Sheep GEMS News Brief 11 – Late February 2025
Gastrointestinal parasites are one of the main health concerns for grazing sheep because available dewormers are largely ineffective due to dewormer resistance. Parasitic worms are a year-round problem in tropical and sub-tropical environments affecting millions of small ruminants world-wide. However, resistance to multiple deworming drugs is common in small ruminants. Infection with gastrointestinal nematode parasites can cause reduced weight gains, fertility, fiber and milk production, as well as cause anemia, diarrhea, and death.
The most pathogenic worm species is barber pole worm or Haemonchus contortus is a blood sucker and can consume as much as 0.2 milliliters of blood per day, which adds up under a heavy infection (hundreds of worms). It causes anemia (white lower eyelids) and bottle jaw (edema under the jaw). Barber pole worm is common in warmer, more humid environments and females can lay 10,000 eggs per day. Another prevalent worm is Trichostrongylus spp. which prefers cooler, wetter environments. These worms can cause diarrhea, weight loss, and sometimes death. Other less pathogenic common worms often fewer in numbers are Cooperia (Cooper’s worm), Nematodirus spp. (thin-necked intestinal worm), and Oesophagostomum spp. (nodule worm). All of these have a similar life cycle which can be completed within 4-5 weeks. Animals pick up infective stage larvae from grazing pastures. Other worms are tape worms, which usually do not cause any production losses. However, deworming generally leads to further dewormer resistance of pathogenic worms. Strongyloides spp. (threadworm; invades through the skin) can be pathogenic initially, but an immune response will minimize further infections (these smaller, larvated eggs should not be counted in a fecal exam). Fecal egg counts (FEC) are useful to estimate worm infection and should only include trichostrongyle parasites (excluding tape worm and Strongyloides) which cannot be differentiated by shape or size.
Young lambs, stressed sheep, and ewes in late pregnancy and early lactation are the most susceptible to internal parasites. Worms can survive drought if they find pockets of moisture in the soil, and during winter conditions. Also, worms can survive in the animal during hypobiosis or arrested development of larvae and development resumes around the time of lambing. “Parasite free” pastures likely do not exist. Thus, it is important to manage sheep accordingly to minimize parasite exposure and maximize natural tolerance. Methods to do this include rotational grazing, withholding grazing especially during the dewy mornings, providing diverse pastures with legumes and forbs, using proper grazing pressure or not overstocking or overgrazing, and use of nematode-trapping fungus (BioWorma®), which reduces larvae survival on pasture. Increased nutrition to susceptible animals increases their tolerance. Parasitized animals require additional protein to repair damage to the gastrointestinal tract. Once an animal becomes heavily infected, treatment with an effective dewormer is imperative. It is no longer recommended to treat the whole flock, but to treat individuals in need identified by FAMACHA scoring or Five Point Check which considers loss in body condition or weight, poor hair coat, and dagginess (accumulation of manure) around the hind quarters. The most promising means to control parasites is through genetic selection for resistance (measured by FEC) or resilience (ability to maintain production or avoid anemia in the presence of an infection). This will be discussed in the next news brief.
For further information contact Joan Burke (joan.burke@usda.gov). For more information on parasites, see www.wormx.info.
Acknowledgements. We thank the many U.S. sheep associations, including the Eastern Alliance for Production Katahdins, the Katahdin Hair Sheep International, the National Sheep Improvement Program, and Katahdin sheep producers, for their contributions to this research. This work is supported by the Organic Agriculture Research and Extension Initiative (grant no. 2016-51300-25723/project accession no. 1010329), and by the Agriculture and Food Research Initiative Competitive Grant (grant no. 2022-67015-36073/project accession no. 1027785), from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.
