Bovine Viral Diarrhoea is one of the most economically significant diseases in both beef and dairy cattle. Unlike many other virus species, the BVD virus can infect a bovine foetus in utero by crossing the placental barrier. If that happens during early pregnancy, specifically in the first trimester, the virus establishes itself in the calf at a time when the calf's immune system is still immature and unable to mount an immune response. At the time of birth, the virus will be present in very high numbers in the blood and virtually every organ and tissue of the calf. Without the benefit of an immune response to fight the virus, the calf will harbour BVDV for the rest of its life and is, by definition, persistently infected (PI).
It is not uncommon for a PI calf to be born, and to grow up, uneventfully. During its stay on the premises, however, it will shed BVDV virus through every bodily fluid, including urine, faeces, nasal excretions, tear fluid, and saliva, thereby not only contaminating the environment but also potentially infecting every cow it gets commingled with.
This creates a significant risk for a large number of animals to become transiently infected (TI) with BVDV. While those animals generally recover, they do present significant economic losses to the producer, particularly since BVDV has been shown to be a part of the 'shipping fever' respiratory disease complex. PI animals are considered to be the single most important and effective transmission mode of BVDV.
Research has shown that even only a single PI animal in a feedlot pen can cause significant per head losses to the producer. This is due to more cows in the sick pen for a longer period of time, higher costs for prolonged veterinary care and drugs, higher death rate, and reduced weight gain. In a cow-calf operation, undetected PI animals will cause the development of more PI animals as they infect susceptible dams early in their pregnancy. Aside from the generation of PI animals, the immediate effect of a BVDV outbreak can be an abortion storm, resulting in the loss of much of that year's calf crop.
Neogen® offers fast and accurate testing for the detection of PI animals. Clients can request a BVDV test offered in combination with subsequent genomic testing. It is the VDX policy that, in the event of a BVDV positive test result, the client will be notified as soon as possible. Since, generally, the diagnosis of BVDV PI means the animal will be culled, clients are strongly encouraged to conduct follow up testing to confirm the diagnosis. Neogen will conduct the follow up testing at no charge, requesting only that a fresh sample is sent to the lab. Confirmatory testing is done both in-house and if required by a third-party lab until an unambiguous confirmation has been achieved.
Neogen can accept several different sample types for testing. While the test method itself is the same for PI detection and TI detection, the distinction is made through the sample type. Since PI animals harbour the virus in their tissues, but TI animals do not, the most appropriate sample types for PI detection are tissue samples, including hair follicles.
In very young animals, serum samples are not a good sample type because there is the potential that maternal BVDV antibodies, passively transferred to the calf through the mother’s milk, will force the virus from the bloodstream into the tissues. This effect is transient since eventually, the maternal antibodies will fade away. If serum is collected during that time of transient passive protection, there will be no virus present and a false-negative diagnosis will be made. If a positive test result is obtained from whole blood, it is impossible to distinguish between PI and TI. Confirmatory testing using a fresh ear notch will have to be conducted to come to an accurate diagnosis of the BVDV status.
BVDV testing is conducted using different methods to accommodate the different sample types and provide optimal test results. The lab can run the PCR test for hair samples and blood cards (excluding young animals as described above). The Allflex TSU samples are processed for the BVDV ELISA. Both tests carry excellent sensitivity and specificity, and the turnaround times are similar.