Myth-Busting BVD Virus Eradication: Is it Possible?

Dr. Michelle Arnold, Ruminant Extension Veterinarian

“BVD” or “Bovine Viral Diarrhea” virus contributes to a wide range of reproductive, respiratory, and digestive system diseases in cattle. Although symptoms of the initial virus infection are typically mild such as fever and possibly off-feed for a day, there is much more going on than meets the eye. In calves, the BVD virus is immunosuppressive, predisposing infected calves to secondary bacterial infections particularly in the lungs, leading to significant sickness and death loss from bronchopneumonia in the stocker/backgrounder sector. In naïve, susceptible (non-vaccinated or poorly vaccinated) adult cows and heifers, infection with the BVD virus often goes completely unnoticed but ultimately results in reproductive failure, including infertility, early embryonic deaths, abortions, stillbirths, malformed calves, and weak newborns depending on phase of gestation when the female becomes infected. If a pregnant, susceptible cow or heifer is infected with the BVD virus between 42-125 days of gestation, the virus will also cross the placenta, infecting her unborn calf. When this calf is born, it is “persistently infected” or “PI” and is a “carrier” of the virus for its lifetime. The dams that experience a transient infection while pregnant will be negative when tested for BVD but their PI calves will test positive.

BVD Persistently infected or “PI” animals are the most unique epidemiological feature of the BVD virus. “PI” animals are the major reservoir for the virus and the reason it continues to exist today. A BVD-PI calf is born with the BVD virus and serves as the primary source of virus transmission because they continuously shed enormous amounts of virus particles throughout their lives in feces, urine, saliva, and nasal discharge. Many die at an early age but if a PI survives to adulthood, virus is also secreted in milk, semen, uterine secretions, and aborted fetal membranes. Approximately half of PI animals appear absolutely normal, and infection can only be detected through testing. The virus is deposited in watering troughs, feed troughs, cattle trailers-virtually everywhere the PI animal goes-and picked up by the other cattle in the pen or herd. Vaccines used in adult cattle against BVD (including those with Fetal Protection claims or “FP” vaccines) will reduce the chance of fetal exposure but protection is never 100%. Use of a modified live (MLV) BVD vaccine, with at least one two-shot (primary and booster) series given to breeding age heifers, is necessary to provide strong BVD PI protection as killed vaccine has not proven effective in this regard.

Currently there is much debate surrounding BVD PI calves, including how to best identify and remove them since one PI animal may expose 200-300 other animals or more to the virus during shipping, in auctions, when commingled in stocker/backgrounder operations and through fence-line contact with neighboring cattle. Several European countries, including Switzerland, Ireland, Scotland, and Germany, have substantially reduced or successfully eradicated BVD virus through national programs based on testing newborn calves and removing those that are PI positive. Of those countries, Germany is probably most comparable to Kentucky’s situation in that they began in 2011 with a high prevalence of BVD, their cattle industry involves frequent trade and transport of cattle, and BVD vaccination is permitted (both killed and modified-live). From 2011 to 2016, the proportion of PI animals in Germany dropped from 0.5% at the start of the control program to just 0.03% PIs remaining, with 48,000 PI cattle removed in 5 years. Although Germany’s “success story” of a mandatory, nationwide control program is enviable, could something similar be implemented in the U.S., including in Kentucky?

To begin, Germany has, as of November 2023, 10.8 million total cattle (3.7 million dairy cows and 625,000 beef cows) with over 4 million calves born annually on just over 127,000 farms. Approximately one quarter of the farms have over 100 head, while a little over 50% of farms have under 50 head. When the control program began in 2011, Germany had a high prevalence of BVD (0.5%) despite vaccination use, and a considerable amount of cattle trade and transport. For comparison, Kentucky has, as of January 2024, significantly fewer head at 1.89 million total cattle (907,000 beef cows) with 920,000 calves born in 2023. Kentucky’s land mass is approximately 3.5X smaller than Germany, with 73,500 farms in the Commonwealth at the end of 2022 with an average farm size of 176 acres. The BVD prevalence in KY is estimated to be, on average, 0.4% or 4 PI animals per 1000 head, most of which are young, lightweight calves.

