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Which dewormer(s) work on your farm? (July 7, 2015)
By Susan Schoenian, Sheep and Goat Specialist, University of Maryland Extension
Worms have developed resistance to all of the dewormers and dewormer classes used to treat small ruminants.
Resistance varies by geographic region and individual farm and is the result of past deworming practices.
Worms do not develop resistance to specific drugs, but instead they develop resistance to the drugs’ mode of action (method of killing worms).
Dewormers in a dewormer class share a mode of action; thus, when resistance develops to one drug, there is cross-resistance to the other drugs in the same class, even if one drug is initially more potent — for example, albendazole versus fenbendazole.
From an industry-wide standpoint, resistance tends to be highest for the benzimidazoles or “white dewormers.”
This dewormer class includes fenbendazole (SafeGuard, Panacur), albendazole (Valbazen) and oxibendazole (Synathic).
Efficacy may be increased with increased exposure to the drug, but there is no guarantee that fasting and/or sequential dosing will be any more effective than a single dose.
Macrocylic lactones are another class of dewormers.
There are two sub-classes: avermectins and milbemycins.
Resistance tends to be highest among the avermectins.
The avermectins include ivermectin (Ivomec), eprinomectrin (Eprinex), and doramectin (Dectomax).
As with the benzimidazoles, efficacy may be increased with increased exposure to the drug.
Moxidectin (Cydectin) is the only milbemycin.
In the Mid-Atlantic region, resistance tends to be less for moxidectin as compared to avermectins and benzimidazoles; however, moxidectin resistance is developing rapidly due to its widespread use and chemical similarity to ivermectin.
The third class of dewormers are cell depolarizers, so-named for the way in which they kill worms. Levamisole (Prohibit) is the primary drug in this class.
It is the most potent and tends to be the most effective anthelmintic on most Mid-Atlantic small ruminant farms.
Less is known about morantel (Rumatel and Positive Goat Pellet) and strongid, the other members of this dewormer class.
No method of parasite control will be effective unless it is backed up by at least one effective dewormer.
Even “natural” or organic methods of parasite control must rely on effective drugs for treatment of clinically parasitized animals.
It is essential that small ruminant producers know which drugs are effective on their farms, as it may be different than the norms of the industry.
There are two ways to determine drug efficacy.
Before and after fecal egg counts can be compared to determine the efficacy of an anthelmintic treatment.
This is called the fecal egg count reduction test.
A fecal egg count reduction test needs to be done for each drug and with multiple animals.
It can be costly and/or time-consuming.
Many producers don’t have enough animals to get reliable results.
The other method to determine anthelmintic efficacy is the DrenchRite test or larval development assay.
This is an in vitro (lab) test that uses wells of larvae to determine drug efficacy.
The test also determines the type of parasites that are present in the manure sample.
The only place (in the United States) that does the DrenchRite test is the University of Georgia.
At a cost of $450, the DrenchRite test may seem like an expensive option.
However, unlike the fecal egg count reduction test, it determines resistance to all drugs simultaneously from a single pooled sample.
Moreover, its cost can easily be justified when you consider the value of a few animals that may die because they are not dosed with an effective drug.
It would only take two 70-pound market goats to cover the cost of the test.
For more information about anthelmintic resistance, visit the web site of the American Consortium for Small Ruminant Parasite Control at www.wormx.info or www.acsrpc.org.
Future deadlines and programs to think about (June 30, 2015)