Test Information
This page contains a summary of key information about the BVD diagnostic tests offered by the four main commercial diagnostic laboratories in New Zealand and the protocols for conducting BVD diagnostic testing. All decisions regarding BVD testing programmes should be made in consultation with your herd health veterinarian. Please refer to the diagnostic laboratory websites for the most up-to-date information about sample collection, sample submission, and test interpretation.
Diagnostic Laboratories
There are four main commercial diagnostic laboratories in New Zealand that provide BVD diagnostic testing:
Gribbles Veterinary Pathology
IDEXX Laboratories
Livestock Improvement Corporation
SVS Laboratories
Diagnostic Test Submissions
The following table summarises the BVD diagnostic tests offered by the four commercial diagnostic laboratories:
Diagnostic Test Protocols
Pooled Serum Antibody ELISA
This test is primarily used to screen larger groups of beef cattle or dry dairy cattle for evidence that animals have mounted an immune response against BVD as the result of being previously infected by the virus. The higher the antibody levels the more likely it is that there is at least one PI animal currently present in the group and these results can inform decisions about whether there is a need to conduct a full PI hunt in the group. Monitoring trends in antibody levels over time can also help farmers to determine whether there have been any breakdowns in biosecurity that have allowed BVD virus to re-enter the herd from outside sources.
Protocol:
Collect individual blood samples from 10 to 15 randomly selected animals from the group into separate serum tubes. The animals must be over 10 months of age and unvaccinated to prevent false positive results from the presence of maternal antibodies and antibodies against the vaccine. This test can be run at any time of year. Performing the test on replacement heifers (10 to 12 months of age) in beef herds will provide information about whether there was a PI calf or PI dam present in the breeding herd that calving season. This test can also be performed on older age groups of unvaccinated cattle as an additional screening or monitoring tool.
Interpretation:
The results are reported as sample-to-positive (S/P) ratios with the following interpretation:
Negative (S/P < 0.17) - There are unlikely to be any PI animals in the group.
Suspect (S/P 0.17 to 0.75) - Animals in the group have been exposed to BVD virus at lower contact rates. There may or may not currently be a PI animal in the group.
Positive (S/P > 0.75) - There likely is (or has recently been) a PI animal directly mixing with cattle in the group.
Herds with Suspect or Positive results should consider performing a PI hunt to find and eliminate any PI animals in the group. Information about the movements of animals into and out of the group may provide additional information about the potential origins of PI animals and/or the likelihood that the PI animal may have already been removed from the group through death, culling, or sales.
Although a negative test result suggests that there are no PI animals present in the group, an individual animal should not be assigned a non-PI status on the basis of a group test result. The only way to confirm a non-PI status is to perform individual animal testing.
Limitations:
Since this test should only be run on animals over 10 months of age, the results for beef herds will be available too late in the season to implement appropriate control measures such as vaccination or PI hunts to prevent the creation of new PI calves if there was evidence of a PI animal having been present in the mixed-age cows or calf crop from that year.
The results only provide a snapshot view into the BVD status of the group. The results may change over time and no longer be valid if new untested animals are introduced into the group or if the animals in the group become pregnant with the potential to be carrying PI fetuses.
This test cannot be used in groups of cattle that have been vaccinated against BVD as part of the herd management programme.
Bulk Tank Milk Screening
These tests are primarily used to screen the milking herd on dairy farms for evidence that animals have mounted an immune response against BVD as the result of being infected by the virus (bulk milk antibody ELISA) and to determine whether there are currently any persistently infected animals in the milking herd (bulk milk PCR). It is useful to perform both tests in conjunction as part of a monitoring programme to assess changes in herd status over time.
Protocol:
Bulk tank milk BVD tests can either be requested by farmers through their milk processing companies (if offered) or samples can be submitted to any of the four main diagnostic laboratories through their veterinarian. The bulk milk antibody ELISA results are not reliable for use in herds that vaccinate replacement heifers and/or mixed-age cows against BVD.
Both tests can be run on bulk tank milk samples at any time of year. Most farms will run one or two bulk milk PCR tests at the start of lactation after most animals have calved so that they can identify and remove PI animals from the milking herd well before the planned start of mating. If bulk milk PCR is being used to screen the milking herd for PI animals, it is important to record which animals were not contributing to the bulk milk tank on the test date (usually because they haven’t yet calved or were being withheld from the tank for other medical reasons). These animals should be individually tested to confirm that they are not PI.
For herds that conduct routine herd milk testing on individual animals, it may be worthwhile scheduling the bulk milk PCR test on the same date as a herd test if possible since this will often provide more accurate records of animals that were present on test date and there will also be individual milk samples already available from the cows to conduct a PI hunt if the PCR results come back positive.
Bulk milk antibody ELISA is typically performed early in lactation before the planned start of mating and again late in lactation before the drying-off period to look for increases in the antibody levels that may suggest the milking herd was exposed to BVD during the high risk periods of pregnancy.
Interpretation:
The results from the bulk milk PCR are generally reported as positive or negative. The test is sensitive enough to detect a single PI animal contributing to the bulk tank in herds with over 1,000 milking cows. A PI hunt should be conducted in positive herds to find and eliminate the PI animals.
The results from the bulk milk antibody ELISA are reported as sample-to-positive (S/P) ratios with the following interpretation:
Negative (S/P < 0.25)
Low (0.25 < S/P ≤ 0.5)
Moderate (0.5 < S/P ≤ 0.75)
High (0.75 < S/P ≤ 1.0)
Very high (S/P > 1.0)
Herds with S/P ratios > 0.75 are considered highly likely to currently have (or have recently had) a PI animal in the milking herd.
