Part 2-The BVD Virus in cow/calf operations-How do I test for BVD Virus?
November 13, 2023
If a calf tests negative, then the dam can be assumed negative. “BVD” or “Bovine Viral Diarrhea” virus is one of the most common and costliest viruses affecting cow/calf herds and backgrounding operations. Control of the BVD virus is best accomplished through implementation of three equally important practices: 1) surveillance testing to detect and remove persistently infected cattle, 2) vaccination to increase herd immunity and 3) implementation of biosecurity measures to reduce virus entry into the herd. This article, the second in a two-part series, will address diagnostic testing strategies, how to correctly interpret results, and how to implement BVD virus prevention measures.
As a reminder, a “persistently infected” or “PI” calf is the result of a pregnant female (cow or heifer) becoming infected with the BVD virus between 42-125 days of gestation. The mature cow or heifer will experience a “transient” BVD virus infection, lasting from a week to 10 days, which is often mild with no overt symptoms of disease. However, 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” and “spreader” of the virus for its lifetime. Although it is often assumed PIs will die young, some survive well into adulthood and can be fed out to slaughter weight or, if female, may become pregnant. If a PI positive mature cow calves, the calf will always be persistently infected, too. This route of transmission accounts for less than 10% of total PI calves born.
A BVD-PI calf is born with the BVD virus and sheds virus particles everywhere it goes for its entire life. Identification and removal of PI calves is critical to stop long-term survival of the virus.
Testing for persistently infected (PI) cattle is easy and inexpensive with the BVD ELISA test. The most commonly used sample for identifying PI cattle is an “ear notch” skin sample. Blood (serum) can also be used although not in calves less than 3 months old. If the ear notch or serum test result is negative, that animal is negative for life and will never need testing again. Any BVD ELISA positive test result should be confirmed by first separating the animal away from the herd and then retesting a second ear notch or blood sample taken 3 weeks after the first sample. True PI animals will still be positive after 3 weeks while those animals with a short-term infection (transiently infected) will test negative on the 2nd sample.
If you suspect BVD virus may be in your herd, work with your veterinarian to come up with a plan for testing and, more importantly, what actions will be taken with the results. To test the herd, the following steps are recommended:
- Test all calves at an early age- It is recommended for calves to reach at least 2 weeks old before taking an ear notch sample. If using a controlled breeding and calving season, test all calves after the last calf is born but before placing the bull in a breeding group in order to remove PI calves from pasture before breeding begins. Ear notches can be stored in the freezer and submitted at one time if desired.
- If a calf is confirmed positive, then test the dam. Remember, a calf can be positive for PI but, in over 90% of cases, the dam will be BVD negative. If calf is negative, then the dam can be assumed negative and does not need to be tested. (See Figure 1).
- Test any cow/heifer without a calf at her side.
- Test all bulls and replacement heifers (purchased or raised).
- Purchased Pregnant Cows and Heifers – Quarantine and test purchased pregnant females and, if negative, they can join the home herd. However, bear in mind that any of their unborn calves could be a PI and all calves must be tested at 2 weeks of age or older, the sooner the better. A better option is to calve out purchased pregnant females away from the home herd and test their calves for BVD virus prior to any mixing with the home herd.
- Remember PIs are considered defective and there is a legal, moral and ethical obligation to either feed them out for personal consumption or euthanize and dispose of these animals without sending/returning them to commerce. Animals that test positive are not to be sold, given away or transported without approval of the State Veterinarian.
In addition to detection and removal of PI animals, prevention of BVD virus in a herd depends on a sound vaccination program to increase herd immunity and biosecurity measures to reduce the opportunities for virus exposure. Vaccines against BVD (including those with Fetal Protection from BVD-PI claims or “FP” vaccines) will reduce the chances of creating a PI calf but protection is never 100%. Vaccines may fail due to problems with the vaccine itself, the cattle to be vaccinated, and/or management errors. The BVD virus is classified into 2 genotypes, BVD virus-1 and BVD virus-2, each of which contains distinct subtypes with genetic and antigenic variation. BVD vaccines may not contain the subtypes of the virus currently circulating in a region and are therefore not fully protective. Problems within the animals themselves may prevent good vaccine response. Cattle that are sick or stressed when vaccinated, in poor nutritional status or too young to produce antibodies will not be protected with vaccination. Finally, 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 temperature either before or during use of the product.
- 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.
Biosecurity measures to reduce virus exposure extend beyond the quarantine and testing of new purchases. 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. It can be spread short distances through large “droplets” (especially saliva and nasal discharge) and is easily transmitted through nose-to-nose contact. Cattle are social animals and will interact with other cattle whenever possible. Most farm/ranch fences are good for marking property lines but not for preventing virus spread. Double boundary fencing with space in-between or installing offset hot wires on both sides of a fence will significantly reduce the risk of transmission. Other potential virus sources include mechanical vectors such as shared farm machinery or contaminated veterinary equipment. Thoroughly cleaning equipment with soap and water is an effective means of killing the BVD virus. Indirect contact with wildlife, vermin, and other domestic animals, especially through urine and fecal contamination of cattle feed and water sources should be minimized as much as possible. Cattle herds are unique entities with different risks for disease on every farm so work with a veterinarian to best protect your herd.
BVD PI testing can be confusing!
When testing calves for PI:
- If the calf is positive, test the dam. The dam’s result may be either negative or positive.
- If the calf is negative, no need to test the dam. The dam will be negative.
When testing cows (dams) for PI:
- If the dam tests positive of PI, her calf and every calf she has will always be positive.
- If dam tests negative for PI, must test her calf. The calf may be either positive or negative.