“We’ve got the same parasites, but they play the game in a different way.” And that, according to Tom Craig, veterinary parasitologist at Texas A&M University, is why cattle producers the country over need to know their worms.

Internal parasites, regardless of genus, generally follow the same life cycle. Inside an animal, the adults produce eggs, which are passed out of the animal. Given the right conditions, the eggs hatch into larvae, which wriggle out of the manure pat and up a blade of nearby grass. There, they wait for an animal to ingest them. Once inside a new host, the larvae become adults, and the process starts over again.

According to Craig, the three internal parasites of most importance to cattle producers are Ostertagia, or the brown stomach worm;Cooperia; and Haemonchus. Depending on your operation, your management focus will change to fit the situation.

Cow-calf operations

Ostertagia is the most pathogenic of the three amigos, and is of most importance to cow-calf operations, Craig says. When it’s ingested, the larvae nestle into the glands in the abomasum wall, where they metamorphose into adults. “When it emerges as a larvae becoming an adult, it largely shuts down the gland it was in for at least a month,” he says. What’s more, surrounding glands become non-functioning as well.

That’s important, because those glands produce digestive acids that help convert food into nutrition. As those glands shut down, appetite is reduced and the animal responds by eating less. So, short of a severe infection, you don’t know you have a problem until you run the cattle across a scale and don’t have the gain you expected.

“The thing about Ostertagia is that it has adapted to the environment,” Craig says. In warmer climates like the South, parasite transmission occurs all winter long. “As soon as it gets to 40 degrees F., and if there’s a little bit of moisture in the pasture, it’s being transmitted,” Craig says. “And you’ll transmit in that part of the world until about the first part of May.”

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The worms picked up in late April and early May go into arrested development, he says. “They know if their babies got out in a pasture in June or July, they’d cook.” So the larvae nestle into a comfortable gland and take a long summer’s nap until September. “Then they say ‘Hey, things are better.’ And they’ll go forth and multiply.”

In the North, it’s the winter that the worms can’t abide, he says. “They go into winter arrest and emerge in the spring. Then we’ve got some states that are in between — some of the worms play the game one way and some play the game the other way to make sure some of them survive.”

However, regardless of your address, Craig recommends deworming once a year, in the spring. Ideally, he prefers to wait until about a month after green-up. That’s because some larvae will survive the winter in the pasture. When it warms up enough for them to emerge, they’re desperate to find a host.

“To me, after the animals have started to graze, then I like to treat them so the pasture won’t have a tremendous number of larvae that have built up over a period of time,” Craig says. However, he realizes that many operations aren’t able to re-gather cattle after turnout and treat earlier. In that case, he recommends a macrolide, which has a residual effect in the animal.

“Here in the South, I like to do it more or less the same time of year, but for a different reason,” Craig says. “I’m after the Ostertagia before they have a chance to emerge in the fall. What you’re trying to do is get the ones that went dormant.”

In the North, Craig generally recommends worming only in the spring. In the more temperate South, where flukes are more of a concern, worming both in the fall and the spring is necessary. Worm cattle in the fall to control flukes, and worm in the spring to control Ostertagia.

And Craig recommends treating both the cow and the calf, or perhaps only the cow if the calves are very young. “If we’ve got some fall calves that are going onto spring pasture and they weigh several hundred pounds, they’re getting enough grass to have some worms,” he says. “If they’re young calves, the first couple months of life, they’re nibbling a little bit and they’ve got a few worms. But they probably need a few just to let the immune system know what it’s supposed to be doing.”

Young calves can be treated later, perhaps at weaning. By the time an animal becomes an adult, it has generally developed an immunity to worm infestations. “Yearlings or first-calf heifers are the ones that are highly at risk of having problems with Ostertagia,” Craig says.