Haemonchosis in Sheep

Haemonchosis is a parasitic disease caused by Haemonchus contortus, often called the barber pole worm. This worm lives in the abomasum, which is the sheep's true stomach, and feeds on blood. Heavy infections can cause rapid blood loss, severe anemia, weakness, poor growth, and death.

This condition is especially important in lambs, periparturient ewes, and sheep grazing warm, moist pastures where infective larvae survive well. One of the hardest parts for pet parents and flock managers is that sheep may look only mildly off until anemia becomes advanced.

Unlike some other intestinal parasites, haemonchosis does not always cause obvious diarrhea. Instead, classic signs are pale lower eyelids, lethargy, bottle jaw, and sudden decline. Because the disease can progress fast, early recognition and a flock-specific plan with your vet matter.

Haemonchosis is caused by ingestion of infective Haemonchus contortus larvae while grazing contaminated pasture. Adult worms in the abomasum lay eggs that pass in manure. In warm, moist conditions, those eggs develop into infective larvae on pasture and are eaten during grazing.

This parasite thrives in many parts of the United States during the grazing season, especially when stocking density is high and pasture rotation is limited. Larvae tend to stay low on forage, so close grazing increases exposure.

Risk is often higher in lambs, stressed sheep, and ewes around lambing because immunity may be weaker. Another major factor is anthelmintic resistance. On many farms, barber pole worms are no longer reliably controlled by every dewormer, which is why your vet may recommend targeted treatment instead of routine whole-flock deworming.

Your vet usually diagnoses haemonchosis by combining the sheep's signs with flock history and testing. Common pieces include body condition, eyelid color using the FAMACHA system, evidence of bottle jaw, and whether the problem is happening during a high-risk pasture season.

Testing often includes a fecal egg count to look for strongyle-type eggs and bloodwork to measure anemia, such as packed cell volume or hematocrit and total protein. In some cases, your vet may also use a fecal egg count reduction test to see whether a dewormer is still working on your farm.

Diagnosis can be tricky because strongyle eggs from different worms look similar under the microscope, and very sick sheep may not always have the highest egg counts. That is why your vet may interpret test results together with clinical signs, treatment response, and flock-level patterns rather than relying on one test alone.

Prevention works best as a flock plan, not a one-time deworming event. Your vet may recommend targeted selective treatment, often using FAMACHA scoring, so only sheep that need treatment are dewormed. This helps preserve a population of susceptible worms in refugia and can slow drug resistance.

Pasture management matters too. Avoid overstocking, reduce close grazing, and rotate sheep away from heavily contaminated areas when possible. Because infective larvae stay low on forage, keeping residual pasture height up can reduce exposure. Good nutrition also supports resilience, especially in lambs and ewes around lambing.

Routine monitoring is one of the most practical prevention tools. Fecal egg counts, follow-up checks after treatment, and watching for pale eyelids or bottle jaw can catch trouble earlier. If your farm has repeated losses, ask your vet about a broader parasite-control strategy that includes grazing management, selective breeding for resilience, and checking whether current dewormers are still effective.