The most important aspect of using dewormers is to conserve their effectiveness for use in animals that truly need them. This can be achieved by using them as little as possible and only when infection levels dictate that intervention is necessary. The old concepts of treating all animals when only a few show signs, or all animals at regular intervals—shorter than every three to four months—is no longer warranted because it promotes dewormer resistance. Even if new dewormers are eventually discovered and marketed, they should not be used indiscriminately because that is how the current dewormer-resistance problem evolved. It would be prudent to establish which dewormers are effective against a worm population on a given premises. This can be achieved by conducting fecal egg count (FEC) reduction testing and should be done by a qualified professional such as a veterinarian, laboratory technician, diagnostic laboratory, etc. FECs are not hard to do, but a microscope is required. The procedures for conducting a FEC are available online (see below). The concept is to do FEC before and 10 to 14 days after deworming treatment. If FECs after treatment are zero, the dewormer is very effective. Any FEC reduction below 95 percent calls a dewormer's effectiveness into question. The most effective dewomer on a given premises should be used only when no other treatment or management options are available, thus extending its useful life on that premises. FEC reduction testing may seem expensive, but it is worth the effort and expense to know what medications are effective on a particular farm. After the most effective dewormer has been selected, it must be used wisely. Some aspects of "smart deworming" include these actions:
- Do not use the most effective dewormer exclusively unless it is the only dewormer that works. Reserve its use for deworming those animals that need it the most and use less effective dewormers otherwise, if at all.
- If rotating dewormers is necessary, do so at yearly intervals and rotate between classes of dewormers. Use the most effective dewormer in each class of dewormers.
- Only deworm those animals that need to be dewormed, not the whole herd. As a general rule, a minority of the population harbors the majority of the worm population; thus, most of the animals may not need deworming. By following this process, much of the worm population is not exposed to the dewormer, and development of resistance can be slowed substantially. This is where the FAMACHA© monitoring system comes into play.
- If there is substantial resistance to all dewormers tested, increasing the dosage or using combinations from different classes (such as levamisole and albendazole) may help; such extra-label use must be made in consultation with a licensed veterinarian and within the confines of a valid veterinarian-client-patient relationship. Another practice that may improve effectiveness is to take animals off feed for 24 hours before administering dewormer. This reduces rumen motility and dewormers pass through the gut more slowly and have more contact time with parasites. Never hold pregnant animals off feed, however.
- Administer the proper amount of dewormer based on accurate body weights of animals to be treated. Confirm accurate administration of oral doses to the back of the animal's oral cavity.
- Do not deworm and move to clean pasture for at least three months because worms that survive deworming are probably resistant to that dewormer. The new pasture will become contaminated with eggs/larvae of resistant worms.
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