Part 1: Using drugs effectively
The parasite resistance issue in small ruminants isn’t new, but it’s still a struggle for many producers. Dr. Dahlia O’Brien, Virginia State University, promotes a multi-pronged approach to managing Haemonchus contortus (barber pole worm), the most economically significant parasite of sheep and goats.
Sheep and goats are especially susceptible to internal parasites due to their grazing behavior and poor immunity. “Sheep graze especially low,” said O’Brien. “They find low areas in the pasture to graze. Goats are natural browsers and tend to eat high off the ground. Shrubs, forages or anything that’s higher than grass is their preference.”
Many goats have access to limited acreage with no browse available, so they’re forced to eat low, which results in picking up larvae as they graze. “Eighty percent or more of parasite larvae are found six inches or below on a blade of grass, so once we force animals to eat low, they’re exposed to parasite larvae,” said O’Brien. “We also know that sheep and goats have poor immune systems.”
Ideally, exposure to internal parasites over time helps sheep and goats develop natural resistance. “In sheep, it’s estimated that by the time they’re around six months of age – and this varies from breed to breed – they start developing some immunity,” said O’Brien. “Goats develop resistance later, and some goat breeds take longer.”
O’Brien explained how internal parasites developed resistance to available anthelmintic (dewormer) products. In the past, producers relied heavily on chemical dewormers to control internal parasites and treated animals every four to six weeks to prevent parasitism. Unfortunately, many animals were not dewormed based on weight and often given less than the correct dose of anthelmintic product, usually because they “weren’t that wormy.”
But under-dosing allows genetically superior parasites to survive treatment. Parasites that survive treatment mate with other genetically superior parasites, promoting resistance to anthelmintic products.
A parasite management program aimed at preventing resistance begins with understanding the three available classes of dewormers, which are grouped based on their chemical structure and mode of action: benzimidazoles, macrocyclic lactones (MLs) and nicotinic agonists. When worms on a farm are resistant to a dewormer product in one group, they’re likely to be resistant to other dewormers in the same group. Some groups include more than one product, but it’s important to select products based on group rather than by product name.
Group 1 (benzimidazoles) include fenbendazole (Safe-Guard®, Panacur®, albendazole (Valbazen® and oxfendazole (Synthantic®).
Group 2 (MLs) include avermectins such as ivermectin (Ivomec®), doramectin (Dectomax®) and eprinomectin (Eprinex®). Milbemycins in this group include moxidectin (Cydectin® and Quest®).
Group 3 (nicotinic agonists) are imidazothiazoles such as levamisole (Prohibit®). This group also includes tetrahydropyrimidines such as morantel (Rumatel® and Positive Pellet® Goat Dewormer) and pyrantel (Strongid®).
Sheep are highly susceptible to internal parasites due to their poor immune systems and tendency to graze low to the ground. Ewes with lambs are even more susceptible and should be monitored carefully to avoid accumulating heavy parasite burdens. Photo by Sally Colby
“Dewormer resistance is when a drug loses its ability to effectively kill worms, and the worms continue to survive in the presence of therapeutic levels of the drug,” said O’Brien. “Every dewormer comes with a prescribed dose, and when you treat the animal with the prescribed dose and the animal isn’t getting better, those worms are resistant.”
Unfortunately, the resistance issue is worldwide. Other countries have developed drugs in other classes, but those products are not available in the U.S. “With numerous farms in the U.S. experiencing not only drug resistance but multiple drug resistance, the American Consortium for Small Ruminant Parasite Control (ACSRPC) conducted numerous research projects and worked on alternative strategies for effective parasite control,” said O’Brien. “The overall goal is to reduce drug use to help slow down the rate of drug resistance.”
O’Brien said unique classes of drugs have been introduced in other countries, and within five years of being introduced, there’s resistance to those drugs.
“We know that worms have developed resistance to all dewormers in all classes,” said O’Brien. “We start suspecting resistance when clinical signs persist after treatment. If you are monitoring fecal egg counts (FEC), those counts remain high after treatment. This is assuming you have used the correct dose for the animal weight.”
The gold standard for determining dewormer resistance is the FEC reduction test (FECRT). Drug efficacy is determined by comparing the FEC of animals before treatment and seven to 14 days after treatment. This test can be done by a veterinarian, Extension personnel or trained producers. An effective dewormer will result in a 95% reduction of eggs.
O’Brien said the test requires at least 10 to 15 animals with at least 250 eggs/gram of feces for accurate results. If individual samples are not available, a pooled sample is an option.
Using dewormers strategically and correctly is the best strategy for slowing resistance.
Part 2 will explain incorporating FEC, FAMACHA and proper deworming technique.
by Sally Colby
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