Nematode stomach (abomasum) and intestinal parasites have become the most important health issue faced by sheep and goat (small ruminant) producers in the U.S. There are three abomasal worms that can infect small ruminants but by far the most important is Haemonchus contortus or the barber pole worm. Haemonchus is a blood sucking parasite that can cause two syndromes, acute or chronic disease. Acute disease is usually seen as severe anemia with no diarrhea and the animal usually being in good body condition. These cases are often most frustrating because the blood loss is so rapid, the animal cannot recover and death can occur quickly. Chronic disease often presents with weight loss (from malabsorption of nutrients), anemia, edema (bottle jaw), lethargy, and sometimes diarrhea (not as common as one would think). Control programs as described below need to be implemented to properly control worm loads and prevent decreased weight gains and animal deaths.

Haemonchus has what is called a direct life cycle in that it only infects one host (a sheep or goat). Adult worms live in the abomasum and lay eggs which are then passed into the environment through the manure. After the eggs fall onto the pasture, they must develop into larvae before becoming infective. The speed of larval development is dependent on heat and humidity. During warm, moist times of the year eggs can develop into infective larvae in about five days, but the process can take up to a couple of months at other times of year.

Once larvae are infective, they must get high enough on the grass to be eaten by a small ruminant. Splashing water from rain usually pushes the larvae up the blades of grass where they are then consumed. Animals are more likely to ingest larvae when pastures are overgrazed and the sheep or goats graze very close to the ground. After infective larvae are ingested, they develop into adults and begin laying eggs in about two weeks.

A unique feature of abomasal worms, including Haemonchus, is that during harsh environmental conditions (cold and/or dry), they can undergo hypobiosis or arrested development. Arrested larvae do not develop into adults but remain in the abomasum until more favorable environmental conditions stimulate their development. This is often why high loads of worms are noted in the spring because many arrested larvae begin to develop into adults and feed on the blood of their host.

Although I will discuss dewormers (anthelmintics) because of the growing concern over parasite resistance, it is extremely important to first describe management techniques that can be performed to decrease worm loads and protect vulnerable animals. It should be emphasized here that dewormers should be used to PREVENT disease and NOT TO TREAT it. When used appropriately, the function of a dewormer as a preventative can be realized. It is also important to mention that the goal of any worm control program should never be to eradicate worms (in fact this is impossible). The goal is to keep worm loads at a level where no deleterioius effects are noted.

1)       Low stocking density – when animals are not crowded and have access to sufficient feed they are less likely to graze very close to the ground and subsequently ingest less larvae. As a general rule, grass should not be any shorter than 3-5 inches. At this length, new growth stimulation of pasture is optimized and it is harder for larvae to reach the tops of the grass blades. The very furthest a larva can migrate is about 6 inches.

2)       Harvesting hay off pasture regrowth – after a pasture has been heavily grazed and begins to regrow, harvest it for hay and most of the larvae will die in the hay making process. The pasture can be considered “clean” after it has been harvested for hay.

3)       Reserve the “cleanest” pastures for the most vulnerable animals – lactating animals and lambs and kids are the most susceptible to worm infections. Allow these animals to graze “clean” pastures that have either been rested for a number of months or are regrown after harvesting hay. Another way to make a pasture “clean” more quickly is to alternate grazing of a pasture between species. If horses or cattle are allowed to graze a pasture after sheep or goats, the horses or cattle will kill Haemonchus when ingested. Domestic livestock do not share worms in general with the exception of Trichostrongylus axei.

4)    Culling – susceptibility to parasites has been shown to be influenced by genetics. For instance, St. Croix and Katahdin sheep breeds are highly resistant to parasitism. This genetic resistance is less dramatic and realized in goats. It has been shown that 80% of egg shedding comes from 33% of the herd. Therefore, if some of the 33% of the high egg shedders are eliminated from the herd, the number of eggs on pastures can be significantly reduced.

5)       FAMACHA system – developed in South Africa, this system uses color of the eye conjunctiva (mucous membrane under the lower eyelid) to assess levels of anemia in sheep and goats. During the the times of year when worm burdens are the worst (warm and/or moist times), sheep and goats’ conjunctiva color should be assessed every two weeks and only those that need to be dewormed are given dewormer. To use the FAMACHA system appropriately, a person needs to be trained by a veterinarian or cooperative extension agent and use a laminated card that has the various levels of anemia one is looking for when checking the eyes. The FAMACHA cards cost approximately $11 and must be replaced every year due to fading. The FAMACHA system greatly reduces the use of dewormers thus saving producers money. It also greatly reduces the incidence of parasite resistance to dewormers because only those animals that need deworming are given the product. Further, when the animals are observed every two weeks, it becomes easier to identify those animals that are more susceptible to parasites and consider culling them.

Although dewormers are a control measure, we will consider it on its own since it is a hot topic. There are essentially three drug classes that are used to kill Haemonchus:

1)      Nicotinic drugs – includes Prohibit (levamisole), Rumatel (morantel), and Strongid (pyrantel)

2)       Benzimidazoles – includes Safeguard (fenbendazole) and Valbazen (albendazole)

3)       Macrolides – includes Dectomax (doramectin), Ivomec (ivermectin), and Cydectin (moxidectin)

One important thing to know is that if you have resistance to one drug on your farm, the worms will be resistant (or will develop resistance quickly) to any other drug in the same class. Therefore, the recommendation is to try and assess whether you have a drug resistance problem on the farm. The only way to do this is a fecal egg count reduction test (FECRT). If you submit fecal samples from 10-15 animals in the herd to us, we will perform a Modified McMaster’s test on the samples. The McMaster’s test is a quantitative test and it will calculate the number of eggs/gram of feces. The dewormer must then be given to the same animals at the appropriate dose and the McMaster’s test repeated in two weeks. If the egg reduction is less than 80-90%, then a parasite resistance problem is likely. A veterinarian can help you analyze the results of the FECRT.

In the face of established resistance, it is often necessary to treat with drugs from two or all three drug classes at the same time to effectively reduce worm burdens. Consult a veterinarian before proceeding with any change in your normal protocol or when you have clinical cases of Haemonchosis.