While Ostertagia is the biggest concern, another genus, Cooperia, can be a problem in young cattle. “By the time that calf becomes a first-calf heifer, you just can’t find Cooperia anymore,” he says.

Cooperia, at heavy enough infections, can cause scours. Unlike Ostertagia, however, Craig says the macrolides don’t work well against Cooperia. White wormers, on the other hand, can be effective.

“The two parasites with cow-calf operations we have to consider are Cooperia in the babies and Ostertagia in the mommies,” Craig says. So, in his mind, if the calves are old enough to benefit from an anthelmintic, treat the cows with a macrolide to help control pasture infestations of Ostertagia and the calves with a white drench to get Cooperia.

What’s more, he says that approach may address a concern that he and others have with anthelmintic resistance in internal parasites. If you treat your cattle and don’t get 100% of the worms, it could mean there are resistance factors at work, he says. “By treating the mommies with one thing and babies with another, we have half the population as a refuge for susceptible worms instead of resistant worms,” Craig says.

Then there were three

The third parasite of importance in cattle is Haemonchus, or the barber pole worm. Sheep and goat raisers are very familiar with this parasite; it’s a major problem in small ruminants and has developed significant resistance to anthelmintics in sheep and goats, says Morgan McArthur, a Milwaukee-based parasitology consultant.

Haemonchus doesn’t affect cattle as severely as it does small ruminants, but it’s still a concern, primarily in stockers and backgrounding operations, he says. And parasitologists are seeing a growing pattern of resistance in both Cooperia and Haemonchus in cattle that is concerning, McArthur says.

Of particular concern are grazing operations in more temperate climates. That’s because conditions are ideal for worms to grow — the temperature and humidity create good pasture conditions for larvae survival. Add to that a high throughput and a continuous grazing management scheme with young stock, and the worms have found their happy place.

Properties with those conditions, McArthur says, are an invitation to a conversation. “And part of that chat,” he says, “has to be, ‘Do we have a problem?’ ”

Unfortunately, he says, there’s no simple way to evaluate whether or not there’s a resistance problem on a property. The only diagnostic at hand is fecal egg counts. “It’s a pretty basic test. Ideally you collect 15-20 fecal samples before treatment and 15-20 samples from the same animals two weeks after treatment, and compare the concentration of parasite eggs. If you’ve got eggs coming through after treatment, it likely indicates the product isn’t working 100%. And how many eggs remain post-treatment is an indication of the degree of resistance.” He says it’s labor-intensive, and can be dollar-intensive, but it’s the only way to evaluate the effectiveness of your parasite management program.

Cow-calf operations are not likely at risk for resistance problems, he says. Mature cattle have developed immunity, while unweaned calves don’t carry much of a parasite burden.

But if you’re in the part of the country where it’s warm and humid, you’re grazing actively, you’ve got young stock, you’ve got an operation where you’ve got concentration, and you’ve mainly been using only one class of product, then it’s worth taking a look, he says.

He suggests that your veterinarian or a parasitologist be a part of the evaluation process. And if testing shows a potential for a resistance problem, don’t just change products, but consider all aspects of your management.

But first, he says, you have to know where you stand. And once you know, then you can begin the conversation on how to deal with it.

“But if you don’t look, you won’t find,” he says.


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