Reducing the Impact of BVD in Dairy Rations

Dr. Doug Hammon, D.V.M., Ph.D.
USU Extension Veterinarian
The Bovine viral diarrhea (BVD) in cattle has been recognized for more than 60 years. BVD virus belongs to the pestivirus family, which includes BVD type I and type II viruses, hog cholera virus, and border disease virus. Although BVD has been recognized as a disease of cattle for decades, we are only now beginning to understand the full impact and cost of BVD in dairy herds.

Impacts of BVD Infection

The detrimental impacts of BVD infections in dairy herds includes decreased milk production, decreased reproductive performance, decreased growth rate, immune suppression and increased risk of other diseases (e.g., diarrhea and pneumonia), unthriftiness, early culling, and increased death loss in young stock. BVD has also been associated with a high prevalence of fevers and poor performance in fresh cows and heifers.

Forms of BVD	Consequences of BVD Infection During Pregnancy
Benign / Subclinical infection	Infection 0 to 45 Days of Gestation: Decreased conception, infertility, partly due to ovarian dysfunction
Persistently Infected (PI)
Fatal Mucosal Disease	Infection 45 to 125 Days of Gestation: Development of persistently infected cattle due to in utero exposure of the fetus to BVD virus
Reproductive Failure - - Decreased follicular development - - Abortion - early embryonic death
Birth Defects	Infection 125 to 175 Days of Gestation: Abortion, fetal malformations, weak calves
Respiratory Disease
Weak Calf Syndrome	Infection 175 Days to Term: calves are typically born normal, but infection during this period may result in weak calves and occasionally abortion.
Immune suppression
Diarrhea

Development of BVD Persistently Infected Cattle

Persistently infected (PI) BVD cattle are created when the dam and her fetus become infected with BVD virus between 45 to 125 days after conception. During this period of development, the immune system of the fetus has not yet developed and the BVD infection is not recognized. The fetus is incapable of recognizing the virus and does not develop antibodies against the BVD. Fetuses infected during this period survive and are permanently infected with the BVD virus, shedding the virus throughout their lifetime.

Most BVD infections that generate PI BVD calves go unnoticed in the mother cows. PI BVD calves come from transient BVD infection of the cow 97% of the time. However, PI BVD heifers and cows that survive to reproduce always give birth to PI BVD calves, which accounts for 7% of all PI BVD calves.

Individual cattle persistently infected (PI) with BVD are the major source of infectious BVD virus in the dairy herd. The prevalence of persistently infected BVD cattle is estimated at 0.05 to 2% of the cattle population, and may be as high as 5 %. With a 2% prevalence of PI BVD cattle, there is a 40% chance of purchasing at least one persistently infected animal in a group of twenty purchased heifers. Testing to identify PI BVD cattle prior to purchase can greatly reduce the risk of BVD infection of your farm.

Fate of Persistently Infected BVD Cattle

Half of all PI BVD cattle die within first year of life, while some live to reproduce. The following is a list of possible outcomes of BVD persistent infection in cattle:

• Death at or near time of birth
• Death due to mucosal disease
• Death from other causes (e.g., scours or pneumonia) due to immune suppression associated with BVD infection
• Survival to maturity with reduced performance
• Survival to maturity and reproduction (PI dams always give birth to PI calves)

Testing to Identify and Eliminate Persistently Infected BVD Cattle

The first question to ask is, do I have BVD in my herd? A history of BVD related health problems would indicate that BVD virus is (or has been) circulating in the herd. Also, herds that purchase heifers and bulls that have not been tested for BVD are at higher risk. The BVD testing strategy employed will depend on whether you are trying to determine if BVD virus is circulating in the herd or if you are trying to identify and eliminate PI BVD animals. It is recommended that you work closely with your veterinarian to develop a BVD testing strategy that suits the goals of your operation. Following are some of the available BVD tests and how they may be used to reduce or eliminate BVD from a dairy herd.

Testing to determine if BVD virus is circulating in the herd.

