Strange things can happen when you try to make one size fit all situations, because it flat doesn’t,” says Tom Craig.

And when it comes to deciding whether or not to worm your cattle this year, trying to make one program fit all situations is particularly fraught with potential problems, says Craig, a parasitologist in Texas A&M University’s College of Veterinary Medicine. That’s because of the profound influence Mother Nature has foisted upon cattlemen in a large chunk of the country this year.

Normally, Craig recommends treating all cattle for internal parasites, and in many regions of the country, that still may be a sound strategy. But, while he still recommends deworming calves, he’s not so sure about deworming mature cows on drought-withered pastures.

“The reason for this is, most years, we’ve got a lot of worms out there in the pasture. This year, I don’t think there are very many,” he says.

Take the brown stomach worm, for example. For the parasite to complete its lifecycle, the cow passes eggs in her manure, where they become larvae. Given the right combination of temperature and moisture, the larvae leave the fecal pat, squirm up a nearby blade of wet grass, and hope another cow comes along and eats the grass. For much of 2011 in the drought areas, those conditions didn’t happen.

That’s not to say, however, that the hot and dry conditions completely sterilized cows and pastures alike. The worms are still there. It just makes the probability of the larvae getting into the animal much lower, Craig says.

But even in harsh conditions, problems can occur. “We saw some wrecks with flukes down on the Gulf Coast last summer,” Craig remembers. “What I think happened is cattle were going into areas where there was something green, even though it wasn’t very digestible. They were going in and picking up vegetation they would normally avoid. Those were the only places the snails could live.” (Read more about flukes here.)

Other internal parasites, like the brown stomach worm and Cooperia, likewise can survive extended drought conditions, he says.

“There was some work done following a drought in Australia about 10 years ago, where they showed a few worms could survive in that fecal pat a year or more,” Craig says. “As soon as it did rain, they got out and got into the animals grazing near the fecal pat. Normally the cattle wouldn’t graze that close to a pat, but that was the only place anything was green.”

That means, given the winter and early-spring moisture that much of the drought area has received, some reinfestation could occur. While Craig has some concern there, he cautions that it’s important to work with your veterinarian to make decisions on a ranch-by-ranch basis.

“I’m an advocate of letting the animals be exposed to a moderate number of parasites so their immune system can turn on. Now when you get to a feedlot, that doesn’t make any difference because it’s an all-in, all-out sort of thing. But cattle on pasture, I think they need some exposure,” he says.

The take-home message is that if there’s even a little green, there will be worms. The question is where’s the tipping point? “Parasites are a numbers game,” Craig says. “Keeping numbers low but not getting rid of all of them is the way the game should be played.”