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October 2006 Issue

Just what is the economic impact of BVDV on the cattle industry?

Bovine viral diarrhea virus (BVDV) is widely reported as one of the most costly viral diseases for the cattle industry worldwide. There have been several studies estimating the economic impact of the disease in beef and dairy cattle. There continues to be, however, considerable discussion and debate regarding the economic impact in specific populations of cattle. This article attempts to summarize key findings from various articles and presentations regarding the economic impact of BVDV.

Background
Bovine viral diarrhea virus is one of the most important pathogenic viruses in cattle. The virus crosses the placenta in infected pregnant cows, causing reproductive losses due to abortions, stillborn calves or calves that die early in life. In addition, when cows become infected between days 30 and 150 of gestation, some calves survive and are persistently infected (so-called PI animals), immunotolerant to the virus. These PI cattle are the main source of BVDV transmission, producing and excreting large amounts of virus. As a result, most eradication and control programs focus on the elimination of PI animals and preventing the transmission of BVDV to other healthy cattle in the herd.

Prevalence of PI animals has been measured at approximately 0.1% to 2%, based on numerous studies around the world. However, despite low prevalence levels in the overall cattle population, the risk of introducing a persistently infected animal into a herd or group of cattle can be quite high in most areas.

The risk of introducing a PI animal is a function of both the prevalence of PI animals in the source population and of the number of animals purchased. For example, if you purchase 500 animals from a population of cattle where the prevalence of PI cattle is 0.3%, you have a 77.7% chance of introducing at least one PI animal. A more complete summary of risk probabilities are represented in Table 1.1

This is critical since exposure to a PI animal, for other animals in the same and adjacent pens, has shown, in a study of U.S. feedlot cattle, to have a detectable adverse effect on morbidity rate compared to non-exposed cattle in a study of U.S. feedlot cattle. This same feedlot study also showed a 43 percent increased incidence of treatment for bovine respiratory tract disease (BRD) in non-PI cattle exposed to a PI animal.2

In addition, numerous studies, summarized in Table 2, have reported significant economic impacts ranging up to $80 or more per head. Recent large U.S. feedlot research trials in Kansas found exposure to PI animals to have a negative impact on both average daily gain and total production costs.3

Cow-calf production

Background Persistently infected (PI) cattle are the major source for BVD infection and disease in cattle that come in contact with them. PI cattle became infected before they were born (before 125 days of gestation) and shed huge amounts of BVD virus throughout their lives. The common ways BVD is introduced into herds are through herd additions that are PI or contact with other PI cattle, including PI calves, yearlings, bulls, females and fetuses carried by pregnant females.

• Any calf, replacement heifer, bull or cow can become temporarily infected with BVD virus for a few days to weeks until their immune system can clear the virus. The disease is usually not fatal by itself, but BVD virus suppresses the immune system and may make infected cattle more susceptible to diseases such as pneumonia, scours, footrot and others. The virus may also cause infertility and/or abortion in susceptible cows, heifers and bulls.
• Testing for PI cattle is different from testing for many other animal diseases in that PI status stays the same throughout the animal’s life. In other words, a non-PI animal will be negative its entire life and a PI animal will remain so its entire life. Because of this fact, PI testing is usually done only once. A test for PI status only needs to be repeated to confirm a positive or if evidence indicates a faulty test. As with all tests, a few false-positive and false-negative results can occur.
“…BVD virus suppresses the immune system and may make infected cattle more susceptible to diseases such as pneumonia, scours, footrot and others.”
• Vaccination is a tool to manage BVD and persistent infection, but it cannot be relied upon to keep a herd free of PI cattle.

Designing best plans for BVD control and eradication strategies

• Meet with your veterinarian to determine or review your BVD goals and current exposure risk.
• Introduce only PI-negative animals into the herd. Calves from purchased pregnant females must be test-negative before the pair is added to the resident herd.
• A number of tests for PI cattle are available; work with your veterinarian and diagnostic laboratory to determine the best testing strategy for your situation.
• Prior to the start of the breeding season, test all calves and all non-pregnant females without calves (aborted, never confirmed pregnant, lost a calf for any reason) that have not been tested previously. Isolate pregnant females until they calve and their calves can be tested.
• Some herds may require annual calf testing.
• Prior the start of the breeding season, test all bulls and replacement females (home raised or purchased) that have not been tested previously for PI status.
• Maintain documentation of test results.
• Work with your veterinarian to design a vaccination program that considers vaccine type and timing. This includes considerations of bull turn-out, pre-weaning, weaning, heifer pre-breeding, and mature cow vaccination to optimize fetal protection.
• Work with your veterinarian, neighbors and replacement suppliers to identify and minimize risk of introducing BVDV to your operation.

