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Finding PIs

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 that

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.

Finding the “needles”

Because of the low prevalence of PI animals, not all producers can justify whole-herd PI screening. However, if ranch history, a significant breech in biosecurity or changes in production practices increase the risk of PI cattle being present, a protocol to screen the herd can be defended based on the likelihood of economic return.

A new effort of the Montana Beef Quality Assurance program is the Montana BVD-PI Herd Screening Project. This past year, the project funded through the Montana Beef Network, has overseen the screening of nearly 35,000 head of cattle from 65 Montana operations. Of those, about 80% were calves sampled at spring branding. The remainders were replacement heifers (10%), weaned calves (5%) and breeding bulls and dams of PIs.

Only 24 PI cattle were found — with one rare “PI-pair” (a first-calf heifer and her calf). But project managers understand the 2006 Montana project didn't represent a statistically valid sampling of the state's cattle population.

Interestingly, the majority of PI calves (62%) were born from first-calf heifers. Conventional wisdom dictates that because BVD virus impinges on reproductive function, it's less likely a rancher will find an older PI cow. Over time, late-breeding cows or those that fail to breed are usually culled from cow herds; therefore, ranchers, through common-sense management, tend to rid PI cows from their herds.

This is one reason why buyers of older cows — especially “put together” cows — should be concerned about bringing disease onto their operations.

The Montana project contracted with Animal Profiling International (API), Portland, OR, as its laboratory collaborator. API promises “next business day” screening results.

API is using “pooled” polymerase chain reaction (PCR) analysis. Their lab pools samples from multiple animals (usually 28 or less) in a strategy that takes advantage of the high sensitivity of the assay while reducing the cost per animal tested.

This screening technique is a “sensitive and specific method of screening cattle for persistent infection with BVDV,” confirms James A. Kennedy, DVM, MS, in the November 2006 Journal of the American Veterinary Medical Association. Kennedy is head of the CSU Veterinary Diagnostic Laboratory, Rocky Ford Branch, and a pioneer in statewide BVD-PI screening programs.

Buying, selling and risk

Given recent advances in BVD-PI diagnostic techniques and reduced screening costs, some cattle feeders are screening all calves coming into their feedyards for PI status. Some feeders are asking ranchers to screen their herds before or at weaning.

Calves, heifers and bulls are also showing up on video sale listings, market Web sites and sale literature as being “BVD-PI-screened” or “BVD-PI-free.”

Most observers believe it's a misnomer, however, to advertise cattle as being “BVD-Free” — as it's nearly impossible to guarantee any animal or herd is free of transient BVD virus. In fact, care should be taken when guaranteeing cattle are “BVD-PI-free,” as no sampling protocol or analytical procedure is immune from error and can be considered 100% perfect.

Nevertheless, Montana project managers have had calls from out-of-state cattle feeders asking for a list of ranchers who have participated in the screening project. They understand the risk of receiving PI cattle from screened herds is nearly nil, for all practical purposes.

It's highly recommended that all ranchers — both seedstock and commercial — who market breeding cattle, screen for PI status prior to sale. If the animals' health management history is unknown, buyers have every right to ask for PI-status screening prior to delivery.

Montana project managers don't necessarily recommend perpetual or annual whole-herd PI screening on a cow-calf operation. They suggest ranchers who whole-herd screen and don't find a PI animal can be “reasonably assured” of minimal PI-risk given the following basic management:

  • Vaccination protocol based on veterinary advice.

  • A sound biosecurity program that includes screening all herd additions before they enter the operation.

  • A firm understanding of the BVD virus and disease transmission.

There may be justification on a herd-by-herd basis for continued whole-herd screening, especially calves from younger cows (2- and 3-year-olds), if:

  • Any PI or transiently infected animal is found in the herd, or if there's unusual sickness, reproductive failures, abnormal births, calving losses, etc.

  • A serious biosecurity breach (in-common grazing situations, fenceline contact with suspect animals, accidental mixing of breeding herds, etc.).

  • Risk for other reasons (suspected vaccination failure or gaps in vaccination protocol).

The Montana BVD-PI Herd Screening Project plans to screen at least 100,000 Montana calves in 2007, with support from the Montana Stockgrowers Association, implementing the same basic protocol as 2006. This will take a concerted effort on behalf of ranchers, veterinarians and others in the industry. But the rewards will be reduced sickness and subsequent treatment for disease, and an improvement in the health of the state's overall calf crop.

The challenge is for other states to develop programs designed to find and eliminate as many BVD-PI cattle as practically possible. This is the most reasonable means of getting a handle on this disease nationwide.

Finally, Montana project managers highly recommend all ranchers work with their attending or consulting veterinarians to develop a herd-health protocol that considers all BVD virus risk factors and incorporates the above management components, including a common-sense biosecurity program.

Clint Peck, Billings, MT, is former Senior Editor of BEEF magazine.

Screening Basics

Screening on the ranch for cattle persistently infected (PI) with bovine viral diarrhea (BVD) should be part of a program involving vaccination and herd biosecurity. The screening protocol assists the producer in finding all PI animals in the herd and reduces the BVD-PI risk in new arrivals.

Some points to remember:

  • Transient BVD virus infection differs from persistent infection.

  • PI calves are “created” during gestation.

  • Born a PI, always a PI.

  • Not born a PI, never a PI.

  • Test the herd before bull turnout.

  • PI females will always produce a PI calf.

  • Mature cows normally don't need screening unless they have a positive PI calf.

  • Individual ID is critical to match all samples with the animal tested and match the calf with its dam.

  • Cattle testing negative for BVD-PI status never need to be retested.

  • PI surveillance should include the necropsy examination of as many aborted fetuses, stillborns and pre-weaning deaths as possible.

  • A plan should be developed to eliminate PI animals from the herd.

For more on the Montana BVD-PI Herd Screening Project, visit and search under “Projects.”

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