Blood samples from calves prior to feeding colostrum can be tested to determine if BVD antibody is present. The presence of antibody indicates that BVD virus is circulating in the herd and that the calf was infected in the uterus. Test six calves every 3 to 4 months.

Blood tests on unvaccinated 6 to 8 month old heifers or steers every 3 to 4 months to determine if BVD antibody is present. The presence of BVD antibody in these “sentinel” animals can be an inexpensive way to determine if BVD virus is (or has been) circulating in the herd and is especially useful as a monitoring tool in larger herds.

Polymerase Chain Reaction (PCR) tests on pooled whole blood samples can be used to identify the presence of BVD virus. It is recommended that 10 blood samples be collected from calves and shipped to the diagnostic laboratory, where they will be pooled and tested for BVD. A positive test indicates that one or more of the blood samples contains BVD virus. The individual samples can then be tested to determine which animal has BVD virus. This may be a cost effective test to determine if BVD virus is present in some herds. The BVD PCR test on pooled samples is available at the Utah State Veterinary Diagnostic Laboratory for $30 per pool.

Polymerase Chain Reaction (PCR) tests on bulk milk can be used to determine if BVD is circulating in the milking string. Research from Cornell University indicates that bulk milk PCR is capable of detecting the presence of a single persistently infected animal within a group of several hundred cows. A milk sample (100 to 250 ml) is collected from the bulk talk, containing milk from two or more milkings, and shipped on ice (not frozen) to the diagnostic laboratory. The Utah State Veterinary Diagnostic Laboratory is currently offering BVD PCR for $30 per sample.

Testing to identify individual persistently infected animals.

Immunohistochemistry test on skin samples is a very valuable test to identify persistently infected BVD animals. This “ear notch” test has been available for a few years and has given us a simple and accurate diagnostic test that can help eradicate BVD from dairy herds. This test is used to identify and eliminate the major source of BVD virus on the dairy: the persistently infected animal. The immunohisto-chemistry test costs $5 to $8 per sample, in addition to the cost of collecting and shipping. Samples are easy to collect: simply obtain a small piece of ear skin using an “ear notcher” and place the sample in a tube with a small amount of 10% buffered formalin. Care must always be taken when handling formalin; it permanently alters tissues, including human skin and eyes. Skin samples need to tested within seven days of collection. As PI BVD animals are the major source of infective virus on the dairy, positive animals need to be eliminated from the herd.

Virus Isolation on whole blood to detect the presence of BVD virus in an individual animal has been the “gold standard” test for many years for identifying PI BVD animals. However, BVD virus isolation generally requires 3 weeks to complete the and cost ranges from $15 to $30 per sample. The accuracy of the “ear notch” test has been shown to be similar to the accuracy of the virus isolation test, is less expensive, and is user friendly.

Vaccination to Prevent PI BVD

The goals of a BVD vaccination program in a dairy herd are: 1) to protect cattle from BVD disease 2) to protect the fetus from becoming persistently infected. Reducing the risk of BVD disease in cattle can probably be accomplished with annual vaccination using either killed or modified live BVD vaccines that provide protection against Type I and Type II BVD virus. However, most research indicates that protection of the fetus from infection requires annual vaccination with modified live virus BVD vaccine. Currently, only modified live virus BVD vaccines are labeled for fetal protection against persistent infection. It is recommended these vaccines be used in breeding cattle, and that the label be strictly followed regarding the timing of vaccination and vaccine handling.

Biosecurity to Prevent BVD virus from Entering the Herd

Keeping BVD out of a herd requires that strict biosecurity procedures be followed. Visitor and vehicle access to the animal production units should be restricted. If visitors are allowed access to animals, clean clothing and sanitized boots or disposable plastic shoe covers should be worn. Because PI BVD animals pose the greatest risk of bringing BVD into the herd, all purchased and incoming animals should be tested for BVD before entering the herd, preferably using the ear notch test. With the high price of replacement heifers and the high cost of BVD associated disease, these biosecurity measures are well worth the effort and expense.

For more information contact Doug Hammon at hammon@cc.usu.edu or (435) 797-1881.