Dewomers are chemicals, or drugs, that have been evaluated and tested for effectiveness and safety for use in animals to remove (i.e. kill) worm parasites. They are also referred to as “Anthelmintics“. For the most part, pharmaceutical companies will not market a dewormer unless it is essentially 100 percent effective. As long as dewormers remain effective at the manufacture’s recommended dosage, control is relatively easy and cost- effective. However, resistance to almost all dewormers has been developed by many worm species. Therefore, reliance on the use of dewormers has become limited.
Only FDA-approved dewormers (see Classes) can be used legally without restrictions. All other dewormers, if used, are extra-label and are subject to specific regulations as outlined by the FDA. Because of public concern over food product residues and environmental contamination with chemicals that may be harmful, the FDA has recently revised the rules and regulations governing use of chemicals in food-animal production. In summary, producers and veterinarians have to pay attention to extra-label use, which means using a product for a purpose other than what it was approved for. Because goats are a relatively minor livestock species, pharmaceutical companies cannot recover the costs that would be incurred for them to pursue approval and labeling. For a veterinarian to use a dewomer extra-label, a valid veterinarian-client relationship is necessary. The veterinarian has to have contact with the animals and make a diagnosis that the parasite situation is potentially life-threatening. The veterinarian has to establish that none of the approved dewormers will work through fecal egg count reduction testing (See Smart Use of Dewormers). Once the approved dewormers have been tested and if none works, then other dewormers can be used extra-label. The Food Animal Residue Avoidance Databank (FARAD) http://www.farad.org/ provides the recommendations on the dosage and withdrawal times for commonly used dewormers. The veterinarian has to take responsibility for prescribing the dewormer, and the producer has to take responsibility for using it properly. In the absence of a valid veterinarian-client relationship, the producer is restricted and cannot legally use an unapproved product extra-label.
The three general classes of dewormers are benzimidazoles, imidazothiazoles and macrolides. The more commonly used benzimidazole dewormers are fenbendazole (Safeguard, Panacur) and albendazole (Valbazen); imidazothiazole dewormers are levamisole (Levisol, Tramisol) and morantel tartrate (Rumatel); and macrolide dewormers are ivermectin (Ivomec) and moxidectin (Cydectin). Of these, only fenbendazole, albendazole and morantel tartrate are currently approved for use in goats. All others would be used as extra-label. A number of these dewormers have gone off-patent and are now marketed under different generic names.
Formulations of dewormers include drench, injection and pour-on. In addition, some dewormers are marketed in feed supplement blocks, mineral mixes, pellets and cubes. For goats, only the drench formulation of fenbendazole and albendazole, and the feed formulation of morantel tartrate are approved for use.
Oral administration is preferred; and with drenches, it is very important to make sure the product is delivered over the base of the tongue. By doing so, the dose is delivered to the rumen where it will be mixed with the ingesta and then distributed evenly throughout the gastrointestinal tract. If the dose is delivered into the front part of the mouth, the animal may spit all or part of it out. Additionally the swallowing or “gag” reflex may stimulate closure of the esophageal groove, causing the product to bypass the rumen. When the rumen is bypassed, the dose goes directly into the omasum (third stomach) and moves quickly through the gastrointestinal tract, thus not allowing sufficient time for the anthelmintic to achieve full effectiveness. The other form of oral administration is in feed products, which does not ensure that all animals will receive an effective dose because individual animals utilize these products differently. Some animals eat more or less than others due to their appetite, their place in the pecking order or their distaste for the formulation — specifically pelleted dewormers, supplement blocks and mineral mixes.
Although it is not recommended to do so, if one elects to use injectable products , injections are subcutaneous and best administered in an area of exposed skin, usually under the front legs, so that it’s possible to see the dose being delivered. It is best to not “tent” the skin. Just lay the needle on the skin and insert it quickly. If the skin is tented, the needle may come out the other side and the injected material will be administered on the skin surface. If the injection is given in an area covered by hair, it can be difficult to ensure that the needle actually penetrates the skin and the dose is delivered appropriately. Sometimes the injected material will run back out of the needle hole, so make sure to press a finger over the injection site for a few seconds to prevent leakage. If one elects to use a pour-on product, which is also not recommended, the material has to be delivered on to the skin. Parting of the hair may be necessary to achieve this, particularly if the hair is long. There are mixed reports as to whether pour-ons, approved for use in cattle only, work on goats. For the most part, they do not seem to be that effective in goats and may also cause skin irritation.
The major problem encountered in controlling nematode parasitism in goats is the genetic resistance that many worm populations — specifically H. contortus (barber pole worm) — have developed to essentially all of our dewormers. Resistance has developed primarily because dewormers have been used and rotated too frequently and many times under-dosing occurs. Continuing to use such a dewormer will increase the selection of more resistant worms which will eventually result in a population of “superworms” that can’t be controlled with drugs. There is no silver bullet one can rely upon. Resistance is genetically controlled, and once established, it is set in the population, and those dewormers can no longer be used effectively.