Emerging concepts in small ruminant parasite control

Longtime sheep and goat producers can readily list all the standard parasite control measures they have been taught over the years:

  • Deworm when fecal examinations are positive.
  • Deworm all animals at the same time.
  • Place animals onto a clean pasture after deworming.
  • Deworm regularly.
  • Rotate dewormers.

Unfortunately, these practices and other factors have contributed to the development of parasite resistance to dewormers. Indeed, in some areas of the South, producers have no effective dewormers and cannot raise sheep and goats without extreme losses to parasites; the common name for a major intestinal parasite — “the bankrupt worm” — has become all too true for them.

Small ruminant public enemy No. 1

Haemonchus contortus, commonly known as the barber pole worm, is responsible for most losses from small ruminant nematode (roundworm) parasitism. (See Table 1 for the most common nematodes). Figure 1 depicts the life cycle of H. contortus. Note that eggs are shed in fecal pellets and several larval molts are required before the parasite reaches the infective L-3 stage.

An organization called the American Consortium for Small Ruminant Parasite Control (ACSRPC) is devoted to addressing the problem of parasite resistance to dewormers. Its research with copper particles and certain tannin-containing plants show promise as potential nonchemical parasite control measures. Nevertheless, producers still rely heavily on anthelmintics (dewormers). Dr. Des Hennessy of Australia coined the term “smart drenching” (deworming) to describe selective treatment of certain animals and the ACSRPC advocates this approach.

The goal of smart drenching is to maintain animal health and production while decreasing the rate of development of parasite resistance to anthelmintics. Smart drenching encourages producers to use dewormers selectively, judiciously and effectively.

Table 1. Common nematodes (roundworms) of sheep and goats.
Scientific name Common name Location
Haemonchus contortus Barber pole worm Abomasum
Marshallagia marshalli - Abomasum
Ostertagia spp (species) Brown stomach worm Abomasum
Trichostrongylus axei and spp Small stomach worm/stomach hairworm Abomasum
Bunostomum trigonocephalum Hookworm Small intestine
Capillaria spp. - Small intestine
Cooperia spp. Intestinal worm Small intestine
Nematodirus spp. Thread-necked worm Small intestine
Strongyloides papillosus Intestinal threadworm Small intestine
Trichostrongylus colubriformis Bankrupt worm/black scour worm Small intestine
Ascaris suum Hog roundworm Bile ducts
Chabertia ovina - Large intestine
Skrjabinema ovis Pinworm Large intestine
Oesophagostomum venulosum - Large intestine
Trichuris spp. Whipworm Large intestine

The smart drenching system

  1. Identify which wormers are effective on your farm. This is done by pre- and post-treatment fecal egg counts (see Reference No. 6 below for the procedure) or submitting fecal samples to Dr. Ray Kaplan’s laboratory at the University of Georgia’s College of Veterinary Medicine for “DrenchRite” laboratory diagnosis of resistance. Resistance is defined as less than 95% reduction in fecal egg counts post-treatment.
  2. Weigh each animal to be treated and administer the correct amount of dewormer to each animal. Be sure to administer the entire dose over the animal’s tongue and to the back of its throat. Consult with your veterinarian when treating goats because extra-label dosages are usually recommended.
  3. Use dewormers from two different classes if resistance is suspected on the farm. See Table 2 for a list of dewormer classes.
  4. Hold animals to be wormed off feed for 12–24 hours before treating with benzimidazoles (Fenbendazole and Albendazole) or ivermectin, doramectin, and moxidectin. This slows the digestive processes, allowing the dewormer to remain in the animal’s body longer for increased effectiveness. Do not hold pregnant ewes or does off feed in late gestation. Benzimidazole effectiveness will be greatly enhanced if the animal is redosed in 12 hours.
  5. Only deworm animals that need treatment. Use the FAMACHA system (described below) to assess animals with clinical anemia due to Haemonchus contortus. For other parasites, base treatment on body condition, age (parasitism is a large concern in younger animals), fecal egg count, performance/production, pregnancy/lactation status (these dams are under higher stress and have reduced immunity), signs of illness and short-term weight gain.