An increase in the bulk milk antibody levels between the start and end of the lactation season may indicate that the herd has been exposed to BVD and experienced an outbreak in the milking herd during the risk periods of pregnancy. These herds should consider testing calves born the following season to ensure that no potential PI animals will be kept as breeding replacements.
Bulk milk antibody levels can remain high for many years in herds that have recently cleared BVD infections depending on the rate of culling and herd turnover. For that reason, the results are more useful as a monitoring tool to check whether animals are still being exposed to the virus over time. In herds that have adequately addressed biosecurity risks to prevent exposure, we would expect the S/P ratios to decline at average rate of about 0.10 units per year. If the S/P ratios remain static or increase between seasons, this may indicate that BVD is getting across the farm boundaries.
Limitations:
The bulk milk antibody ELISA results are unreliable for herds that vaccinate replacement heifers and/or milking cows since these animals will produce high levels of antibodies in their milk in response to the vaccine. False positive results can occur if farmers are unaware of the vaccine status of cattle in their herds. Furthermore, the results from a single positive test can be difficult to interpret since antibody levels can remain high for several years after the herd has cleared infection.
The results from these tests will only provide information about the BVD status of animals that were contributing to the bulk milk tank on the test date. Dairy herds may still have PI animals among the fetuses, calves, replacement heifers, and dry stock which can cause subsequent outbreaks if and when they join the milking herd.
Any time untested animals enter the herd or new calves are born into the herd, there is a risk that herds can become positive for BVD despite having previous negative test results. Ongoing monitoring through annual bulk milk testing is important for identifying changes in herd status.
Individual Animal Testing
These tests are primarily used to identify individual animals that are actively infected with BVD by looking for either viral proteins (antigen ELISA) or viral genetic material (PCR). Since PI animals shed extremely large quantities of virus throughout their entire life, these tests have almost 100% sensitivity in being able to detect PI animals any time they are performed. Once an animal tests negative for the virus, it can be assigned a non-PI status for life and does not ever need to be tested for BVD again. This is because animals can only become PI if they become infected with BVD as fetuses between days 40 and 120 of the pregnancy.
Protocol:
Collect individual blood, milk, or tissue (ear notch) samples from animals that need to be tested for BVD virus. PCR can be performed on any sample type from animals almost immediately after birth. If antigen ELISA is being used, only ear notch samples should be submitted for calves under 35 days of age to prevent interference from maternal antibodies in colostrum. Once animals are over 35 days of age, antigen ELISA can reliably be performed on any sample type.
These tests can be performed at any time of year, but it is preferable to identify and remove any PI animals from the herd before the planned start of mating to prevent susceptible dams from getting infected during high risk periods. A convenient time to test calves is at tagging or disbudding when it is easy to collect ear notch samples. For dairy herds, BVD tests can be performed on individual milk samples collected for routine herd milk testing to significantly reduce the costs associated with traditional sample collection and processing.
It is highly recommended that all purchased or leased breeding cattle (replacement heifers, mixed-age cows, and bulls) be individually tested to confirm that they are not PI before arriving on the property. Any calves born to purchased pregnant cattle should be tested as soon as possible after birth to ensure that they are not PI animals. Animals brought onto the property for grazing or fattening should also ideally be individually tested to minimise the risk of infecting breeding cattle on the property.
Interpretation:
The reporting of these test results varies between the four diagnostic laboratories and the different test methodologies, but animals will generally be classified as being negative or positive.
If the animal tests negative for the virus, it can be assigned a non-PI status for life. If an animal tests positive for the virus, this means it is either transiently infected or persistently infected with the virus and can be assigned a suspect PI status. The only way to definitively confirm a PI status is to repeat the test in 3 to 4 weeks. Animals that are negative on the second test were most likely transiently infected and have now recovered. Animals that are still positive on the second test are PI animals and should be removed from the herd as soon as possible.
If the animal has high levels of viral antigen or genetic material and/or if the animal has suggestive clinical signs of being PI (stunted growth, ill-thrift, history of multiple illness), some farmers will choose to cull on the basis of a single positive test result rather than incur the additional costs and risks of keeping the animal in isolation until it can be re-tested.
Limitations:
Although both antigen ELISA and PCR have excellent diagnostic performance for detecting PI animals, there may be a rare false negative result amongst the thousands of tests that are being run each year and there may also be clerical errors in labeling the samples and/or recording the results against an individual animal ID number. It is highly recommended that herds conduct additional annual monitoring tests (pooled youngstock antibody testing and/or bulk tank milk testing) to catch any potential breakthroughs.
Since PI animals shed very large quantities of virus into their surrounding environment, there may be a rare false positive result from sample contamination if there is not good hygiene when personnel are moving between animals to collect samples. Animals that produce a positive result should be re-tested in 3 to 4 weeks to confirm whether or not they are PI.
Point-of-Care BVD Testing
The antigen ELISA is also available as a lateral flow device (IDEXX BVDV Point-of-Care Test) that can be used to test ear notch samples directly on farm and produces results about an animal’s BVD infection status within 30 minutes. This test is most useful in clinical situations where persistent BVD infections are suspected as an underlying cause for an animal having severe clinical signs such as fever, lethargy, loss of appetite, ocular discharge, nasal discharge, oral lesions, and diarrhoea. Knowledge of the animal’s BVD status may influence immediate decisions around whether to treat or cull the animal.