– Michelle Arnold, DVM (Ruminant Extension Veterinarian, UKVDL)
Concern is mounting in KY regarding the identification and subsequent movement of cattle persistently infected with the Bovine Viral Diarrhea virus (or “BVD-PI” animals) into livestock sales. The BVD virus is known to cause severe immunosuppression and also works synergistically with other viruses to make them more deadly, resulting in substantial respiratory disease and death loss in the stocker/backgrounder industry. What is largely unrecognized is the effect of a BVD-PI calf on the cow/calf operation where it was born or raised. Infection can cause reproductive disease (delayed breeding, abortions, malformed calves, PI calves), respiratory disease, enteric (“gut”) disease and immunosuppression (destruction of the white blood cells needed to fight infection). This article addresses some of the common myths surrounding this virus by explaining the nature of the virus, its broad impact and the difficulty of controlling it through vaccination alone.
Myth #1: Since BVD is a virus, it does not last long in the environment and dies quickly when it freezes.
The BVD virus is a “single-stranded RNA virus” which is very stable under moist and cool or cold conditions. It is not affected by freezing and can easily survive at least a week in the right environment. Its enemies are soap and water and hot and dry conditions. It can only be spread short distances through large “droplets” (especially saliva and nasal discharge) and cannot be spread by the wind.
Myth #2: As a backgrounder, if the calves make it past 30 days after arrival and I have two rounds of vaccine in them, I am “home free”.
Not necessarily. The BVD virus can easily mutate or change while reproducing itself and has the ability to pick up pieces of other viruses and stick them inside its own genetic material. This can lead to rapid change (mutation) from a low virulence strain (not very “mean”) to a killer virus. If a PI animal remains in the pen, he continually sheds BVD virus that can mutate. Infection with this newly formed strain may result in a respiratory break after 30 days and can cause significant sickness and death. After infection, it takes an average of 14 days to clear the virus from a “transiently” infected calf but it may last up to 28 days or more.
Myth #3: PI calves are easy to identify because they are stunted, grow poorly and usually die young.
If it were only that easy! PI animals may have congenital defects or may appear completely normal. To illustrate, the prize-winning bull in the 2000 Wisconsin State Fair was tested and found to be a PI. The “PI” animals are the major reservoir for the virus and the reason BVD disease continues to exist. Given the importance of this issue, it is essential to understand what a “PI” truly is and how one is found. The problems begin when a pregnant cow or heifer is infected with the BVD virus between 42-125 days of gestation. The virus crosses the placenta, infecting the unborn calf. When this calf is born, it is “persistently infected” or a “PI” calf and can be considered a “carrier” of the virus for its lifetime. Most PIs are born to heifers who were naïve at the time of exposure. PI animals are the primary source of virus transmission because they shed an extremely high number of virus particles throughout their lives in feces, urine, saliva, nasal discharge, milk, semen, uterine secretions, and aborted membranes. The virus is deposited in watering troughs, feed troughs, cattle trailers-virtually everywhere the animal goes-and picked up by the other cattle in the pen or herd. Although it is often assumed PIs will die young, some survive well into adulthood and have calves or can be fed to slaughter weight.
Myth #4: I tested all of the calves born this year in my herd and found one PI calf. The vet euthanized the PI so my other calves (that all tested negative) should be fine.
One PI calf usually indicates big problems on a cow/calf operation. Any fetus infected while in the uterus by the virus but did not become a PI will still not be normal. The virus commandeers cell functions to produce more virus that normally are used for fetal development of immune tissue. The virus destroys endocrine tissue and may destroy 20-80% of the thymus gland, an important driver of immune function in calves. These calves will have increased respiratory disease, poor performance, and reproductive issues if they reach sexual maturity. Bulls infected before sexual maturity may have BVD virus persist in the testes and produce BVD-infected semen.
Myth #5: BVD-PI is a problem for the backgrounder; there is really no economic benefit to a cow/calf producer to find a PI calf in his/her herd, especially if forced to euthanize a positive calf.
Bottom line: if one PI calf is out in the pasture constantly shedding virus during breeding season, many (if not all) of the cows/heifers will be exposed to the virus during the highest risk time that may result in very expensive clinical herd problems such as:
- Poor reproductive performance/ rise in infertility (despite good nutrition and fertile bulls)
- Decrease in overall pregnancy rate and % pregnant after the first service. This “delayed breeding” is often blamed on the AI technician, a dud bull or hot weather when really it is a viral problem.
- Abortions, stillbirths, neonatal deaths, and weak calves
- Physical abnormalities (dummy calves, eye defects, cleft palate) in neonates
- Calf loss due to pneumonia or scours before weaning
Other possible sources of the BVD virus in a cow/calf herd include introduction of new cattle (including bulls) into the herd without testing for BVD, fence line contact with feeder calves or the neighbor’s herd, and populations of wild animals (such as deer) on the farm. Show cattle can bring the virus back when they return to the farm. A calf purchased from a sale to put on a cow who lost her calf at birth may be PI. A purchased pregnant cow or heifer may be negative for BVD yet she is carrying a PI calf.
Myth #6: I vaccinate my cows annually against BVD so my herd is fully protected.
Unfortunately, no. Vaccines against BVD (including those with Fetal Protection claims or “FP” vaccines) will reduce the chance of fetal exposure but protection is never 100%. Vaccines may fail due to problems with the vaccine itself, the animals, and/or management errors. The current BVD vaccines available contain BVDV 1a and BVDV2a strains. These vaccines were quite effective when strains 1a and 2a were the most prevalent types. However, the most common type of virus circulating now on farms in the US is BVDV1b so the vaccines are not as protective. Problems within the animals themselves may prevent good vaccine response. Animals that are sick when vaccinated, too stressed to respond, in poor nutritional status or too young to produce antibodies will not be protected with vaccination. A PI calf within a herd will suppress immune response from vaccine in all of the other calves it contacts. Finally, yet importantly, management errors are an all-too-common cause of vaccine failure. These may include:
- Not giving 2 doses of killed vaccine as described on the label
- Improper mixing of vaccine (shaking violently rather than swirling)
- Failure to use modified live vaccine within 1 hour of mixing (VERY COMMON ERROR)
- Inappropriate storage either before or during use of the product (must be kept cool)
- Use of expired vaccine
- Use of soap, detergent, or disinfectants to clean the inside of multi-dose syringes used to inject modified live vaccine (inactivates vaccine)
- Poor timing: The immune system needs two weeks to develop a protective response from a vaccine before challenged with the virus.
Diagnostic testing for BVDV PI is inexpensive and easy. 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 young stock (calves less than 3 months old). Any BVD ELISA positive test result (at the UKVDL) should be confirmed 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 still be positive after 3 weeks while transiently infected will test negative. Other laboratories may have different protocols so check the laboratory on the need for confirmatory testing. Remember PIs are considered defective and there is a legal, moral and ethical obligation to dispose of these animals without sending/returning them to commerce.
If you have questions, please contact Dr. Michelle Arnold at 859-257-7190.
Much of the information in this article was from a recent webinar by Dr. Julia Ridpath, an internationally-recognized expert in bovine respiratory disease and BVD. This webinar was made available to extension veterinarians to help educate all parties involved in cattle production on the many faces of this disease.