The benefits of smart drenching are threefold: fewer animals are dewormed, so costs are reduced; there is less pressure on parasites to develop resistance; and more parasites in the “refugia” remain susceptible to dewormers. The refugia is the portion of the parasite population not subjected to dewormers and therefore not under pressure to develop resistance; it includes parasites in untreated animals as well as eggs and larvae on pasture. According to Dr. Kaplan, the refugia provides a pool of sensitive genes that dilutes resistant genes selected for by deworming (see Figure 2).

Table 2. Anthelmintic (Dewormer) Classes for Nematodes (Roundworms)
Class Examples
Benzimidazoles fenbendazole, oxibendazole, albendazole, mebendazole
Avermectin / Milbemycins ivermectin, eprinomectin, doramectin, moxidectin, others
Imidazothiazoles / Tetrahydropyrimidines levamisole, pyrantel, morantel, others

The FAMACHA System

The FAMACHA System originated in South Africa. It is a method of identifying individual sheep and goats that are heavily parasitized, based on physical evidence of anemia caused by Haemonchus contortus. A colored chart (see Figure 3) is placed next to an animal’s conjunctiva (pinkish tissue inside the lower eyelid) to assess each animal’s level of anemia.

A scale of one to five is used; a score of one is the reddest and most healthy and a score of five is palest and most anemic. Animals with scores of four and five should be treated or culled; those with scores of one or two do not need treatment; various factors will help a producer decide whether or not to treat those with a score of three.

This system helps producers identify the parasite equivalents of “Typhoid Mary” in their herds: 20% to 30% of animals harbor 70% to 80% of the herd’s worms and are responsible for the majority of environmental contamination with worm eggs. The FAMACHA System helps identify heavily parasitized individuals so producers can make appropriate management decisions (treat or cull).

Parasite control through management

Nonchemical parasite control measures will become even more important as resistance to dewormers grows. Here are some key practices that can help producers reduce the need for chemical deworming:

  1. Never graze pastures below 3 inches. Infective parasitic larvae live in water droplets on pasture plants and are much more common in the lower 3 inches of forage.
  2. For similar reasons, try not to let animals graze on wet pastures.
  3. Rotate pastures and allow as much rest time as possible between regrazing — at least three weeks; six months is much more effective in ensuring larval death.
  4. If possible, practice multispecies grazing. Only a few parasites are transmissible between species, so following sheep or goats with horses, for example, will help reduce the number of small ruminant parasites on pastures.
  5. Do not overstock animals — never graze more than six to eight small ruminants per acre of irrigated pasture.
  6. Select for animals that are healthy and resilient despite parasite infections; cull individuals that require repeated deworming to survive.
  7. Do not feed animals directly on the ground.
  8. Protect feed troughs, water sources and trace-mineral salt feeders from manure contamination.
  9. Provide as much browse as possible for goats.

If you must deworm

Unless you decide to use intensive genetic selection for parasite resistance and cull affected animals heavily, you will need to deworm some animals when warranted. First of all, know what dewormers are effective in your herd or flock (see step 1 of smart drenching above).

Another recommendation is to confine and deworm herd/flock additions with a dewormer from each of the three major classes simultaneously; drylot these new animals and do not allow access to pastures and the rest of the herd/flock until fecal egg counts are negative. Use dewormers sparingly and intelligently to prolong their effectiveness on your farm. Practice the non-chemical means of parasite control mentioned above to increase the sustainability of your small ruminant flock or herd.

As ACSRPC member Dr. Steve Hart stated recently at the Northwest Oregon Dairy Goat Association’s annual conference regarding parasite resistance in the Northwest: “Y’all are in good shape right now. Don’t ruin it.”

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