Rational thinking would cause one to figure, in a drought as epic as this year’s, that worm larvae have a snowball’s chance of surviving. Pastures have a crisp, toast-like patina and there isn’t enough moisture and green grass to entice any self-respecting Ostertagia larvae to leave that nice, homelike cow patty and brave the elements.

One would be wrong. A drought does not play fair. Neither do parasites.

“What’s interesting about drought is it seems to increase the risk of internal parasites,” says Gerald Stokka, Extension livestock stewardship specialist at North Dakota State University. “I think it has to do with grazing activity.”

The life cycle of Ostertagia ostertagi, the brown stomach worm, is pretty straightforward. Adult worms, living in the animal’s abomasum, produce eggs that are passed out in the manure. The eggs hatch in the fecal patty and the juvenile larvae wiggle up a nearby blade of grass and wait for another animal to come along and ingest them. Back in the animal’s stomach, they grow to adults and start the whole process over again.

“In normal situations, cows are pretty fussy,” Stokka says of their grazing habits. “They don’t normally eat right up next to a fecal patty.”

But when you have dry conditions and there’s less forage to eat, cows will eat pretty much everything, and the grass is greener next to a cow patty. “So some of the parasites may not last near as long once they hatch out, but because of grazing behavior, the risk of internal parasites actually increases during dry weather.”

That sets the cow and her calf up for several unsavory things to happen, says Phillip Kesterson, a Bridgeport, NE, cow-calf veterinarian. Internal parasites affect cattle two different ways, he says. One is that they reduce the animal’s appetite; the other is that they suppress the animal’s immune system.

“On a relative basis, with feed being short and animal condition being tough, there’s possibly a greater advantage to deworming now as opposed to when times are good,” he says. If you take a little away from an animal when forage is lush and plentiful, the animal can still flourish.

“But if you take a little away from an animal that’s already marginal, that’s a problem. Those parasites are taking a little bit away from that animal on a minute-by-minute basis; if feed quality and quantity is reduced, that’s bad.”