What to do with a positive test result

• Immediately consult with your veterinarian to determine herd goals and appropriate measures.
• If any calf comes back as a suspected PI animal it may be euthanized immediately or held in isolation until a second confirmatory test is run. If a calf is confirmed as a PI, it should be euthanized immediately to eliminate the major source of BVD virus to the breeding herd.
• The mother of PI positive calves should be tested (there is a chance that the dam is also a PI).
• Cows with PI calves that are not PIs themselves may not need to be culled.
• Work with your veterinarian, neighbors, and replacement supplier to identify and minimize risk of introducing BVDV to your operation.

Two ways PI calves are produced

“When producers were asked if they'd received any benefit from the program (Colorado BVD Control Program), every one of them said they had,” Kennedy explains. “Some attributed the benefit to increased dollars for their calves and some to decreased illness in the herd.”

Patsy Houghton, Heartland Cattle Co., McCook, NE, also sees the value of BVD biosecurity and PI elimination. “The cost of even low PI prevalence in cow herds can be very, very high,” she says. Houghton says ranchers should look at BVD PI control as an investment, not an expense. And, she's a firm believer in addressing the point of BVD infection—the mother cow. “If we can keep the BVD virus from reaching the fetus, we'll prevent any more BVD PIs from being produced,” she says.

Vaccination alone will not solve BVDV problems.

AVC Contact: Dr. Bob L. Larson, Chairman, AVC BVD Committee
E-mail: rlarson@vet.ksu.edu

Stocker/feedlot production

Background

• PI cattle are the major source for BVD infection and disease in cattle that come in contact with them. PI cattle became infected before they were born (before 125 days of gestation) and shed huge amounts of BVD virus throughout their lives.
• Any feeder animal can become temporarily infected with BVD virus for a few days to weeks until their immune system can clear the virus. The disease is usually not fatal by itself, but BVD virus suppresses the immune system and makes infected cattle susceptible to diseases such as pneumonia, scours, footrot and others.
• Vaccination is a tool to manage BVD, but it cannot be relied upon to prevent disease if PI cattle are present.

Designing best plans for BVD control and eradication strategies

Meet with your veterinarian to determine or review your BVD goals and discuss control options Consider sourcing previously tested cattle Consider arrival testing Initiate or continue a modified live (MLV) vaccination program Euthanize or isolate PI cattle away from other cattle Discreetly inform the source of cattle when PI test result is positive (may involve veterinarian-to-veterinarian contact)

“In our trial, five of 24 — or 21% of the test pens — had at least one PI animal,” Hessman says (Dr. Bill Hessman, Haskell County Animal Hospital, Sublette, KS).

BVDV-PI cattle are more likely to become chronically ill or die than non-BVDV-PI cattle. Also, the probability for initial treatment with respiratory disease was 43% greater for cattle exposed to BVDV-PI cattle in the same pen or an adjoining pen.

Vaccination alone will not solve BVDV problems.

AVC Contact: Dr. Bob L. Larson, Chairman, AVC BVD Committee
E-mail: rlarson@vet.ksu.edu

Improvements to the U.S. HerdChek* BVD Antigen Test Kit

We are pleased to announce several improvements to the U.S. HerdChek BVD Antigen Test Kit that will be available starting later in 2006.

How to ensure consistent performance with xChek* software

When running ELISAs, a good measure of test performance is the reproducibility of the positive and negative control. xChek offers a report that displays not only data on the control replicates, but also control tracking over time.

It is important to review control data often to ensure that values are not trending or exhibiting excess variability. Control values for a specific lot should be similar on a run-to-run basis.

To run a control tracking report from xChek:
• Click on Report > Control Tracking
• Highlight the test and check by well (% diff) > OK. A control tracking report will be displayed
• Specify a date range if necessary

We recommend that the difference between control replicates be no greater than 20%.

Another key component of the control tracking report is that it also tracks the lot number of the kit being used. This is very useful information when evaluating quality control data. The kit lot information can be entered into the software in two ways: either through database assays or in plate view at the time that you are reading your plate.

Events around the world

• Minneapolis, Minnesota, USA—October 12–19, 2006
  USAHA/AAVLD
• Barcelona, Spain—October 17–18, 2006
  AVEDILA/Expoaviga
• Pau, France—October 25–27, 2006
  ADILVA
• Foz do Iquacu, Brazil—October 25–27, 2006
  XIII Congresso Latino Americano de Suinocultura—Pork EXPO
• Lyon, France—November 8, 2006
  Emerging Diseases Congress
• Hannover, Germany—November 14–17, 2006
  EuroTier 2006: International DLG Exhibition for Livestock & Poultry Production and Management

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*HerdChek and xChek are trademarks or registered trademarks of IDEXX Laboratories, Inc. in the United States and/or other countries. All other product and company names are trademarks or registered trademarks of their respective holders.