Rethinking deworming of beef cattle

by Sally Colby

Whether you’ve been routinely applying a pour-on as cattle come through the chute or administering an injectable, parasitologist Dr. Ray Kaplan wants cattle producers to know they are likely contributing to the issue of parasite resistance to dewormers.

Kaplan, professor at the University of Georgia College of Veterinary Medicine, said the goal in managing parasites, in addition to keeping animals healthy, is to maintain the use of existing products and not create resistance.

“The current strategy for managing worms in cattle has come to rely completely on dewormers,” said Kaplan. “And really with just one class of dewormer – the majority of doses administered are the macrocyclic lactone (ML), or ivermectin types. These are amazing drugs – they’re so amazing that the parasitologist responsible for discovering the activity of ivermectin received a Nobel Prize.”

MLs are highly potent, very broad spectrum and they kill both internal and external parasites. “Their pharmacology makes it easy to administer them via different routes, including pour-ons,” he said. “Over time they’ve become inexpensive.”

However, access to widely available, highly effective drugs has resulted in the use of these products replacing critical thinking about parasite control, and the consequences have been severe.

“The main consequence is that worms have evolved resistance to dewormers,” said Kaplan. “This is a problem in all species of livestock, and a problem in all the different drug classes. There have been no new dewormers sold for cattle since ivermectin came on the market in the 1980s. There have been newer ML products, but they’re all closely related to ivermectin.”

Resistance means the drugs are much less effective than they were in the past. “While the approach of using dewormers frequently was successful for a while in optimizing cattle productivity and health, because the drugs are not as effective as they used to be, the levels of control and benefits are not what we expect,” said Kaplan.

The worms of concern in cattle include stomach and intestinal roundworms and liver flukes. Tapeworms are common, and are the one worm visible in feces, but rarely cause disease unless animals are overwhelmed with them. Some worms are more damaging than others and cause serious disease and production loss.

“Of all the species that infect cattle, Ostertagia (brown stomach worm) is by far the most damaging in terms of health consequences and production consequences,” said Kaplan. “The worm species that predominates in your cattle depends on several factors – mostly where you live (climate) and the time of the year.”

To ensure producers understand the basis of timely and effective deworming, Kaplan reviewed the lifecycle of Ostertagia: worms live in the gut, cows poop on the pasture and worm eggs are left in poop. “The fecal pat of a cow is a perfect home for a worm,” he said. “There’s a crust on the outside to keep it moist and protect it from becoming too hot or cold.”

First stage larvae (L1) develop in the egg then hatch, which takes 24 to 48 hours and is temperature dependent. After a few days, worms develop into second stage larvae (L2). Both first and second stage larvae are free-living in the feces, can only survive in the protective environment and are susceptible to environmental damage. While larvae are in the fecal pat, they ingest bacteria and store energy in their intestinal cells. After a few more days, L2 larvae develop into third stage larvae (L3), which is the final stage in the free-living environment. “That’s the parasitic stage,” said Kaplan. “The L3 has to make its way out of the feces and onto the pasture, which requires moisture. If it’s completely dry, the L3 won’t get through the crust.”

The L3 larva has specialized abilities and can no longer eat. “It has a second coating that works like a space suit,” said Kaplan. “The wall that covers the worm is a double coating that helps protect the L3 from the environment. The L3 will now be in the free world outside the fecal pat and has to survive long enough to be ingested by a cow. It can survive in that environment for many months, and even years. They’ll burrow into the soil where they’re protected.”

Noticeably absent from the L3 larva is that it has no legs – it can’t crawl. Kaplan said 75% of infective parasite larvae are in the bottom two inches of the pasture. Another 15% of larvae are in the next two inches of pasture, which means 90% of all parasite larvae are in the bottom four inches of pasture.

“All they can do is wiggle, and it’s a nicer environment,” said Kaplan, describing traits of L3 larvae. “It’s protected from cold, heat and ultraviolet rays. They survive much longer if they stay low. But there’s a population of L3 that slowly come out as the fecal pat disintegrates over time, and cows ingest L3 during grazing.”

The benzimidazoles were the first group of widely used dewormers, introduced in the 1960s. Some producers likely remember using the product thiabendazole (TBZ). These products are no longer used on large livestock. Although there are numerous deworming products, today there are just three groups, or classes, of dewormers.

Dewormers currently used are known as “white wormers,” including Safe-Guard, Panacur, Valbazen® and Synanthic®. MLs are commonly known as ivermectins and include various products. The third class is nicotinic agonistics such as levamisole and Rumatel®. “Each one of these groups works by a completely different mechanism of action,” said Kaplan. “They kill the worms through a different biochemical pathway. Within each group, the drugs work similarly, therefore the mechanism of resistance within a group is virtually the same. If you get resistance to one drug in a group, there’s resistance to all the drugs in that group.”

Kaplan added that within a group of drugs there are slight differences in potency and how they interact with the body, but overall, the similarities outweigh the differences.

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