The German control program began in 1998 as a voluntary effort run independently by the 13 individual federal states. Costs were high and essentially no progress was made over a 10-year period. In 2008, the German government unveiled a consistent, nationwide BVD eradication program with two major objectives; 1) the fast and efficient removal of PI animals and 2) the establishment of certified BVD virus-free farms. Beginning in January of 2011, the eradication program was implemented with the following rules:

  1. Mandatory testing of all newborn calves within the first 6 months of life, shortened to 1 month of age from 2016-2021.
  2. Immediate elimination of all detected PIs.
  3. Only BVD negative animals could enter commerce/be sold. In 2016, movement restrictions were imposed on farms with BVD, including pregnant animals could not be sold until after calving and a negative test result of the offspring since a negative dam can deliver a PI positive calf.
  4. Prevent reinfection on negative farms through implementation of biosecurity and vaccination protocols.

To remove PI cattle, a case definition was required to describe what legally constitutes a PI animal. A persistently infected (“PI”) animal was defined as:

  1. Tested positive for the BVD virus antigen with an ELISA test (primarily by ear notch in calves or blood test in adults) or the BVD virus genome with PCR. If desired, producers were allowed to test any positive animal a second time, up to 40 days later, to differentiate PIs from transiently (short-term) infected (“TI”) animals.
  2. All offspring of a PI positive dam were considered positive without the need for a test since a PI positive cow will always produce a PI positive calf.
  3. Any cattle diagnosed with mucosal disease, a fatal form of BVD that involves severe and bloody diarrhea, rapid weight loss, ulcers in the mouth, nose and interdigital areas of the hoof, and death, only occurs in PI cattle.

Between the start of mandatory testing in 2011 and 2022, there were approximately 5 million BVD tests run per year, including all calves born, any follow-up confirmatory testing of positives, and any imported cattle. The proportion of PI animals was reduced each year starting with 0.5% (23,792 PI calves among 4.9 million newborn calves) in 2011 to less than 0.001% (55 PIs among 4.3 million newborns) in 2022. In the first 5 years, PI animals were found on over 8000 farms in 2011 and only 324 farms in 2016, meaning more than 99.8% of all German cattle farms had no PI animals detected in 2016. The final phase of eradication involves molecular sequencing of the virus in the remaining PI animals to trace back to their herds of origin and contact herds.

In summary, BVD virus exposure can be quickly and substantially reduced, primarily through early testing and removal of newborn PI calves before they ever leave the farm of origin. However, many questions remain as to how a control program would be implemented and the effect this reduction in PIs would have on overall cattle health. BVD-infected cow/calf herds experiencing losses in reproductive performance and higher calf morbidity and mortality would ultimately benefit from diagnosing and eliminating BVD virus but at the cost of testing and subsequent culling of PI animals. Is there sufficient value to the industry in removing BVD-PIs to compensate the cow/calf sector for those additional costs? Should producers receive indemnity payments for PI calves that must be euthanized and, if so, what is a PI calf’s value? Are calves that test BVD negative worth more? It is important to understand that even one PI in a pen of cattle or in a cattle pot results in continuous virus exposure for the rest.

In the stocker/backgrounder world, BVD virus is just one contributor to the bovine respiratory disease (BRD) complex involving numerous pathogens (viruses and bacteria) interacting in many ways in a wide range of management and environmental conditions across multiple types of operations. Although BVD is not the ultimate cause of death, its immunosuppressive impact increases the severity of infections by other BRD organisms and often increases morbidity and mortality rates, especially in recently weaned, lightweight calves. However, it is not known how much BRD’s impact would be reduced if enhanced BVD control could be achieved. The most commonly used sample for identifying PI cattle is skin, usually taken as an ear notch. Blood (serum) can also be used but not in calves less than 3 months old. Any BVD ELISA positive test result can be confirmed, if desired, by segregating the animal and retesting a second ear notch or blood drawn at least 3 weeks after the first sample. True PI animals will remain positive after 3 weeks while transiently infected (“TI”) will test negative. Remember PIs are considered defective and there is a legal, moral and ethical obligation to dispose of these animals without sending or returning them to commerce. In Kentucky, transportation or sale of BVD positive animals is prohibited by law unless approved by the State Veterinarian. Positive animals may be euthanized, immediately slaughtered (does not affect meat), or quarantined and fed to slaughter in an isolated location or permitted feedlot.

Source: Ohio State University