Bulk Milk Tank Testing
Detection of BVDV Persistently Infected Lactating Cows
BVD PCR and Virus Isolation (VI) are performed on each bulk tank milk sample. Performing both tests together increases the sensitivity for detecting BVD.
- Maximum of 400 cattle in one sample to avoid dilution of BVD virus. String sampling, (submitting multiple samples) is recommended for dairies milking more than 400 head.
- Minimum of 200 ml of fluid milk per sample.
- Sample containers need not be sterile; tap water clean plastic soda bottles are fine.
- DO NOT FREEZE; freezing milk will lyse the somatic cells and decrease the sensitivity of the detection assays (PCR and VI).
- Do not allow milk to clot or add preservatives.
- Samples need to arrive at the AHDC Monday-Thursday, within 24-48 hours of collection. Fermentation occurs naturally in stored milk, and may interfere with the PCR results.
A BVD positive bulk tank indicates:
- Acutely infected cattle circulating BVD OR
- One or more persistently infected cattle OR
- Both acute infection and PI animals
The bulk tank sample is centrifuged and a pellet of somatic cells is collected and used for sampling. Simply testing the fluid milk by PCR is not adequate, as we have detected samples that were PCR and virus isolation positive on the somatic cells, but PCR negative on the fluid milk sample. The BVD PCR result is reported in 2-3 days, but the virus isolation results are reported in 7-10 days, because the virus has to grow (or not grow) in cell culture.
An accurate list of all animals represented in the bulk tank or string samples should be generated at the time of sampling, any pen changes tracked, and all of the animals that leave the milking herd, including deaths, following the collection of the milk sample(s) should be individually sampled. (Appropriate samples from individual cows include: ear notch [OK to ear notch carcass], EDTA whole blood, or serum for BVD Antigen Capture ELISA to detect PIs). This is to guard against the possibility that a persistently infected (PI) animal is culled prior to testing the entire lactating group as indicated by a positive bulk tank test. The failure to find a PI animal under that circumstance would appear to represent a false positive test result when in fact the positive animal was culled prior to the herd test. A positive test result is significant, but negative results must be interpreted with caution, particularly without information regarding the PI status of non-lactating animals in the herd. Testing for BVDV in the individual animal in the absence of hard evidence for its presence in the herd is expensive and may discourage future testing when a valid reason does exist. Bulk tank testing is one method to generate the needed "hard" evidence.
Information on the development of the BVD bulk tank testing at the AHDC
Testing of milk from the bulk tank has been an accepted practice for many years particularly with respect to the detection of bacteria and mycoplasma. In 1995, Radwan, Brock, Hogan, and Smith at the Ohio Agricultural Research and Development Center published a report (Vet Microbiol 44:77-92) indicating that bulk milk samples may be a useful sample for detecting BVDV persistently infected animals in the lactating herd using PCR. Of the 136 herds that they tested, 33 were PCR positive, but none were found positive by their virus isolation procedure using the somatic cells as the test sample.
At the Animal Health Diagnostic Center, we were interested in this report, but we approached implementing this test with caution. We began limited testing using PCR, but we also did parallel testing by virus isolation. We were able to detect BVDV in somatic cells from the bulk milk sample by PCR, but we were also able to detect the virus by our standard virus isolation procedures (culturing viable "somatic" cells).
In >95% of the cases, we had agreement between PCR results and the virus isolation tests. This finding is in sharp contrast to Radwan et al. where all PCR positive samples were virus isolation negative. We still continue to use both tests on the somatic cells from the bulk tank because we recognize that no test works in all situations. Also, milk from acutely infected cattle in the bulk tank sample may cause either a PCR positive result or grow in virus isolation; we do not have research data on bulk tank samples to compare acutely infected cattle with PIs. This discrepancy between the two tests may be due to residual non-infectious viral nucleic acid (PCR detectable) at the end of an acute infection.