9. Public health importance

Zoonotic tuberculosis in cattle is a public health concern worldwide. The prevalence is estimated at 0.5–1% in developed countries and 10–15% in developing countries [58]. In developing world, high levels of human immunodeficiency virus (HIV) and poverty, especially in Sub-Saharan countries, are contributing factors. Consumption of raw or undercooked products, and especially milk, from infected cattle is the main cause of nonpulmonary tuberculosis [59]. M. bovis is excreted in milk of about 1–2% of infected cattle in large numbers such that a single infected cow can contaminate bulk milk by 100 cows to cause infection in susceptible humans [60]. Social-cultural factors, for instance, the tradition by pastoral communities to consume raw blood and milk and raw or undercooked meat and meat products, are risk factors [1]. It is estimated that in Africa, 90% of milk is consumed either raw or fermented, thus increasing the risk of transmission [61]. Cervical lymphadenitis is the commonest manifestation of oral infection (Figure 3). Inhalation of infected dust particles or aerosols shed by infected cattle is the second important route especially in rural pastoralist communities. Abattoir workers, farmers, milkers, veterinarians, and animal handlers are also exposed to this mode of transmission [58, 62]. Infection in wildlife puts hunters, trappers, and zoo workers at risk. Trans-cutaneous transmission may occur through handling of infected carcasses [47].

Figure 3.

Raptured lesion (arrowed) in the left retropharyngeal lymph node of a tuberculosis patient infected by M. bovis. ©2018 JKN Kuria.
