Worm Infections in Goats and Sheep – Sept ’16

September 2016 Newsletter PDF

Worms Got Your Goat?

by Carrie Bargren, DVM

In the world of goats (and sheep), poor health, diarrhea, weight loss, anemia and poor production and reproduction are most commonly caused by the same thing: internal parasites! The gastrointestinal nematodes are a class of parasites that are the most challenging, frustrating and economically significant. There are five major nematodes found in pastured goats and sheep, of which Haemonchus contortus or the Barber-Pole worm, is the most important clinically and economically in the United States, and the most common.

A nematode’s life cycle involves both the environment and the gastrointestinal tract of the animal. The female worm deposits eggs into the manure before it is passed. Once on the ground, the eggs hatch within the manure in warm, high moisture conditions and move through several stages of development. Early stages are highly susceptible to dry conditions and extreme temperatures, which help reduce pasture contamination, but the infective larval stage is resistant to these conditions and will survive for months in manure on the ground. Following a heavy rain, heavy dew or even on a humid day, the larvae move out of the manure, climb nearby grasses and are eaten by unsuspecting goats. Once in the rumen, the larvae dig into the stomach lining, undergo additional development before moving out of the lining and become reproductively mature adult worms. Thus the cycle continues.

Fortunately, Wisconsin winters are very good at cleaning pastures of larvae each year. But Haemonchus larvae already inside a goat are able to go into a static state, similar to hibernation, where they stop developing once environmental conditions are poor. This allows them to survive until spring when they continue their life cycle, mature and reproduce.

Most infected goats do not have any outward symptoms until they are heavily infected. These include anemia, diarrhea, weakness, poor growth, weight loss, swelling under the chin and chest, and death. The anemia is the result of the Haemonchus contortus larvae feeding on red blood cells while burrowed in the stomach lining.

Infection is diagnosed by a fecal test at the veterinary clinic, which identifies nematode eggs and gives a rough estimate of the severity of infestation. A Fecal Egg Count at a diagnostic lab will provide a count of the eggs per gram of feces.  This result is used in treatment decisions and to monitor infection control in the herd. For those infected with Haemonchus contortus, the FAMACHA scoring system compares the color of the inside of the lower eyelid to a scale which indicates the amount of blood loss in each animal: a red inner eyelid means the goat is healthy with a low level or no infection while white means ‘very anemic’ with a high infestation and the goat should be dewormed.

If a goat is heavily infested with worms, can’t she just be treated with dewormer and recover? It’s a lovely thought, and in the mid-1900’s when anthelminthics (dewormers) were introduced, it was that simple and they were heavily and frequently used. However, some of the cunning little nematodes formed a resistance to the most commonly used dewormers. When these products were used, all but the resistant worms would die. These super-worms survived and produced more resistant worms. For some classes of dewormers, resistance to one kind means that the nematode is resistant to the other dewormers in that class. Resistance is now widely documented to Fenbendazole (Safeguard, Panacur), Albendazole (Valbazen) and Ivermectin (Ivermectin Sheep Drench), which essentially make them useless. The two products that are currently still effective are Moxidectin (Cydectin Drench for Sheep) and Levamisol (Levasol), but resistance to these could develop at any time. Therefore, they should be used sparingly, only twice per year: once after the first hard frost and again before the ground begins to thaw.

At these two times per year, ideally, the only animals that should be dewormed are those with a poor body condition or FAMACHA score. Healthy animals might have a small amount of worms that won’t cause them trouble and should not be dewormed. By not deworming them, and therefore not exposing the worms to a dewormer, the worms in the animal will not develop resistance to the drug.  Other acceptable times to deworm are soon after a goat has kidded; young kids (since their immune systems are not developed) and any new additions to the herd. New animals should be kept separate for 72 hours after deworming. The only other time to use a dewormer is to save the life of a severely infected animal. Alternate dewormer products every year to further reduce the development of resistance. Once resistance is acquired, it is a change in the nematode’s DNA and cannot be reversed. A Fecal Egg Count performed once prior to deworming and again 10-14 days after will indicate whether the dewormer is still effective.

Across the herd, not every goat will have the same level of infection: 30-35% of the animals carry the majority of the nematodes. Some goats are inherently less susceptible to infection and this trait can be selected for by keeping and breeding those that don’t need frequent deworming while removing those that require constant deworming due to heavy infestations. The practice takes time, but can be highly beneficial for the herd long term.

As dewormer products become less and less effective, the main focus on controlling nematode infestations should be on pasture management. Goats prefer to browse rather than graze, which is ideal for controlling reinfection. Browsing involves eating twigs and leaves from shrubs which are elevated off the ground whereas grazing is eating grass and other plants at ground level. The benefit of browsing in reducing reinfection is that the goats are not eating where they defecate and are less likely to eat the larvae that work their way out of the manure and climb nearby grasses. When goats eat forages that are over six inches tall, nematode infestation is reduced. An added benefit to browsing is that the tannins in shrubs and
forages have been shown to reduce the amount of worm eggs excreted in manure. Take a similar approach when feeding hay: use a hay feeder to keep hay off the ground and away from manure. Just make sure it prevents the goats from climbing in, because they will try and defecate in the feeder, which defeats the purpose of the feeder. An ideal goat pasture is on a hill or slight incline to keep the ground dry and make larval development difficult, and full of brush and shrubs for the goats to browse.

Grazing cattle or horses with the goats reduces the amount of eggs found on pasture. Horses and cattle cannot be infected with goat nematodes and vice versa, so if a cow eats a goat nematode egg, she is not infected and there is one less infective egg that a goat might eat. Short cycle crops can be grown on the pasture to break the nematode life cycle. Rotational grazing is possible, but requires a large area of land since each area grazed will need three months of down-time to reduce larval counts significantly, which is impractical for most small herds.

Haemonchus contortus and other nematodes create a perplexing, frustrating and often devastating problem within a goat herd, compounded by the worms’ increasing resistance to dewormer products. Pasture management should be a priority, and selective deworming should be used to supplement good practices, but the balance of both will vary from herd to herd. Working closely with a veterinarian well-versed in small ruminants will help you find that balance and maintain a healthy herd, no matter how big or small.