Haemonchus contortus is the most economically significant parasite of sheep and goats throughout much of the United States and the world, due to the severity of the parasitism and the emerging anthelmintic resistance. Haemonchosis also affects New World camelids. It is a common cause of death in all these species, and often takes the practitioner and owner by surprise, as clinical signs can be subtle. Rapid diagnosis, strategic prevention, and pasture management, along with a thorough understanding of the pathophysiology of haemonchosis facilitates the development of herd health protocols to control this deadly disease.

The life cycle of Haemonchus is simple. Infective L3 larvae, ingested by the host on pasture, develop into adults in the abomasum and produce eggs that are passed in the feces. The eggs hatch in the feces and undergo two molts, becoming L3 larvae that can migrate up blades of grass in drops of moisture. In proper conditions, it takes only 3-4 days for an egg deposited in feces to develop to the L3 stage. Depending on temperature and moisture, L3 larvae can survive on pasture up to 6 months. Adult worms only survive several months in the host.

Haemonchus contortus favors warm moist climates; however, it has adapted successfully to most of the United States. Optimum conditions for H. contortus exist from May through September in Urbana, Illinois; the season is even longer in the Southeast. In the winter, larvae become metabolically inactive, undergoing hypobiosis. Most animals succumb to haemonchosis in the spring, due to the combined assault of larvae emerging from hypobiosis and the immunosuppression of late pregnancy. This results in the periparturient rise in egg shedding, resulting in numerous infective larvae on pasture at the time when young animals are most susceptible. Although all ages of sheep and goats are susceptible to haemonchosis, recently weaned animals are usually the most vulnerable. Besides age, other factors that increase susceptibility include overgrazing, dense stocking rates, and inadequate nutrition, particularly protein intake.

The most common clinical signs are failure to thrive and weight loss. As worm burdens increase, more severe signs, such as anemia, hypoproteinemia, submandibular edema (bottle jaw), weakness, and collapse, may develop. Unlike other gastrointestinal nematodes, H. contortus does not usually cause diarrhea. Due to the nonspecific signs and lack of diarrhea, haemonchosis is often undiagnosed until death. The death can appear sudden, even though the course of infection may have been prolonged.

Clinical pathology findings include anemia, hypoproteinemia, and eosinophilia. In one experimental study, mean packed cell volume dropped to 15% in infected sheep 35 days after infection, and mean eosinophil count increased from <5% to 25% of circulating white blood cells 56 days after experimental infection. Eosinophilia decreased from days 63-77 of the experiment, which may have resulted from localization of eosinophils to the abomasum. A significant correlation between eosinophilia and worm burden has been demonstrated.

Clinical diagnosis of haemonchosis is usually based on clinical signs and fecal flotation results. Clinical observation is perhaps the most useful tool in diagnosing and managing haemonchosis, but requires close examination. Because anemia is the principle clinical problem, mucous membrane pallor is one of the easiest ways to monitor sheep, and has proven effective as a diagnostic approach. The FAMACHA system uses a 5-point scale to gauge ocular mucous membrane color, which correlates with packed cell volume in sheep. Owners and veterinarians must be trained in the system and use the official cards, but the FAMACHA system is practical on-farm, and can be used to select animals for strategic de-worming. The FAMACHA system also has been successfully used in goats and camelids. The importance of regular application of FAMACHA scoring cannot be over-emphasized. Many animals submitted to the ADDL with fatal haemonchosis had recent satisfactory FAMACHA scores.

Although fecal flotation is a valuable diagnostic tool to assess gastrointestinal parasitism, H. contortus eggs cannot be distinguished easily from those of other strongylids. Most pastured sheep have some level of gastrointestinal nematode infection, so the presence of strongylid eggs in feces is expected. Counting eggs per gram of feces is useful in developing treatment and control strategies. The quantitative McMaster’s technique, using a McMaster slide with a chamber beneath a printed grid, allows enumeration of eggs per gram of feces. The McMaster’s technique is easily learned by veterinarians and producers, and can also be used to assess anthelmintic effectiveness by measuring the reduction in fecal egg count 7-14 days after deworming.