-level infection. In comparison, Bowridge said in his experience, Suffolk sheep are the most susceptible to parasites. “I gave them [Suffolks] the same number of larvae as the St. Croix lambs and the Suffolks’ FEC went up to 70,000 – almost twice that of the St. Croix lambs.”

Bowridge dewormed the Suffolks, introduced another parasite infection two weeks later, and the sheep still couldn’t clear the infection. “There’s a genetic/breed underpinning of how parasite resistance works,” he said. “Some breeds are more resistant to parasites.”

He explained that the mechanism of parasite resistance is immunity – an animal’s ability to respond to the parasite and generate an adaptive response that provides life-long protection.

WVU operates one of the largest goat tests in the country. Bowridge noted that 275 goats from cooperating breeders across the nation have been nominated for the 2023 goat test. “Once the goats are here, I give them a triple dose of dewormer,” said Bowridge. “They get Cydectin, Valbazen® and Prohibit®. Then we give each of the goats 5,000 larvae when they start the test.”

If resistant animals can be identified and used as sires, the industry can make improvements within flocks or herds. “There are differences within any breed,” he said, “and how different they are allows producers a selectable range to place emphasis on both FECs and parasite resistance as well as growth, ribeye area and other important production traits.”

Over the past 20 years, Bowridge’s work has shifted to Katahdin sheep, and he’s watched the breed go from being on the “rare breed” list to the most popular sheep breed in the U.S. “The breed has put a lot of emphasis on using estimated breeding values (EBV),” he said. “Through the use of EBVs, we can select sheep that are more parasite resistant than others.”

Bowridge noted two time points to determine parasite resistance. “We have a weaning worm egg count reduction,” he said. “This is the animal’s genetic potential for reducing FEC at weaning. The second, and more powerful, is the post-weaning worm count reduction. Post-weaning is any time after 90 days of age.”

One study looked at FEC on pasture using parasite resistant rams. “We used sires with EBV that predict parasite resistance to generate lambs that are parasite resistant,” said Bowridge. “It’s confirmation that genetic breeding value works and can be applied.” Researchers also found no difference in growth rate regardless of sire status or being on pasture.

“What we didn’t expect is death loss differences,” said Bowridge. “The first year, we had a massive outbreak of Clostridium type A. Of the high sired lambs, we lost about 30% of lambs. On the low sires, we only lost about 10% of lambs. This is death loss that has nothing to do with parasitism – it’s after seven days of age and up to two weeks post-weaning.”

These findings are significant and prove that resistance has a critical role in overall animal health. “During environments with high pathogen load, particularly with Clostridium A, we were losing more lambs that are more parasite-susceptible,” said Bowridge. “In the second year, we vaccinated for Clostridium type A, had a reduced death loss, but still get a consistent up to 9% difference in death loss on a yearly basis that has nothing to do with parasites. That got us thinking – are parasite resistant sheep more disease resistant?”

Parasite resistance is an animal’s ability to resist a parasite infection and relies on its ability to generate an immune response to the parasite. “The early thought is when you select for parasite-resistant sheep, you’re selecting for a better immune response to parasites,” said Bowridge. “But since we saw fewer animals dying pre-weaning, we thought maybe selection for improved resistance to parasites improves immunity to pathogens in general.”

He added that every year since the Clostridium outbreak, he has been breeding divergently, still vaccinating for Clostridium type A, and is seeing about a 5% difference in death loss between the low lambs (low value for parasite resistance) versus lambs that are more susceptible.

To measure animal’s ability to resist disease, Bowridge looked at aspects such as ewe culling rate based on mastitis. “We have to be able to measure response to disease other than parasitism,” he said. “An easy way to do this to look at their response to vaccination and whether there are differences based on genotypes.”

When looking at FECs at weaning, Bowridge found that high EBV sheep have higher FECs than low EBV sheep, and the same was true with lambs. “So the EBVs work, and this is where it gets cool,” he said. “Low EBV sheep generate more antibody to vaccination than high EBV sheep, and low lambs have more antibody than the high FEC lambs.”

For a positive outcome in selecting for parasite resistance, the first goal is to lower FECs. “It’s reducing the number of animals that suffer from parasitism,” said Bowridge. “One of the easiest ways to do that is to apply genetics. When we use lower FEC EBV sheep, we get greater parasite resistance, reduced death loss to weaning, greater lamb survivability, lower mastitis in brood ewes, greater ewe longevity, greater antibody response to Clostridial vaccines and greater colostrum and IgG production.”

by Sally Colby