Authors: Respectively, Extension Livestock Specialist, Clayton Livestock Research Center; and Extension Veterinarian, Department of Extension Animal Sciences and Natural Resources, both of New Mexico State University.
Cattle persistently infected (PI) with bovine viral diarrhea virus, commonly known as BVDV, represent a low-prevalence but high-cost disease affecting all segments of the industry due to limited monitoring and difficulty of diagnosing PI animals. This publication focuses on transmission routes and periods of susceptibility to BVDV, how to determine if an operation is low- or high-risk for contracting BVDV infections and PI animals, and how to develop a bio-security plan to protect an operation against BVDV and PI animals.
Prevalence of BVDV and PI Calves
Though a validated strategy to determine the status of BVDV and PI does not exist, it has been estimated that up to 10% of beef cow herds have at least one PI animal, and 1% of all calves born in the U.S. are born PI. In 2002, a survey of randomly selected beef herds in five geographically diverse states (Alabama, Nebraska, Nevada, North Dakota, and Ohio) found that 3.9% had at least one PI animal, and nearly 3 out of every 1,000 calves were PI. Approximately 20 to 50% of PI calves are expected to die prior to weaning. Therefore, the prevalence rate of PI calves moving from the ranch to a stocker pasture or feedlot is 0.08 to 1.95%. Most PI calves will die before 24 months of age.
Bovine Viral Diarrhea Virus
Clinical symptoms of BVDV include profuse diarrhea, fever, nasal discharge, and ulcers on the inside of the mouth and between the toes.
Bovine viral diarrhea virus is commonly classified as a reproductive disease due to immense losses experienced with first trimester abortions, fetal abnormalities, pre- and post-natal growth retardation, and the birth of PI calves. However, the disease is classified as a transition disease because of its impacts to the stocker and feedlot phases. The virus can infect cattle through three infection routes: in utero, exposure to PI animals, and acute infections.
In utero Infection
Depending on the stage of gestation when a cow is exposed to and subsequently infected with BVDV, infection may result in fetal resorption, abortions, PI animals, or normal calves with a strong immunity.
Exposure to BVDV during the first 60 days of pregnancy will most likely result in a dead fetus that will be absorbed, aborted, or stillborn. Exposure to BVDV between days 60 and 150 of pregnancy most often results in PI calves. After approximately 150 days of pregnancy, the bovine fetus develops a functioning immune system that can mount protection from most strains of BVDV. Therefore, when cattle are exposed during the third trimester of pregnancy, the likelihood of fetal infection or abortion substantially decreases, but is not eliminated.
Fetuses infected with BVDV before 125 days of gestation do not recognize the virus as an infection; instead, it is recognized as a normal circulating agent. As a result, the virus is incorporated into the fetal genetic make-up so the calf does not mount an immune response. Ultimately, the virus survives and the calf will shed BVDV through all body secretions for the remainder of its life. These calves are termed PI or persistently infected. Persistently infected calves are a source of BVDV exposure to all cattle they contact.
Persistently infected animals are the primary reservoir for BVDV in all segments of the industry. These calves do not always present the clinical symptoms of the virus, but are often "poor doers" with a suppressed immune system, making them more susceptible to other calfhood diseases, and resulting in higher sickness and death rates and a lower weaning percentage per exposed cow. However, some PI calves appear normal, making diagnosis without laboratory aid impossible.
Acute (short-term) infections of the virus occur across all classes of cattle. Infected animals only shed the virus for approximately two weeks. Acute infections occur in the cow herd, but are the most common in stocker and feedlot cattle, primarily due to exposure from commingling with PI calves. The immunosuppressive effect of BVDV on calves is important in the battle against the complex of respiratory diseases experienced in high stress and confinement situations following weaning and shipping.
The virus typically enters an animal via close contact with an infected (acute or PI) animal via an oral or nasal route. Most commonly, acute or PI cattle shed the virus in mucus, saliva, feces, and urine. The virus can also be transmitted by contaminated water troughs, feed bunks, needles, rectal sleeves, semen, and biting insects. Acutely infected bulls will shed the virus in their semen for approximately two weeks; PI bulls can constantly shed the virus in their semen.
