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October 2013
Potential solutions to highly resistant worms in sheep and goats



By Steve Hart
Goat Extension Specialist
Langston University

Highly-resistant (to dewormers) worms occur due to the overuse of dewormers, importing resistant worms in newly acquired animals, and lack of an effective parasite management program.  A parasite management program is not xx ml of YY dewormer every zz days. 

A parasite management program uses the principles of biology of the worm and manages pastures and animals in such a way to suppress worms. This implies a plan of certain management actions. This would include such things as rotational grazing with a 40-day (or more) rest period, not grazing within 4 inches of the ground, and using browse and/or sericea lespedeza. Higher stocking rates encourage worms.


Finding an effective way to deworm animals (discussed later) will only provide short-term relief for the problem and not solve the problem in the long-term.  If you have significant dewormer resistance, you are going to have to develop a parasite management plan if you want to stay in the sheep/goat business for the long-term

Other management practices that can be useful to control worms by reducing the number of infective larvae on a pasture include baling hay which removes most of the infective larvae from the pasture (the larvae in the hay bale die within a month).  Tillage, such as used to plant wheat or sorghum-sudan pasture for grazing, is an effective method of cleaning pasture, since larvae that are tilled an inch under the soil do not survive. 

Grazing with another animal species (horses or cattle; sheep and goats share the same parasites and will not help one another) is another way to clean larvae off the pasture. Pasture rest (at least 6 weeks during a hot summer, longer in cooler climates) will also clean a pasture, especially when the weather is hot.  With improved pasture species, the pasture forage quality will decline with this amount of rest.  Solutions include making hay on the pasture while it is rested, grazing with another animal species, or even mowing to keep the grass in a vegetative stage. 


There have been several cases where resistant worms have been replaced with susceptible worms, but this requires superb management and the services of a parasitologist. But, without an effective parasite management program that minimizes the use of dewormers, this is a solution for only a few years.

Another way to overcome parasites is to select animals that are more resistant to parasites. This begins by culling animals.  Fifteen percent of the animals in the herd carry 50 percent of the worms,whereas 20 to 30 percent of the animals in the herd carry 70 to 80 percent of the worms. If you get rid of the animals that carry the most worms, then there are many fewer infective larvae on the pasture to infect the rest of the animals, and you have prevented these animals from contributing their weak parasite resistance genes to the herd. 


Everyone knows that some of their animals are always wormy, and those are easy to identify and cull. There are two ways that wormy animals can be identified. One is by doing FAMACHA© across the season and the animals that require the most deworming can be culled. One should categorize animals by litter size (singles, twins and more) and cull those within each group that require the most deworming. The reason for categorizing by litter size is that does/ewes that raise more kids will often show more symptoms of worms.  


Another way to identify the animals with the most worms is to do a fecal egg count on all animals and cull the ones within each litter size with the highest fecal egg counts. The buck or ram accounts for 50 percent of the herd genetics. You should take several fecal egg counts during  the summer from your bucks/rams and get rid of those  with the highest fecal

egg counts. FAMACHA© could also be used, but fecal egg counts are better for the bucks since fecal egg counts are more accurate and can be justified for the bucks/rams due to their importance. 

Yearlings will tend to have higher fecal egg counts since their immune systems are not fully developed. This should be factored into selection. If you identify the most resistant rams/bucks using this method, they should be bred to the best females (determined by FAMACHA©) and the resulting offspring saved as replacements.

How to kill resistant worms
First, you need to incorporate the support of the animal’s immune system which will require good nutrition to support an immune response. There are three classes of dewormers: benzimidazoles (Panacur®, Safeguard®, and Valbazen®), cell-depolarizers (Prohibit® and Rumatel®) and avermectins and milbermycins (Ivermectin and generics, Eprinex®, Dectomax® and Cydectin®). 


In general, Valbazen® is the most potent benzimidazole and Cydectin® is the most potent member of the avermectins and milbermycin group. The use of the more potent members may prevail over resistant worms. Doubling the dosage will help only little, increasing the percentage of worms killed often by only 25 percent. Goats should be given twice the sheep dose of Benzimidazoles and the Avermectin/milbermycin group. They should be given only one and a half times the dose of Prohibit® due to the narrower margin of safety. 


Animals can be given combinations of dewormers at the same time, one from each of the different classes of dewormer (usually the most potent members). These can be quite effective against resistant worms.


Benzimidazoles can be given every 12 hours for several doses to increase the effectiveness.  The last dose can be given with a dewormer of another class to increase the effectiveness. Copper oxide wire capsules have been shown to be effective against the Barberpole worm (Haemonchus contortus).  

The only long term solution to highly-resistant worms is an effective parasite management program. Combinations of dewormers may work for a while, but will fail without an effective parasite management program. It is possible to raise sheep/goats without dewormers (they did exist before dewormers were available and some people manage to raise them without dewormers), but the weak ones died, they rotated themselves, did not graze close to the ground, and goats browsed and ate a variety of plants. It is our management that has caused parasite problems. 

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