Trichostrongylosis in Sheep: Black Scours and Pasture Worm Burdens

Trichostrongylosis is a parasitic disease caused by Trichostrongylus worms living in the abomasum or small intestine of sheep. Merck Veterinary Manual lists Trichostrongylus species among the common gastrointestinal parasites of small ruminants, with T. axei affecting the abomasum and other species affecting the small intestine. These worms are picked up while grazing and can build into a meaningful pasture burden over time.

In sheep, this parasite is classically associated with dark, loose manure called black scours, along with poor weight gain, reduced appetite, and a thin, unthrifty appearance. Some sheep carry a moderate burden with few obvious signs, while lambs and stressed adults may become noticeably ill. The disease is often more about lost production and chronic poor thrift than sudden collapse.

Because several internal parasites can circulate in the same flock, trichostrongylosis is often part of a broader strongyle worm problem rather than a single-parasite issue. That is why your vet usually looks at the whole flock picture, including pasture use, body condition, age group, fecal results, and whether current dewormers are still working.

Trichostrongylosis starts when sheep swallow infective larvae while grazing. The eggs are passed in manure, hatch on pasture, and develop into infective larvae under favorable conditions. Warmth, moisture, and repeated grazing of the same ground help parasite numbers rise. Cornell parasite-control resources note that strongyle larvae can develop to the infective stage in about a week under moist, warm conditions, which helps explain why pasture contamination can build quickly in the grazing season.

Heavy stocking density, short grazing height, and limited pasture rest all increase exposure. Sheep forced to graze close to the ground are more likely to pick up larvae. Lambs are especially vulnerable because they have less immunity, while adults may show milder signs unless nutrition, weather stress, lambing, or other disease lowers their resilience.

Another major cause of ongoing problems is anthelmintic resistance. Merck notes that resistance in small-ruminant nematodes is widespread and that repeated use of the same drug class increases selection pressure. In practical terms, a flock may appear to be 'dewormed' but still carry a meaningful worm burden because the product no longer works well on that farm.

Your vet usually starts with the flock story: age group affected, pasture history, deworming history, body condition, manure quality, and whether the problem is seasonal or persistent. A physical exam helps assess dehydration, weight loss, and overall thrift, but diagnosis usually needs testing because many sheep parasites produce overlapping signs.

The most common first test is a quantitative fecal egg count. This estimates strongyle eggs per gram of feces and helps your vet judge whether parasite burden is likely contributing to disease. Because strongyle eggs can look similar under the microscope, your vet may also recommend larval culture or species differentiation when the exact parasite mix matters.

If treatment failure is suspected, a fecal egg count reduction test is especially useful. Cornell describes this as paired testing before treatment and again about 10 to 14 days later to measure how well the dewormer worked. In some cases, your vet may also recommend bloodwork, body condition scoring, or necropsy of a deceased sheep to rule out other causes and confirm the parasite picture.

Prevention focuses on reducing pasture exposure and using dewormers more carefully. Merck recommends targeted selective treatment to help slow resistance, rather than automatically treating every animal on a fixed schedule. In sheep, treatment decisions may be guided by clinical signs, body condition, dag score, fecal egg counts, and flock history. This approach helps preserve refugia, meaning a portion of the worm population remains unexposed to dewormers and is less likely to select for resistance.

Pasture management also matters. Avoid forcing sheep to graze very short forage, reduce overcrowding, and give paddocks enough rest when possible. Mixed-species grazing and mowing can help in some systems, and Cornell extension materials emphasize weighing animals accurately for dosing and checking dewormer performance regularly.

Quarantine new arrivals before mixing them into the flock. Cornell recommends keeping new animals separate for at least three weeks and confirming they are not shedding significant parasite eggs before joining the main group. Work with your vet to build a seasonal parasite plan for your farm, because the best prevention program depends on climate, stocking rate, lambing pattern, and the resistance history on your property.