Parasites can also suppress cattle's immune response.

Parasites in cattle can do more than just nip at a producer's feed dollar. They can also take a big bite out of the effectiveness of a herd health program.

That's why producers should consider delaying vaccination for two to four weeks after parasite control is administered on moderate to heavily parasitized calves and yearlings, says a USDA researcher.

A moderate to heavy parasite load can suppress an animal's immune system, leaving it less able to fight off infection or fully respond to vaccination. That's on top of the usual, more well-known effects of parasites — appetite suppression, reduced production and lowered weight gain, says Lou Gasbarre, an Agricultural Research Service immunologist in Beltsville, MD, who studies nematode infections of cattle.

“If really interested in maintaining good herd health, producers have to keep their herd's parasite populations under control,” Gasbarre says. He advocates timing animal vaccinations to coincide with when the animal's immune system is most able to capitalize and build immunity to the target diseases.

“The two keys to an effective vaccination program are a good nutrition program and effective parasite control,” Gasbarre says. As a general recommendation, he says health vaccination of calves and yearlings should be delayed two to four weeks after treatment with a broad-spectrum anthelmintic.

This, he says, allows the animal's immune system sufficient time to recover from the effects of moderate to heavy parasite infection. The drawback, of course, is that calves must be physically handled twice, Gasberre points out.

“Right now, we don't have enough evidence that a light load of parasites would have the same effect on an animal's immune system,” he adds. But, the delayed vaccination appears to make sense for calves and yearlings with moderate or higher parasite loads.

He says a “moderate” infection level exists in birth-to-weaning-age calves if just one calf in a 20-calf sample of the herd is found to have a fecal egg count of 200 or more eggs/gram of manure. In yearlings, that threshold is 50 eggs or more/gram in a 20-yearling sample.

Bert Stromberg, University of Minnesota professor of parasitology, agrees with Gasbarre's recommendation to delay vaccination. While handling cattle a second time is a drawback, he says research shows the benefit of the practice.

Stromberg says he's promoted the concept in Minnesota producer meetings this past winter. “The reaction was good for the most part. Some producers felt they probably had enough contact with cattle in the spring that the practice was feasible. Others felt it wasn't,” he says. “In a perfect world, this is what I'd do.”

Ostertagia Plays Its Tricks

The most problematic parasite, Gasbarre says, is Ostertagia ostertagi, also known as the brown stomach worm. It's particularly adept in causing immunosuppression in cattle. It works like this.

As an animal develops, a fine balance is struck in the immune system between antibody responses and those based on responses by cells of the immune system. The first responds to extra-cellular challenges such as bacteria and parasites. The latter is designed to fight intracellular challenges such as viruses, malignant tumors and some protozoal parasites, Gasbarre says. It's the balance between the two that provides the animal with a broad spectrum of immunity.

Parasites, particularly Ostertagia ostertagi, upset this balance at moderate to heavy infestation levels, Gasbarre says.

Typically, when an immune cell comes in contact with a tumor or virus, it releases communicators called cytokines to the other cells of the body. These cytokines determine the animal's exact immune response, Gasbarre says.

“Parasites, and particularly Ostertagia, invoke very strong responses of special cytokines from cells called Th2 cells. These cytokines are very good at making an antibody response,” Gasbarre says. “But those Th2 cells turn off the response against viral agents which are stimulated Th1 cells.”

The result is that the animal can be left susceptible to viral challenge. But, Gasbarre points out, the effect depends on the total parasite load. Even then, the effect may be masked.

“If you were to vaccinate an animal with a moderate to heavy parasite load, you would still get an antibody response, which would lead you to believe that everything was fine. But, in actuality, moderate to heavy parasite loads have compromised the cellular response,” he says.

The equation is further clouded by the fact that that the effect across the herd won't be all the same.

“Parasites can interfere in the immune system response of some animals, while others suffer minimal effect, and some may have absolutely no effect,” Gasbarre says. “It's like cigarette smoking in humans. If you smoke, your chances of contracting cancer are greater, but you won't necessarily get it. In this same way, parasites raise the odds of animals being susceptible to infections and/or not responding to vaccination.”

Gasbarre stresses that a lightly parasitized herd on a good nutritional plane is not at risk.

“But, in areas of heavy parasite loads, producers should watch vaccination times to ensure they make use of their herd's immunity. They should manage to minimize the number of parasites. The idea isn't to completely get rid of them but to keep them well below the threshold for any kind of potential effect.”