Milk as a diagnostic fluid

Over 40 years there have been profound changes to the Australian dairy production environment. The number of farms decreased from 21,989 in 1980 to 5055 in 2020,^{1, 2} milk production per cow increased from 2888 L/cow per year or 1.9 million cows producing 5.49 million L per year, to 6311 L/cow per year or 8.8 million L from 1.4 million cows. Many dairy farms represent assets valued in the $10 to $100 million or more. The average herd has increased from 85 to 274 cows. Consequently, farm management has less time to engage with the individual cow. These changes influence the delivery of veterinary services as the individual cow now represents a much lower proportion of the enterprise asset value. However, herd health and productivity are critical to an enterprise and farmers are committed to stewardship of their cattle. The challenge for the veterinary profession is to deliver cost-effective services that identify, monitor, and mitigate risks to herd health and productivity. Such services must be designed to deliver better outcomes with greater labour efficiency. In this series of reviews, we evaluate the value of bulk tank milk which provides a readily available and contemporary indicator of herd status of health and production and, where appropriate, compare the value of bulk milk testing to that of individual cow testing, to determine the mastitis,³ viral,⁴ and metabolic status⁵ of herds. We provide quantitative and qualitative reviews of tests that may do this. We also note two coincident and valuable scoping reviews of this area,^{6, 7} one of which includes data on the value of bulk tank milk for parasite evaluation.⁷

Bulk milk somatic cell counts are routinely utilised by processors and veterinary advisors to assess milk quality and udder health. Because this assessment does not capture cows with clinical mastitis, diagnostics at the cow level may also be needed to manage udder health. Additional markers of inflammation or the humoral immune response are primarily available at cow level, except for antibody testing for Mycoplasma bovis, which can be conducted at bulk milk level to support biosecurity efforts. Elevated somatic cell counts are primarily due to intramammary infections, and its causative agents, including those with antimicrobial resistance, can be detected through culture or PCR. Specificity of PCR for contagious pathogens (Streptococcus agalactiae, Mycoplasma bovis) is high (0.90) but sensitivity is variable (0.15–0.99) unless repeated bulk milk testing or cow-level testing is used. For pathogens that may be cow-derived as well as environmental (Staphylococcus aureus, Streptococcus dysgalactiae, Streptococcus uberis), sensitivity of bulk milk testing is low (<30%).³ New technologies such as matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) or loop-mediated isothermal amplification (LAMP) offer new insights into intramammary infection but are not applied at the bulk milk level yet.

Bulk tank milk was very effectively used in Australia to evaluate herd status with an ELISA test to eradicate bovine leukaemia virus. Interpreting results from bulk milk tests requires careful consideration of complexities, including the test target, prior virus exposure including vaccination, duration of antibodies in milk or virus shedding, and other relevant factors. For direct detection of pathogens or the immune or metabolic responses to pathogens, increased herd size and prevalence of organisms or response has significant testing implications. Specifically, as herd size increases the probability of the assay detecting the presence decreases due to potential for dilution. Studies identified the limits of detection for different assays and estimates for herd sensitivity (0.44–0.97) and herd specificity (0.42–1.00) for pestivirus ELISA.⁴ The use of PCR to detect persistently infected animals detected up to 1 animal in 1000, however, other studies indicated a lower sensitivity. Bulk tank milk can also be used to detect bovine alphaherpesvirus-1 (BHV-1) which is prevalent in Australia, while the use during a foreign animal disease outbreak is arguable.

There are valuable bulk tank milk assays that can monitor the herd nutritional status.⁵ The bulk tank milk urea and protein content provide useful indications of herd nutrition using routine milk testing from milk processors. These tests provide indicators that encourage further investigations of nutritional influences on herd fertility but are unlikely to provide strong diagnostic value. The fat to protein ratio has a high specificity, but poor sensitivity for the detection of fibre insufficiency and acidosis on an individual cow basis and is useful as an alert to evaluate other indicators of acidosis in a herd. Integrating herd recording demographic information with Fourier-transformed mid-infrared spectra (FT-MIR) can provide tests that are useful to identify cows with metabolic disorders and these tests will become more available in Australia. Selenium, zinc, β-carotene, and vitamin E status of the herd can be determined using bulk tank milk and could be combined to monitor herds.

Bulk tank milk is an under-utilised resource that can be used to improve the health and productivity of dairy herds. There is considerable potential to increase the availability and adoption of bulk tank milk as part of an efficient integrated farm service. Some of the tests readily available and reported on a daily basis include milk volume, somatic cell counts, fat, protein, and milk urea. More can be done with these data to evaluate herds. However, there appears to be increased scope for testing for milk minerals, vitamins, and assays for mastitis and viral pathogens. These assays can provide cost-effective rapid means of monitoring herds and have the potential to be integrated with statistical monitoring methods to automate detection of changes in herd status and equip veterinary services with new tools to assist farms.

The authors declare no conflicts of interest or sources of funding for the work presented here.