Unpasteurised fresh and souring dairy products form an essential component of household
diets throughout many rural communities in southern Africa. The presence of milk-borne
zoonotic pathogens such as Mycobacterium bovis (M. bovis), the causative agent of bovine
tuberculosis and zoonotic tuberculosis in humans, constitute a public health threat, especially
in remote areas with poor disease surveillance in livestock and highly compromised
human health due to HIV/AIDS.
In this study we used culture to determine the longevity of M. bovis in experimentally inoculated
fresh and naturally souring milk obtained from communal cattle in the KwaZulu-Natal
province of South Africa. The effect of bacterial load and storage temperature on the survival
of M. bovis was evaluated by spiking mixtures of fresh milk and starter soured milk
(aMasi) culture with three concentrations of bacteria (102, 104, 107 colony forming units/ml),
followed by incubation under controlled laboratory conditions that mimicked ambient indoor
(20°C) and outdoor (33°C) temperatures and periodic sampling and testing over time (0-56
M. bovis cultured from samples of the fresh and souring milk was identified by PCR analysis.
At the highest spiking concentration (107cfu/ml), M. bovis survived for at least 2 weeks at 20°C; but, at all concentrations in the 33°C treatment, M. bovis was absent by three days
after inoculation. Logistic regression analysis was used to assess the effects of bacterial
concentration and time since inoculation, as well as determine the potential half-life of M.
bovis in raw souring milk. Given the most favourable tested conditions for bacterial survival
(20°C), approximately 25% of mycobacteria were alive after one day of storage (95% CI: 9-
53%), giving an estimated half-life of M. bovis in raw souring milk of approximately 12 hours
(95% CI: 7-27 hours).
This study demonstrates that M. bovis may survive in fresh and souring milk for periods of
time that represent a risk of exposure to people consuming these products, as well as
domestic or wild animal populations that have reported opportunities to consume homemade
unpasteurised dairy products. The temperature at which the milk is soured and stored
substantially affects the survival time of M. bovis.