Strongyle nematode control in small ruminants

By Dr. Laura Williams, Parasitology Section Head
Washington Animal Disease Diagnostics Laboratory (WADDL)

Anthelmintic resistance is a growing concern in small ruminants. Many factors affect the development of anthelmintic resistance, but veterinarians and small ruminant producers can lessen the impact of anthelmintic resistance by implementing strategic deworming schedules formulated using objective measurements of worm burdens. The general aim is to treat animals as few times as possible to avoid excessive selection pressure for creating resistant populations of nematodes. At the same time, there should be enough deworming treatments to avoid parasitic disease in herds and flocks. Detailed information about individual farms including parasite history and management structure is necessary to develop useful anthelmintic recommendations, but key elements can form a basic scaffold from which to build a specific deworming program.

Overall, appropriate pasture management is the most important factor in reducing strongyle nematode burdens. Ruminants shed strongyle eggs in the feces, and the hatched larvae contaminate the pasture, infecting ruminants that graze these pastures. Treatment of grazing animals in spring, early in the grazing season, helps to prevent initial pasture contamination and reduce pasture parasite levels over the course of the summer. Strongyle nematodes prefer the warm, wet spring and summer weather. The larvae thrive on pasture throughout these seasons and the adult worms readily produce eggs in the gastrointestinal tract all summer long. Whether small ruminants require treatment during the summer is dependent on many factors, including weather and farm conditions.

The best and most objective way to determine whether mid-summer anthelmintic treatment is necessary is to perform a quantitative fecal float. Ruminants shedding high levels of strongyles (typically greater than 200-300 eggs per gram) should be treated to prevent further pasture contamination. Small ruminants harboring Haemonchus contortus, also known as barber pole worms, are the most at-risk for severe disease and death. The eggs produced by H. contortus cannot be differentiated from other strongyle nematodes on a fecal float but can be identified specifically using the Haemonchus contortus fluorescent assay available at WADDL (Figure 1).

Haemonchus contortus eggs identified using the fluorescent assay.
Figure 1. Haemonchus contortus eggs identified using the fluorescent assay.

Additionally, anthelmintic efficacy can be evaluated on individual farms by performing a Fecal Egg Count Reduction Test (FECRT) during the summer season. This test involves submission of a fecal sample before deworming and again 10-14 days after deworming to ensure that egg numbers are decreasing following anthelmintic treatment. All of these testing modalities to quantify strongyle egg counts, detect H. contortus, and evaluate FECRT to assess anthelmintic efficacy are available at WADDL, and can be used in the development of strategic anthelmintic schedules on an individual farm.

More information is available through the WADDL website. Veterinarians and producers alike can work together to lessen the impact of anthelmintic resistance and ensure the future of small ruminant production in our region!