Q.
What is a combination treatment (of dewormers)?
A.
It is now recommended that clinically-parasitized sheep/goats be given combination treatments. A combination treatment is when you given more than one dewormer (different drugs) to the same animal at the same time. The treatment is sequential. One drug is given after the other. In some countries, there are deworming products that contain more than one drug active, but in the US, each drug in the combination treatment must be purchased and administered separately, ideally in a separate syringe or drench gun.
The reason behind a combination treatment is to kill as many worms as possible. By giving more than one dewormer at the same time, you get an additive effective. Let's say an animal has a fecal egg count of 1000 eggs per gram. The first drug kills 80% of the worms. There are 200 eggs left. The second drug kills 70% of the worms. There are 60 eggs left. The third drug kills 60% of the worms. There are only 24 egg left -- versus 200 if the most effective drug was given singly.
The best time to use combination treatments is before the worms have become highly resistant to the dewormers (less than 60% fecal egg count reduction). A combination treatment won't be very effective unless each of the drugs in the combination has high enough efficacy against the targeted species of worms, usually Haemonchus contortus (barber pole worm) in most parts of the US.
There is some evidence that when combination treatments are used in conjunction with other best management practices, there may be a reversion back toward susceptibility of the dewormers. A recently published study showed that a combination treatment (moxidectin + levamisole) was still effective after four years of continuous use.
In the US, there are three classes of dewormers: benzimidazoles, macrocyclic lactones, and cell depolarizers. In a combination treatment, you should give the most potent drug from each class. This is usually albendazole (Valbazen®) + moxidectin (Cydectin®) + levamisole (Prohibit®). A two drug combination is sometimes given. In this case, the two drugs should be moxidectin and levamisole, since resistance to the benzimidazoles is usually the highest. On-average, the avermectins (e.g., ivermectin) have the next highest resistance, which is why moxidectin (another macrocyclic lactone) is recommended for the combination treatment.
All drugs in the combination should be given orally (drench formulations). Pour-ons and injectables are less effective and promote drug resistance. The drug should be deposited over the tongue using a syringe with a long metal nozzle. Full doses of each drug should be given. Dosage should be based on accurate weights. If you don't have a scale, you should use weigh tapes to estimate weights. It is better to slightly overdose than to underdose the drugs. Underdosing is a primary cause of dewormer resistance.
On large farms/ranches, there is a desire to mix the drugs, so you don't have to calculate so many dosages and fill so many syringes. This should not be done. Not all of the drugs in the recommended combination treatment are chemically compatible. Plus, you may not use all the dewormer combination at once. It is not known if the mixed drug would maintain its efficacy if it were used hours, days, even weeks later. Resist the temptation. It's not that hard to use multiple syringes or drench guns. If you don't draw up a dose of dewormer for each animal, the drench gun should be callibrated for the heaviest animal that might be dewormed.
Combination treatments should not be given to all of the animals in the flock/herd. Only clinically-parasitized sheep/goats, as evidenced by FAMACHA©, the Five Point Check©, or other criteria, should be given combination treatments. If every sheep/goat in the group is given the combination treatment, all of the surviving worms will be resistant to all of the drugs/drug classes used in the combination treatmaent. You'll end up with "SUPER RESISTANT" worms. It's important to maintain refugia. Refugia are worms that have not been exposed to the dewormers; thus, remain susceptible to treatment. This is especially important if treated animals are being moved to a clean pasture.
The withdrawal period for a combination treatment is the withdrawal period for the drug with the longest withdrawal. This is usually moxidectin (Cydectin®). In the US, all the dewormers recommended for a combination treatment are FDA-approved for sheep. However, they are not approved for goats (or camelids). Combination treatments for goats (and lllamas/alpacas) require extra label drug use and a valid veterinarian-client-patient relationship. Only veterinarians have the legal authority to use or prescribe drugs extra label.
There are no additional (safety) risks when giving more than one dewormer at the same time. The precautions that exist for giving an drug singly are the same as when the drug is given as part of a combination treatment. Drugs should always be administered according to the label unless a qualified veterinarian prescribes otherwise. The label on the package or container may not be accurate when the drug is used extra label.
06/06/2021
Additional reading
Combination Dewormers: the Time is Now
Sheep dewormer chart
Goat dewormer chart
Camelid dewormer chart