Bovine Viral Diarrhea: How to Minimize Losses From This Often-Invisible Threat
Each year, bovine viral diarrhea (BVD) costs the U. S. dairy industry an estimated $35 - $65 million per one million calvings. Yet in many operations the source of the disease goes undetected.
That’s because of the silent way the disease is often transmitted—by cows that have it without displaying any symptoms.
“BVD often spreads through persistently infected (PI) cows,” explained Scott Smith, DVM, partner in The Dairy Authority, in Greeley, CO The Dairy Authority is a veterinary practice that specializes in dairy production medicine and also operates an extensive diagnostic lab.
“If a pregnant female becomes infected between day 40 and day 120 of gestation, the fetus doesn’t ‘see’ the virus as foreign, so it doesn’t fight it,” Smith explained. “Therefore, the organism can live in the animal forever, creating what we call a persistent infection. PI animals will not clear the virus and will shed virus every day of their lives.”
Because these animals are often asymptomatic, they go unnoticed, infecting other cows every day. That’s how the disease sustains itself.
“PI animals might get sick, but they might not,” Smith elaborated. “They can be perfectly healthy and be shedding the virus to herd mates, who can develop clinical BVD, which causes diarrhea and respiratory disease. It is also an immunosuppressive disease, which makes affected animals more susceptible to other viral infections or severe bacterial infections, such as salmonella, manheimmia, pasteurella or others.”
Dealing with transient infections
The invisible form of the disease affects about one in 1,000 dairy cows, according to Smith. But the PI cows are just part of the problem. Non-PI cows that have an active infection comprise a much greater percentage of the herd. Those infections are dubbed “transient,” as opposed to persistent.
Animals with transient infections get sick from BVD, either with diarrhea or pneumonia. They mount an immune response, clear the virus and usually get well, Smith said, although a small percentage will die from the infection. In certain instances, mortality can be significantly higher. Those that live will have immunity for life.
The risks are higher for pregnant cows and fetuses, he offered. Exposure of the fetus to the virus prior to day 40 of gestation can result in abortion. Later exposure can cause birth defects or weakened calves with higher mortality rates. The virus can even cause cerebellar hypoplasia, a condition that causes tremors and typically results in euthanasia.
“The unborn fetus is much more susceptible to BVD than older animals,” explained Smith. “It may only take a few hundred viral particles to make a fetus sick; in older animals, many thousands of organisms are required.”
He said transiently infected animals should be isolated temporarily to prevent further spread of the disease. Any treatment required is usually for secondary bacterial infections.
Reducing susceptibility in the herd
There are several elements in an effective BVD control plan. They include virus detection, biosecurity, biocontainment and vaccination. “When there are lapses in any or all of those areas, the incidence of BVD goes much higher,” Smith asserted.
So, how do you find PI animals when they display no symptoms?
Smith asserted that there are several accurate tests to identify BVD persistently infected animals. They include an antigen-capture ELISA test, which he said delivers rapid, accurate results to pinpoint PI cows. This is usually done via a skin biopsy (ear notch) or serum. Another test, an immunohisto chemistry (IHC), is run on a skin sample, from which a stained slide is created to reveal the presence of the virus. A third test, a PCR (polymerase chain reaction) analysis is less desirable for identifying PI animals because, unlike the first two tests, transiently infected animals can also have positive results with the PCR test.
The next question is, how do you decide which cows to test?
Smith suggests testing young animals, for two reasons. First of all, testing young calves can give you two results for the price of one test. “If you test a calf and s/he is negative, that means the mother is also negative, because a PI cow will always give birth to a PI-positive calf and cannot have a negative calf. On the other hand, a PI-negative cow can give birth to a PI-positive calf if she gets infected while pregnant. For this reason, we always recommend testing the dam of every PI calf to sort this issue out.”
Secondly, testing young animals allows the producer to remove persistently infected animals from the herd and euthanize them sooner, reducing the amount of time they can expose other animals to the virus.
Identifying persistently infected animals is critical, Smith stated, because “if producers have unknown PI animals, they are always going to have an issue with BVDV in their herd.”
Biosecurity and biocontainment
One area of concern in dairy operations is animal movement. Smith said herds bringing in new animals are more likely to have issues with BVD. He says the first thing producers can do to put themselves ahead of the game is to know their own herd’s BVD status.
“Most producers work directly with their veterinarians, who know the herd and are familiar with BVD control programs—as well as what can happen when new animals are introduced,” Smith explained. “The vaccination program, as well as the health status of all animals, should be reviewed and discussed.”
It’s also prudent to know that same information about any animals you bring into the herd—before you make a purchase. While that’s not always easy, the producer can ask to review vaccination records and BVD tests on the cows being purchased.
Post purchase, new animals should be isolated for three - four weeks so that any transiently infected animals can clear the virus. The new group should also be checked to detect PI animals before they are commingled with the herd. PI animals should, of course, be culled and destroyed to avoid the spread of the disease.
If purchased cows are already milking, the animals must be integrated into the herd more quickly. Consequently, Smith said his lab often runs PCR tests on milk and pools it to detect the presence of the virus. Then, if BVD is detected, they conduct individual tests on all cows in the group. Typically, the tests only identify PI animals, he asserted. If the tests come back negative, “we can feel pretty confident there is no PI infection.”
Though vaccination programs are not 100 percent effective, they are still an important component in BVD prevention.
“Vaccinations alone will not keep BVD out of your herd. Nevertheless, you want to make sure you have a complete and up-to-date vaccination program. These vaccines are designed to prevent BVD and are important in increasing the herd immunity level.”
New additions to the herd are generally vaccinated after being tested either during the three - four week isolation from the herd or at a later date, depending on herd vaccination protocol.
Bovine Viral Diarrhea is an ever-present problem for dairy producers. However, through proper implementation of biosecurity and biocontainment procedures, as well as testing and vaccination, they can reduce the unpleasant surprises and losses caused by this often-invisible disease.
Tips for Controlling BVD
• Know the status of your herd before bringing new animals in
• Make sure you have a complete and up-to-date vaccination program
• Isolate newly purchased animals three - four weeks before introducing them to the herd
• Test all new animals for BVD
• Remove positive animals from the herd immediately and destroy them
• Vaccinate new animals that are PI negative