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June 2013
Haemonchus contortus thrives in upper midwestern United States



by Richard Ehrhardt, Ph.D.
Small Ruminant Extension Specialist
Michigan State University

In the United States, there is a perception that the gastrointestinal nematode (GIN), Haemonchus contortus (common name= Barber Pole worm), is a dominant parasite of sheep and goats in certain geographical areas (southeastern USA) and not as significant in other regions including the Upper Midwest region (loosely defined as Ohio, Indiana, Michigan, Wisconsin, Minnesota, Iowa, Illinois). 

While the origin of this perception is unclear, we offer evidence that it is inaccurate, as we consistently find that Haemonchus dominates the parasite population in large flocks surveyed throughout the year in Michigan. Understanding the GIN population structure is important in developing effective integrated control programs, as it dictates the choice of GIN infection control strategy.

It is clear that Haemonchus thrives in warm, humid environments which present optimal egg hatching and larval development conditions. In Michigan, there is great diversity in climate given the range of latitude (42nd-46th parallels) and proximity to the one of four Great Lakes; however, despite this diversity in microclimate, there are significant stretches of warm, humid weather in all that are conducive to Haemonchus development.

The extent and duration of cold during a Michigan winter (in all microclimates) kills the vast majority of the free-living, terrestrial population of Haemonchus as confirmed by studies performed on the Michigan Agriculture Experiment Station in the 1940’s. However, while the free living population is decimated, the population living within the animal is doing just fine as it enters a state of arrested development or hypobiosis, allowing it to survive until warm weather arrives the following spring (see figure 1).


This arrest wanes markedly at birth which in many flocks/herds is coincident with warm spring weather, thus quickly repopulating pastures and propagating a year round life cycle even in cold, frozen climates.

Based on this well documented life cycle, it was not a surprise to find Haemonchus as a major player in the GIN population in the Upper Midwest; however, what did surprise us was how it dominated the GIN population in the flocks we surveyed at all times of the year (figure 2).

Figure 1.  Haemonchus enters arrested development within small ruminants during winter in cold climates allowing it to survive over the winter and re-infect pastures the next spring.

Caution should be used in extrapolating this finding to the entire region as it is likely that greater GIN population diversity could be found if a larger sample representing a broader range of flock size was surveyed. However, it is striking how consistently Haemonchus dominated the GIN population in these large flocks, which represent a large number of ewes (approximately 3,600 ewes). These flocks varied in location from the far North to the South and represented both pasture based systems as well as those with significant confinement rearing periods. 

These findings corroborate field reports in the Upper Midwest indicating that Haemonchus is dominant GIN species. Furthermore, these findings suggest that targeted selective treatment programs targeting Haemonchus (i.e. FAMACHA©), are appropriate strategies for use in many flocks/herds in this region.


Figure 2.  GIN larval population composition in 4 large (>500 ewe) flocks in Michigan during different seasons. Larval population composition was determined by counting larvae cultured in the laboratory obtained from replicates (n≥3) of composite samples of feces. These flocks represent a diversity of climate and sheep farm management system within Michigan.

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