Tracking BVD trends with national testing data
Creating a national BVD test result database could help us keep better tabs on the BVD situation in New Zealand and make it easier for farmers to confirm the BVD status of animals they are trading. However, we need to get better about recording accurate animal and herd identification numbers as well as reasons for testing on the laboratory submission forms to make the data more meaningful.
Background
Historically, most of the data we have on BVD prevalence (how common BVD is) in New Zealand has come from small scale studies where researchers have either analysed BVD testing records from a veterinary practice management database or proactively recruited herds to volunteer for testing. This means that the information is often limited to a small geographic region or a small time period, which makes it difficult to monitor the distribution of BVD positive herds in New Zealand across space and time.
Almost all BVD diagnostic testing in New Zealand is currently performed by one of the four major commercial veterinary diagnostic laboratories: Gribbles Veterinary, IDEXX Laboratories, Livestock Improvement Corporation (LIC), and SVS Laboratories. The databases maintained by the labs contain the most up-to-date information we have on how often herds from across New Zealand are testing for BVD as well as the results from these tests.
Study Objectives
In this study, we explored the feasibility of pooling data from the four diagnostic laboratories to see what additional insights we could generate about the current BVD situation for New Zealand and how the data could be better utilised to support future BVD control efforts.
Did you know……….
Once an animal has been individually tested for BVD virus and found to be negative, it can be certified as being non-PI for life because PI animals can only be created before birth. Testing calves at the same time as they get their NAIT tag placed is a great opportunity to get rid of PI animals before they cause major problems in your herd.
Methods
Each of the diagnostic laboratories provided a data extract containing anonymised records of all submissions for BVD diagnostic testing from the 2015, 2016, and 2017 calendar years including information on the geographic region, sample type, test requested, and test results. Because of differences in how the laboratories set cut-off values for the diagnostic tests and how they stored the information in their databases, there was a lot of work involved with processing the data to allow it to be combined.
Results
There were a total of 59,007 unique BVD diagnostic test requests submitted over the three-year period, which included 39,920 bulk milk antibody ELISA tests, 27,832 bulk milk PCR tests, 3,229 serum antibody ELISA tests, 9,132 blood/serum/tissue antigen ELISA tests, and 7,122 blood/serum/tissue PCR tests. Because there was often no information on the animal production type (beef or dairy) or identification numbers, it was difficult for us to estimate the total percentage of dairy and beef herds that were performing BVD tests each year in New Zealand. Conservatively, we think that about 65-70% of dairy herds are performing annual screening tests compared with only about 5-15% of beef herds depending on the estimates that are used for the total number of commercial herds in New Zealand.
About 10% of samples that were submitted for individual animal virus testing came back positive for BVD, but this is likely much higher than the true prevalence of PI animals since these animals were probably tested because there was a strong suspicion that they had BVD. Only about 30% of herds with positive antibody-ELISA screening had records of any follow-up testing being performed to identify PI animal in the herd. Of the subset of 116 known beef herds with a positive antibody-ELISA result that had a subsequent antigen or virus test result of individual cattle, 25 (21.6%) had at least one antigen- or virus-positive sample. Of the 89 known dairy herds with a positive antibody-ELISA result that had a subsequent antigen or virus test result, 14 (17.5%) had at least one antigen- or virus-positive sample. Possible explanations for why no PI animals were found in the other 80% of herds could include (1) that the animals had already been removed from the herd from death or culling due to poor performance or (2) that not all animal in the herd were individually tested to identify PI animals.
Clinical Relevance
Although the combined data from the diagnostic laboratories contained valuable information about the frequency of test requests and prevalence of positive results across New Zealand, many laboratory submission forms were missing key data on the animal or herd identification numbers, herd type (dairy or beef), and reasons for conducting the test (routine annual surveillance versus testing in response to a suspected outbreak situation). This makes it difficult to use the data in its current form to understand more about the BVD situation in New Zealand.
Other countries with national BVD control programmes have easy systems in place to automatically link tests results from the diagnostic laboratories with their equivalents of the NAIT database, which has allowed them to keep better tabs on how the prevalence of disease is changing over time in response to BVD control measures. With all the IT infrastructure development from the Mycoplasma bovis eradication in New Zealand, it would be relatively straightforward to establish a similar national database.