If you're a brown stomach worm (Ostertagia), life is good.

That's particularly true if you're an Ostertagia with a southern accent. A combination of last summer's drought and a wet, cool winter created conditions ideal for heavy Ostertagia loads this spring, according to Tom Craig, Texas A&M University professor and parasitologist in the Department of Veterinary Pathobiology.

Cattle grazing drought-stressed pastures last year likely picked up a heavy load of Ostertagia when the rains began last fall, Craig says, because the cattle were forced to graze closer to fecal pats than they would under more normal conditions. “When it did start to rain in the autumn, there was an emergence of quite a few worms from the fecal pat,” Craig says. “And cattle were eating quite a bit closer to fecal pats because they didn't have any choice. And I think they probably picked up a load.”

In fact, he says he saw some cases of clinical ostertagiosis last fall, which is unusual. “I haven't seen clinical cases in quite a while.”

And that means cattle are likely carrying heavy Ostertagia loads this spring. “I'm looking for this to be one of those years where we're really picking up a high number of parasites,” he says.

Life cycle

Ostertagia's life cycle is direct, in that it doesn't use an intermediate host. Cattle pick up infective larvae as they graze, which enter the stomach lining and mature into egg-laying adults three to four weeks after they're eaten.

The adult Ostertagia then shed eggs and infected cattle pass eggs in the manure. With favorable weather conditions, the eggs hatch and develop into infective larvae in about 14 days. If conditions are hot and dry, the larvae will perish fairly easily, Craig says. In the South, this winter's wet, cool conditions were ideal for Ostertagia, Craig says.

Once they become infective, the larvae develop a protective coating and leave the fecal pat to wiggle their way up a blade of grass where they can be picked up by grazing cattle.

This wiggling is enhanced by a film of moisture from rain or dew. In the North, Ostertagia will overwinter under the snow and emerge in the spring when the temperature warms up. In the South, with its much milder winters, the infective larvae can be active all winter long. And since the worms aren't killed by frost, they'll wiggle around all winter if moist conditions exist. They may not wiggle as fast, Craig says, due to the cooler weather, but they're still out there wiggling.

If you've got cattle grazing, you're making for some happy worms. “And, of course, with cattle grazing closer to the fecal pat and closer to the ground, the higher the chance of transmission becomes.”

Why worry?

According to James E. Strickland, University of Georgia Extension veterinarian, it's estimated that annual parasite-related losses to the livestock industry total more than $100 million. But because most parasite losses are subclinical and losses go unnoticed and largely unmeasured, the figure could be much higher.

So even though your cattle may not show clinical signs of parasites, the damage is being done. In the case of Ostertagia, which is the most common internal parasite of cattle, the stomach and intestinal linings are damaged by irritation and inflammation, Strickland says. Therefore, cattle don't get the proper nutrients, due to decreased digestive and absorptive ability of the mucosal surface.

University trials indicate paybacks from internal parasite control of $25 to $200/head, Strickland says. That's because healthy cows do better.

According to “Fifty Years of Pharmaceutical Technology and Its Impact on the Beef We Provide to Consumers,” written by Rod Preston and Tom Elam, studies have shown that an effective internal parasite control program has several positive outcomes:

  • Beef cow weights and body condition scores (BCS) are improved; body weight increases 20-30 lbs. and BCS improves 0.2-0.4 points.

  • Conception rates increase.

  • Calf weaning weights are improved 20-40 lbs.

  • Heifers show improved growth rate (about 0.1 lb./day), reduced time to puberty (33% more reach puberty at 14 months of age) and improved conception rate.

Then comes spring

“Normally, we expect when the grass starts to emerge in the spring, the level of infection will go up considerably,” Craig says, because the larvae wiggle a lot faster as the temperature warms up.

That means spring is the ideal time to control internal parasites. Knocking back parasite loads now will help clean up pastures this summer and minimize the economic effect that internal parasites can have on cattle production.

Craig recommends treating in April or May. As the temperature gets warmer heading into summer, the Ostertagia residing in the cattle will go into a sort of hibernation.

“They go into the stomach and rest all summer long and come out in the autumn,” Craig says. “And that's why this time of the year is so important. If we can kill those guys while they're resting, we don't have the damage they do when they emerge from the glands (in the stomach where they've been resting) and we don't have the contamination of the pastures.”

This ability by Ostertagia to go into an arrested state varies by geography, he says. “They go into arrest in the spring and come out in the autumn in the South. In the North, they do the opposite. They go into arrest in the winter and come out in the spring after the grass has started growing.”

Then there are the states in between. “The problem with producers in the center states is that we've got some worms in the population that are winter arrest and some that are summer arrest,” Craig says.

Regardless of where you live, he says, spring deworming accomplishes the same goal; your cattle are ready to grow and gain on summer pasture and the worm load in your pastures going into the fall and winter is less.

For cattlemen who have fall-calving herds, Craig says it can pay to treat both the cows and calves. “If those calves are pretty good size coming into late spring, getting up to several hundred pounds, treating both the cows and calves is certainly called for,” he says. “If we've got really young calves at side, don't bother with the calves; put the emphasis on the cow.”

Typically, Craig says, if you worm in the spring, there's no need to retreat in the fall, unless you're fighting flukes. “If you're in a fluky area, then attacking the flukes (in the fall) but using something that gets the other worms as well is worthwhile,” he says. “In areas that are more marginal, I really doubt if it's needed.”

However, he cautions that having a conversation with your herd-health veterinarian is critical. “Veterinarians in local areas have a much better idea of what is happening,” he says, particularly if they're familiar with your herd.

It may be tempting to take your chances and hope the weather this fall and winter isn't conducive to larvae survival. Craig says that's a risky gamble. “I think we need to plan as if things are going to be right for the worm instead of right for us,” he says.

“I'd say, over seven years, there will be one year that you'd break even, one year where it might cost you a little bit more to deworm than you'd gain, and the other five years, you've made a heck of a lot. And we don't know which one of the years it's going to be before time.”

But he does know this: 2010 isn't the year to skimp on your internal parasite control program. “I really think that this year is a year that without fail, we've got to do some deworming before we get to the summer,” Craig says.