Q.
Which dewormer should I give to my sheep/goat(s)?
A.
The one(s) that work(s). When the animals need it.
Dewormer resistance
The worms (especially barber pole) have developed resistance to all of the dewormers and dewormer classes. In the US, there are three classes of dewormers: benzimidazoles (SafeGuard®, Valbazen®), macrocyclic lactones (Ivomec®, Cydectin®), and cell depolarizers (Prohibit®, Rumatel®). When worms develop resistance to one of the drugs in a class, cross resistance soon follows.
Dewormer resistance varies by farm and geographic area. Resistance tends to be worse in moist climates where more frequent deworming is generally required, due to the greater parasite challenge. It’s likely to be higher on farms which deworm frequently, practice whole flock/herd treatments, and/or fail to follow other best management practices. Some farms have resistance to all of the dewormers and dewormer classes, whereas some farms may still have efficacy with two or more dewormers.
You can tell if you have dewormer resistance on your farm by doing fecal egg count reduction tests. Collect fecal samples before and after deworming from at least 10 animals. Testing a few animals will tell you if treatments were effective, but it won't really tell you if you have resistance to the dewormer(s).
Sample the same animals each time. The fecals can be analyzed individually or combined into a pooled sample for analysis. Several land grant universities now offer low-cost fecal egg counting ($5/sample) for this purpose as well as genetic evaluation. Go to https://www.wormx.info/lowcostfec to learn more.
Compare the before and after fecal egg counts. A fecal egg count reduction of less than 95 percent means there is resistance to the drug. Less than 80% efficacy means there is significant resistance. Below 60% and the drug may no longer be effective as the sole treatment.
Another way to test for dewormer resistance is to have a DrenchRite® test done. In this labor-intensive lab test, a pooled fecal sample is submitted. The worm eggs are allowed to hatch and different concentrations of the dewormers are applied to the larvae. For information about the DrenchRite® test, send an email to avatta2@lsu.edu.
Combination treatments
It is now recommended that clinically-parasitized animals, as determined by FAMACHA©, the Five Point Check©, or other criteria, be given combination treatments. A combination treatment is when you give a dewormer from each drug class to the animal at the same time, one after the other. The goal of a combination treatment is to kill as many worms as possible. By using several drugs sequentially, you get an additive effect.
For the combination treatment to be most effective, you should use the most potent drug from each class. Usually, this is albendazole (Valbazen®) + moxidectin (Cydectin®) + levamisole (Prohibit®, Leva-Med®). Ideally, use a separate syringe or drench gun for each drug. Do not mix the drugs, as they are not all chemically compatible. In other countries, combination products may be commercially available, but in the US, each drug must be purchased and administered separately. Use all drench formulations and be sure to dose based on accurate weights. If you don’t have a scale, use weigh tapes. Do not underdose. Observe the withdrawal period for the dewormer with the longest withdrawal.
All of the drugs in the recommended combination treatment are FDA-approved for sheep, but they are extra label for goats and camelids. In other words, you must have a valid veterinarian-client-patient relationship. Only veterinarians are allowed to use or prescribe drugs in an extra label manner.
All of the dewormers in the combination treatment are safe when given according to the label. Dewormer dosage charts can be found on the web site of the American Consortium for Small Ruminant Parasite Control. Go to https://www.wormx.info and click on topics, then dewormers. Albendazole (Valbazen®) should not be given during the first 45 days of pregnancy or within 45 days of removal of males. Levamisole (Prohibit®) has the lowest margin of safety at 3-4x, and there are anecdotal reports of it causing late term abortion in goats.
05/29/2021
Additional reading
Combination treatments: the time is now - WormX
Managing dewormer resistance: Best Management Practices - WormX