Health management programs are popping up around the country to investigate the role of screening cattle for persistent infection (PI) of the bovine viral diarrhea (BVD) virus. This is a disease that costs the U.S. cattle industry an estimated $2 billion annually.

The focus is mostly on whole-herd screening of cattle to find PI calves. It's usually an effort of finding needles in a haystack — but “needles” that can do a lot of damage on individual ranches and greatly impinge on the reputation of a cattle herd. The impact of at least one PI animal in a herd has been conservatively reported to range from $14-24/cow/year.

The programs also emphasize how important ranch biosecurity is in preventing BVD and in the “elimination” of PI calves. They also emphasize the relationship between sickness in cattle and the quality of the end product. BVD as a disease is strictly a cattle-management problem and has no human health implications.

The main negative health effects of the BVD virus are it can inhibit conception and cause abortion in susceptible females. Beyond reproduction, the concern centers on suppression of the animal's immune system, which causes infected animals to be more susceptible to other diseases. In some cases, BVD's effect on the immune system is more critical than the direct acute effects of the virus.

Effective BVD-PI control becomes a “management-over-medicine” situation, which should begin at the ranch so PI animals can be eliminated before they enter commerce. BVD-PI elimination also has significant beef quality and consumer confidence implications according to the research by Gary Smith, Colorado State University (CSU) meats scientist and a top authority on beef supply chain management.

“Each time you treat an animal for sickness in a feedyard, you run the risk of losing a quality grade and tenderness score,” Smith says.

“Creation” of PIs

Management and control of BVD in cattle herds must consider two ways the virus spreads. The first is horizontal transmission, when a transiently (temporarily) infected animal releases a virus through bodily secretions and enters a susceptible animal through the mouth or respiratory tract. The second is vertical transmission from an infected dam's bloodstream to her fetus during pregnancy.

Calves with persistent BVD virus infection can develop in the uterus if the heifer or cow is exposed to the virus during gestation. In fact, this is the only way a PI animal is “created.” Most data suggest the critical exposure period is 40-125 days after conception.

Fetal infection can lead to fetal death, the birth of a normal calf or the birth of a PI calf — meaning the infection lasts the entire life of the animal.

Once a calf is born PI with the BVD virus, it will always be PI. If an animal is not PI at birth, it can never become PI. Breeding-age PI females will always produce a PI calf and will also remain a source of horizontal transfer of BVD virus.

Although a high percentage of PI calves die at or near birth — or at least by weaning — many PI calves survive and can appear healthy. But PI animals usually have a very high and persistent amount of virus circulating in their blood and other fluids, shedding BVD virus continually.

It's critical that BVD management at the ranch focus on removing PIs from the herd before cows are bred, to reduce the risk of exposing fetuses to PI animals. In fact, a heifer or cow may not be PI, but her calf can be PI. That's why most ranch screening programs should first focus on finding PIs in the new calf crop. If a PI calf is discovered, then the dam can be tested for PI status.

Of course, aborted fetuses, dead calves, deformed calves, heifers that won't breed or stay bred, or other suspect cattle should be screened for PI status routinely. Tissue samples from freshly dead cattle can be submitted for analysis.

The scientific literature indicates BVD-PI has a clustered distribution, which means a few herds may contain several PI cattle but most herds contain only normal, non-PI cattle.