Defining Low- or High-Risk Operations
Beef cattle operations that could be classified as low-risk fit two major criteria: 1) a closed herd status and 2) having a vaccination program aimed at preventing fetal infection and/or transfer of BVDV. Because BVDV exposure occurs through contact with and/or exchange of bodily secretions, commingling cattle within a given pasture (e.g., newly received cattle, community grazing allotments, stray cattle, etc.) or across-fence contact with neighboring cattle constitutes an open herd. The likelihood of having a 100% closed herd in the Southwest is very small. Low-risk BVDV operations originating from either a closed herd and/or on a preventative vaccination program are not likely experiencing a decline in reproductive performance (from normal) and tend to maintain a high weaning rate per exposed cow.
High-risk operations, including those from open herds with no vaccination program designed to combat BVDV, tend to experience a decrease in pregnancy rates despite good nutrition and bull fertility, and a higher than normal incidence of sickness and death in calves prior to weaning despite good sanitation and nutrition. High-risk herds have an increased exposure to cattle infected with BVDV, which results in a higher percentage of cows and calves being infected with BVDV.
Developing a BVDV Prevention Plan
Three major principles that should be adopted in a BVDV prevention plan include 1) enhancing immunity, 2) preventing exposure to at-risk animals, and 3) eliminating PI carriers from the herd.
Protecting the fetus from infection may be the most effective means of battling BVDV and PI in the cow herd. The goal of a vaccination program designed to combat BVDV and PI in cow-calf operations should be to prevent fetal infection and enhance immunity from the cow to the calf (colostral immunity).
In general, BVDV occurs in multiple types and strains. Therefore, an effective vaccination program should involve adequate cross-protection against the multiple types and strains of BVDV. It is important to note that vaccination programs do not rid a herd of PI. However, if cattle have developed some immunity to BVDV, when they are exposed to the virus it will not spread as quickly.
Both killed and modified-live virus (MLV) vaccines are available to provide protection against fetal and acute infections. Modified-live virus vaccines require at least a single dose, and induce a stronger, longer-lasting immune response that provides cross-protection against different strains of BVDV. Killed vaccines require at least two doses (three weeks apart), and induce a weaker, shorter-lasting immune response, usually not providing effective cross-protection against different strains. Both killed and MLV vaccines require annual boosters to maintain a strong immune response.
The most common recommendations to induce fetal protection against BVDV infection involve the use of MLV vaccines that have a fetal protection claim and include protection from both type 1 and type 2 BVDV. These vaccines have proven to be the most effective if administered four to eight weeks prior to the beginning of the breeding season. Open replacement heifers should receive three doses of MLV between branding and the beginning of the breeding season to ensure active immunity. Nursing calves are usually protected if their dams received properly timed vaccinations against type 1 and 2 BVDV, and if the calves received adequate colostrum as newborns. However, colostral immunity has been shown to lose its protective effect beyond seven months of age, thus requiring further vaccination against BVDV for greater protection. Most vaccination programs for newly received stocker and feedlot cattle include the MLV type 1 and 2 BVDV vaccines administered in combination with vaccines to combat the entire bovine respiratory disease complex.
Preventing exposure to at-risk animals is an essential component in a BVDV prevention plan. All newly purchased cattle, including pregnant cows/heifers, replacement heifers, and bulls, should be quarantined and tested prior to exposure to the resident cow herd or pen mates. Many tests exist to identify PI cattle. However, skin tests (most commonly taken from an ear notch sample) analyzed by immunohistochemistry are utilized to accurately distinguish between a PI and a normal animal that will clear the virus from its body. There is no known maternal interference with the skin test, so calves can be tested within the first week of life. Upon collection, samples should be individually identified and kept cool or frozen until they are submitted to an accredited laboratory for analysis. The average cost is about $5 per sample.
While pregnant cows may test negative for BVDV, if exposed to the virus prior to day 150 of gestation, calves may need to be skin tested soon after birth.
Eliminating PI Carriers
High-risk herds should undergo the most stringent screening program to identify and eliminate PIs. The American Veterinary Council recommends testing all cattle for the presence of PI before the breeding season in high-risk herds. Cattle that test positive for BVDV should either be sold for slaughter or euthanized. Beef from BVDV-infected cattle is safe for human consumption. It is highly recommended that producers evaluate suspected calf deaths for the presence of BVDV.
A BVDV screening and prevention program should be adequate to meet the needs of individual operations. If there is no herd history of BVDV, the most common sense approach would entail implementing an effective vaccination program and testing only new cattle. Conversely, if BVDV is suspected or has a history on the operation, a more aggressive screening protocol may be necessary to reduce the prevalence of BVDV and PI animals. Contact your local veterinarian for assistance in developing a BVDV prevention program for your herd.
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Printed and electronically distributed November 2009, Las Cruces, NM.