Trichostrongylosis in Sheep
- Trichostrongylosis is a parasitic worm disease caused by Trichostrongylus species, often called stomach or bankrupt worms, that live in the abomasum and small intestine of sheep.
- Common signs include poor weight gain, weight loss, diarrhea or soft manure, rough fleece, reduced appetite, and general thrift loss. Young lambs and heavily stocked pasture groups are often hit hardest.
- Your vet usually confirms the problem with a quantitative fecal egg count, herd history, body condition findings, and sometimes follow-up testing after treatment to check for dewormer resistance.
- Treatment is not one-size-fits-all. Your vet may recommend selective deworming, supportive care, nutrition changes, and pasture management based on severity, season, and resistance patterns on your farm.
- Prompt veterinary guidance matters if sheep are weak, dehydrated, rapidly losing condition, or if multiple animals are affected at once.
What Is Trichostrongylosis in Sheep?
Trichostrongylosis is a gastrointestinal parasite disease caused by Trichostrongylus worms. In sheep, these small roundworms live in the abomasum and small intestine, where they irritate and damage the lining of the digestive tract. Merck Veterinary Manual lists Trichostrongylus among the more pathogenic common small-ruminant parasites, with medium-high disease potential.
These worms are often grouped with other strongyle-type parasites because their eggs look similar on routine fecal testing. That means a sheep may have trichostrongylosis alone, but mixed infections with other stomach and intestinal worms are also common. The result is often a slow, frustrating pattern of poor growth, weight loss, and reduced flock performance rather than one dramatic sign.
For many flocks, the biggest impact is economic and management-related. Lambs may fail to gain as expected, ewes may lose condition, and pasture contamination can build quickly when weather favors larval survival. The good news is that your vet can help match care to the situation, from targeted testing and treatment to longer-term parasite control planning.
Symptoms of Trichostrongylosis in Sheep
- Poor weight gain or weight loss
- Diarrhea or soft manure, especially in heavier intestinal involvement
- Rough fleece or poor body condition
- Reduced appetite and lower feed efficiency
- Lethargy or decreased grazing activity
- Ill thrift in lambs or recently stressed sheep
- Dehydration from ongoing diarrhea
- Weakness, marked decline, or multiple affected sheep in the group
Trichostrongylosis often causes a slow decline rather than a sudden crisis. Many sheep first show poor growth, a dull fleece, and loose manure. In heavier infections, especially in lambs, you may see obvious weight loss, weakness, and dehydration.
Contact your vet sooner if several sheep are affected, if lambs are falling behind, or if any animal is weak, not eating, or drying out from diarrhea. Severe parasite burdens and mixed worm infections can become much more serious when nutrition, weather stress, or dewormer resistance are also part of the picture.
What Causes Trichostrongylosis in Sheep?
Sheep develop trichostrongylosis after grazing infective larvae on contaminated pasture. Adult worms in the digestive tract lay eggs that pass in manure. Under favorable moisture and temperature conditions, those eggs hatch and develop into larvae on pasture, where they are eaten during grazing. This direct life cycle is typical of trichostrongyle parasites.
Risk rises when sheep are stocked densely, graze short forage close to manure, or stay on the same pasture without enough recovery time. Warm, moist conditions usually help larvae survive longer. Young lambs are especially vulnerable because they have less developed immunity, and stressed adults can also show more disease.
Mixed parasite burdens are common in real-world flocks. A sheep with Trichostrongylus may also carry Haemonchus, Teladorsagia, or other strongyle worms. That matters because signs can overlap, and dewormer resistance has been reported across all major anthelmintic classes used in sheep. For that reason, your vet may recommend a flock-specific plan instead of routine whole-group deworming on a fixed schedule.
How Is Trichostrongylosis in Sheep Diagnosed?
Diagnosis usually starts with a flock history and physical exam. Your vet will look at age group affected, pasture exposure, body condition, manure quality, growth rate, and whether recent deworming worked as expected. Because strongyle-type eggs look similar, diagnosis is often made as part of a strongyle parasite problem rather than by egg appearance alone.
A quantitative fecal egg count is one of the most useful tools. This test measures eggs per gram of manure and helps estimate parasite shedding in the group. It is more informative than a simple positive-or-negative flotation. Your vet may sample several sheep in the same management group to understand how widespread the problem is.
In some cases, your vet may recommend a fecal egg count reduction test after treatment. This compares egg counts before and after deworming and helps detect anthelmintic resistance. If sheep are very sick, additional testing may include packed cell volume, total protein, hydration assessment, or necropsy in losses to sort out mixed infections and other causes of diarrhea or poor thrift.
Treatment Options for Trichostrongylosis in Sheep
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Targeted veterinary exam or herd consultation
- Quantitative fecal egg count on representative animals
- Selective deworming of clinically affected or high-shedding sheep
- Basic supportive care such as improved hydration access and easier feed access
- Short-term pasture move to lower-contamination ground when feasible
Recommended Standard Treatment
- Veterinary exam with flock-level treatment plan
- Quantitative fecal egg count before treatment
- Targeted anthelmintic treatment chosen for likely efficacy on that farm
- Supportive care for diarrhea, dehydration risk, and body condition loss
- Nutrition review, including protein and forage quality support
- Follow-up fecal egg count or recheck to assess response
Advanced / Critical Care
- Full veterinary workup for severely affected sheep
- Lab monitoring such as packed cell volume, total protein, and hydration assessment when indicated
- Aggressive supportive care, including fluids or intensive nursing support
- Necropsy or additional diagnostics for deaths or unclear herd outbreaks
- Post-treatment resistance testing and broader parasite-control redesign for the flock
- Management review for stocking density, pasture rotation, and high-risk groups
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Trichostrongylosis in Sheep
Bring these questions to your vet appointment to get the most out of your visit.
- You can ask your vet which sheep in the group should be tested first and how many fecal samples would give the most useful picture.
- You can ask your vet whether the signs fit Trichostrongylus alone or a mixed strongyle infection.
- You can ask your vet which dewormer classes are most likely to work on your farm right now.
- You can ask your vet whether a fecal egg count reduction test is needed to check for resistance after treatment.
- You can ask your vet which sheep should be treated now and which can be monitored instead.
- You can ask your vet how pasture height, stocking density, and rotation may be affecting reinfection pressure.
- You can ask your vet whether lambs, thin ewes, or recently lambed sheep need a different parasite-control plan.
- You can ask your vet what follow-up timeline makes sense for rechecks, repeat fecal testing, and prevention.
How to Prevent Trichostrongylosis in Sheep
Prevention works best as an integrated parasite management plan. That usually means combining pasture management, targeted testing, selective treatment, and nutrition support instead of relying on routine deworming alone. Merck notes that resistance has been reported in all major drug classes used for sheep gastrointestinal parasites, so preserving drug effectiveness matters.
Practical steps include avoiding overstocking, reducing close grazing on short forage, and giving pastures time to rest so infective larvae decline naturally. Mixed-species grazing can help in some systems because many sheep parasites do not complete their life cycle the same way in other livestock species. Good protein intake also supports the sheep's ability to cope with parasite challenge.
Regular quantitative fecal egg counts can help your vet track pasture contamination and treatment success. Many flocks also benefit from treating only the animals that truly need it, rather than the whole group every time. That approach helps maintain refugia and may slow resistance development.
If your flock has repeated worm problems, ask your vet to help build a season-by-season plan. The best prevention program is the one that fits your pasture setup, climate, lambing schedule, and local resistance patterns.