​Maryland Small Ruminant Page

The DrenchRite® test (larval development assay) can also be used to detect drug resistance [3]. The DrenchRite® test utilizes eggs isolated from the feces of infected animals and tests for resistance to all drugs simultaneously. A pooled fecal sample (>350-500 epg) from at least 10 animals is required. The DrenchRite® test can also be used to identify worm species [3].

 

Drenching

Another way to slow dewormer resistance is to avoid underdosing and exposing worms to sub-therapeutic levels of the dewormer. To avoid underdosing, animals should be weighed and given doses that are appropriate for their weight and species. Goats usually require higher doses of anthelmintics than sheep and other livestock because they metabolize the drugs more quickly. In fact, it is common to give goats 1.5 to 2 times the sheep (or cattle) dose of an anthelmintic, with the 1.5 dose being for levamisole, the anthelmintic with the narrowest margin of safety.

 

Sheep and goats should always be dewormed with drench (liquid) formulations. Drenches are FDA-approved, safer, and generally more effective [4] than other formulations. Oral medications also have shorter withdrawal periods. For example, the withdrawal period for a subcutaneous injection of moxidectin (Cydectin®) is 132 days in goats, whereas the withdrawal period for the oral drench is only 17 days (goats) [7].
 

Sheep and goats should be dewormed using an oral dosing syringe with a long, metal nozzle. The syringe can be a single dose syringe or an automatic syringe that refills after each treatment. If an automatic (backpack) drench gun is used, the equipment should be calibrated for the heaviest animals in the group not the average. Drenching equipment should be regularly checked to make sure it is delivering the desired level of medicine.
 

The drug should be deposited over the tongue into the esophagus. If the medicine is deposited into the animal's mouth, this may stimulate the closure of the esophageal groove, causing the medicine to bypass the rumen and be less effective. Fasting animals prior to drenching can also increase the effectiveness of certain dewormers (e.g. benzimidazoles, ivermectin) [6]. Another strategy is to give two consecutive doses 12 hours apart [6]. The longer the drug is in the rumen, the more effective it will be.

It goes without saying that all drugs should be used according to the manufacturer's instructions. Anthelmintics that are not properly stored may be less effective. Anthelmintics should not be left in the barn, but should be stored at a constant temperature, ideally in a refrigerator. While no one likes to discard expensive medicines, outdated drugs may be less effective and thus contribute to the development of resistant worms. Drugs should never be mixed.
 

Quarantine drenching
Because newly purchased animals can be as source of anthelmintic resistance, all new arrivals should be kept in a quarantine pen and be dewormed with drugs from two (levamisole + moxidectin) or three (albendazole + levamisole + moxidectin) anthelmintic classes. The idea is to kill all the worms so that no resistant worms survive, as worms pass resistant genes onto their offspring.

 

Leaving some animals untreated and/or keeping animals on the same pasture will help to dilute the worm populations and maintain “refugia” (untreated worms). Maintaining refugia is the key to slowing drug resistance. If you treat at pre-breeding, pre-lambing, or post-weaning, be sure to leave some animals untreated. Another good option is to keep livestock in the barn or drylot for 24 to 48 hours after treatment with an anthelmintic. This will ensure than any eggs that are present in the gut have passed out into the feces.
 

Of course, any practice that reduces the need for deworming will slow drug resistance. Rotational grazing can reduce or increase the need for deworming, depending upon how it is practiced. The keys are not allowing pastures to become too contaminated with infective worm larvae and allowing sufficient rest periods to enable worm eggs and larvae to die off. Removing a hay or silage crop will result in a clean pasture for grazing. Co-grazing small ruminants with cattle or horses can have beneficial effects on both pasture and parasite management.

 

Maintaining a sufficient grazing height of at least 3-4 inches will reduce ingestion of infective worm larvae, as it is estimated that approximately 80 percent of worm larvae is in the first two inches of vegetative growth. Along the same line, animals that browse will be exposed to less infective worm larvae. Some forages (e.g. Sericea lespedeza, chicory) have been shown to inhibit parasite development. Zero grazing will obviously reduce (if not eliminate) the need for deworming; however, it will prevent animals from developing immunity.

 

Genetic selectionSpecies, breeds and individuals within a breed differ in their ability to handle parasitic infection. Goats tend to be more susceptible to parasites than sheep. Hair sheep (e.g. St. Croix, Barbado) are more resistant to parasites than their woolly counterparts. The resistance of hair x wool crosses (e.g. Katahdin) tends to be intermediate between hair and wooled breeds. The Gulf Coast Native is the only wooled breed of sheep with documented parasite resistance.

 

While less data is available on goats, Kiko, Spanish, and Myotonic goats tend to be more resistant to parasites than Boers, Nubians, and other breeds. Though the level of heterosis has not been documented, crossbred individuals will tend to be more parasite-resistant than their purebred counterparts.

Any flock or herd can be selected for reduced parasite infection. Parasites are not evenly dispersed in a flock or herd. The 80-20 rule states that 20 percent of the animals are responsible for 80 percent of the worm infection (fecal egg counts). By selecting animals that shed fewer eggs and require less frequent anthelmintic treatments, a producer can reduce anthelmintic use and slow drug resistance, while developing a more resistant herd.

 

Parasite resistance, as quantified by fecal egg counts, is a moderately heritable trait. Parasite resilience, as quantified by packed cell volumes (and estimated by FAMACHA© scores) is less heritable, but correlated to parasite resistance.

The most important strategy is to use a ram or buck that has documented resistance to internal parasites. Unfortunately, selection programs for resistant sheep and goats are still in their infancy. The Katahdin breed is the only breed (in the U.S.) in which EBV's are calculated (via LambPlan) for parasite resistance (fecal egg counts). The primary goal of the Western Maryland Pasture-Based Meat Goat Performance Test is to identify bucks that are both resistant and resilient to internal parasites. Virginia Tech's Southwest AREC's hair sheep ram test includes an evaluation for parasite resistance.
 

A new anthelmintic [11]

There is a new anthelmintic called Zolvix® (monepantel). It is the first anthelmintic in a new class of anthelmintics called Amino-Acetonitrile Derivatives (ADD). Zolvix® is effective against the worms that are resistant to the other anthelmintic classes. However, like all dewormers, resistance to Zolvix® is inevitable. In fact, resistance may already have been detected in a hobby herd of goats in New Zealand.

Zolvix® is available in most countries, but not yet the United States, where it is awaiting FDA approval. When Zolvix® becomes available, it needs to be used in a manner that preserves its effectiveness for as long as possible. Resistance is inevitable, but the rate at which it develops is not.