Bulk Tank Milk Cultures Can Provide Useful Information

Bulk tank milk (BTM) culturing can supply two important types of information:

1) presence or absence of a bacterial group, and,
2) identification of predominant bacterial groups in BTM.

The more often BTM is sampled, the more useful the information. Samples taken over consecutive days or weeks are most useful. Extreme caution should be taken when interpreting results from a single BTM sample.

The first question to ask when interpreting BTM cultures is whether or not the samples are positive for Streptococcus agalactiae, Staphylococcus aureus, or Mycoplasma spp. Presence of these pathogens in BTM almost always indicates the presence of infected quarters in the herd. However, negative culture results do not necessarily mean that the herd is negative for infections caused by these pathogens.

Another question that can be answered is "What are the predominant bacterial groups in the BTM sample?" Bacterial isolates from BTM are typically a heterogeneous mixture of various taxonomic and ecological groups. Theoretically, any bacterial isolate from BTM could arise from an intramammary infection. The probability of an isolate originating from a mammary infection is dependent of the bacteria. For example, coliforms and environmental streptococci may originate from mammary infections, but more commons sources of elevated counts caused by these bacteria are milking wet udders, organic soil in milk lines, cracked inflations, inadequately heated wash water, and inadequate cooling of milk.

A note of caution: BTM cultures can not be used to predict the number of quarters infected within a herd, and therefore are not useful as indicators of mastitis prevalence in a herd. Bulk tank milk cultures can be valuable supplements to quarter milk samples, but never a substitute for determining infection incidence and prevalence based on quarter milk samples.

Source: NMC Publication "Laboratory and Field Handbook on Bovine Mastitis" (1999) pg. 171.

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