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Be Selective in Deworming

Parasite Drugs (Anthelmintics)

Anthellmintics are drugs used against parasites the either kill the egg laying adults or kill the larvae before they become capable of  laying the harmful eggs. While that sounds like it cures all of the problems with the parasites, the reality is the parasites have developed a resistance to almost all of the known drugs that are used against the parasites. Research after research has stated that the resistance is coming from the drugs being overused and/ under dosing of a drug when they were used. This resistance parasites have to these drugs makes the fight against them extremely hard because it takes away one of the key tools breeders use to have for helping control the parasites.

 Classes of Drugs

Benzimidazoles – (the white drenches). While these are effective against many types of internal parasites, it is low against the Haemonchus parasite. There are several drugs in this family. They are:

A big problem with the Benzimidazole type of drugs is if parasites become resistant to any one of these drugs, they are also resistant to the others even if you have not been using them.

 When giving benzimidazoles by mouth it is better to hold the animals off feed for 12 to 24 hours before treatment (don't remove water, just food). The drugs will not pass so quickly through the GI tract and active levels will be maintained in the body longer.

 The Benzimidazoles (Safeguard®, Panacur®, Valbazen®, Synanthic®), also called "white dewormers" are broad spectrum and safe to use. They are effective against tapeworms. Albendazole is effective against adult liver flukes, but should not be used in pregnant or lactating females


The two drugs in this class are ivermectin and moxidectin.

The Macrolytic lactones or "avermectins" (Ivomec®, Dectomax®, Quest®, Cydectin®) are the newest family of drugs. They are broad spectrum and have a wide margin of safety. They are also effective against external (biting) parasites, including nose bots. Moxidectin is a persistent-activity dewormer that continues to kill worms after it is administered.


The main drug in this family is levamisole and has shown to be very effective against the round worm. There has been less parasite resistance shown against levamisole but that could change with breeders starting to over use it as they have all of the other drugs.

Levamisole (Tramisol®), also called a "clear dewormer" is broad spectrum and effective against arrested larvae. However, it has a narrower margin of safety, especially in the injectable form. Pyrantel (Strongid®) is only effective against adult worms. Moratel (Rumatel®) is an oral feed additive and is only effective against adult worms.


“The most important aspect of using dewormers is to conserve their effectiveness. This can be achieved by using them as little as possible and ony when infection levels dictate that intervention is necessary. The old concepts of treat all animals when a few show signs or all animals at regular intervals (shorter than every 3-4 months) is no longer warranted because it promotes dewormer resistance.” Louisiana State University.

Research and the goat experts use to recommend deworming the entire herd and then moving them to a new, clean pasture. That is no longer the thinking that is coming from the recent research. The problem the “deworm and move to a clean pasture” strategy has is related to the breeding of the parasites inside the stomach. That is the only place breeding occurs that result in the eggs. If a goat only has the most resistant parasites in their system, the breeding can only produce similar strong, resistant parasites. However, if some parasites are the less resistant ones that breed with the stronger, resistant parasites, the product will be parasites that are not as resistant as the most resistant ones.  

 Recent recommendations are to deworm only the animals that really need it and leave them in the same pasture for a few days before rotating them. This helps keep some of the less resistant parasites in the system to help minimize the number of “super parasites” in your herd.

 Goats generally metabolize drugs faster than other animals such as cattle.  This means the drugs may go thru the goats system faster than other animals. Since most parasite drugs are actually developed and approved for animals other than goats, the directions and doses may not be best for goats. A larger amount of the drug may have to be given to be effective and instead of giving an injection or pouring it on, it generally need to be given to goats as a drench.

 Another recommendation for breeders to consider is to dry lot the herd for a day or two after they have been dewormed to minimize the number of eggs and larvae that survived the deworming are not being dropped in the new pasture.

Per-Parturient Rise (PPR)

A phenomena called the “per-parturient rise” (PPR) in fecal egg output. This occurs at or around kidding time and extends through most of the nursing period. Because the kidding and nursing are stressful conditions, the dam’s immune system is compromised. Furthermore, nutrients are partitioned preferentially to support mammary and fetal development and then lactation, which also decrease the animals’ ability to generate an effective immune response to worm infection. This allows the existing  worms to increase the number of eggs laid, thus increasing the number of eggs deposited in the feces.

 Do not deworm and move to clean pasture (no animal grazing for at least 3 months) as those worms that survive deworming are probably resistant and then the new pasture will become more highly  contaminated with eggs/larvae of resistant worms

Refugia (untreated worms)

Worms that are not treated are called “refugia.” The concept of refugia has been largely overlooked in the past. Having some worms in refugia (not treated) insures that a level of genes remain sensitive to dewormers. (Kaplan, n.d.) A surviving population of untreated worms dilutes the frequency of resistant genes. Consequently, when a dewormer is required, it will be effective because the worms will be susceptible to treatment. (Kaplan, n.d.) When fewer numbers of animals receive treatment, the refugia population remains large. The more refugia, the better. Sustainable techniques, such as FAMACHA©, fight drug resistance by increasing refugia. In contrast, several practices accelerate drug resistance. They include frequent deworming (more than three times a year), under dosing (often caused by miscalculation of body weight), treating and moving to clean pasture, and treating all animals, regardless of need. These practices lead to resistance because they decrease the number of worms susceptible to dewormers (refugia).

 Since no dewormer is 100 percent effective 100 percent of the time, worms that survive a dose of dewormer are resistant to that dewormer. Frequent deworming increases the rate resistance develops. Each time animals are dewormed, the susceptible worms are killed. The strong ones survive and lead to a population of very resistant worms. Underdosing causes larger numbers of stronger worms to survive. The weakest, most susceptible worms are killed. But because of the weak dose, more of the stronger worms will be able to survive and reproduce, creating a population of stronger worms. Once an animal has been treated, only resistant worms remain. If the animals are moved to a clean pasture they deposit only resistant worms on the pasture. There are no susceptible worms to dilute the worm population. Treating all animals regardless of need ignores the importance of refugia and will lead, in time, to a population of worms unkillable by dewormers.

Anthelmintics should not be used indiscriminately. Frequent deworming is costly. It accelerates the development of anthelmintic-resistant worms and leads to a false sense of security, which may result in unnecessary production losses and animal deaths. The routine use of anthelmintics is prohibited under the new National Organic Standards.

Another concept that has also been reported to have some success In improving effectiveness is to take animals off feed for 24 hours before administering the dewormer. This will reduce rumen motility and the dewormer will pass through the gut slower and have more contact time with the target worms.