5.3 Transmission and risk factors

Infected animals shed Mycobacterium via respiratory aerosols, milk, saliva, feces, urine, and discharging lesions. The main route of infection in cattle is mainly through the inhalation of infective aerosols. This is supported by high frequency of tuberculous lesions found in the respiratory tract and associated lymph nodes [46]. Transmission is facilitated by close contact between animals and therefore the production system plays an important role. Intensive livestock farming, referred to as zero-grazing, promotes close contact between animals. In extensive production, such as practiced by nomadic pastoralists in arid and semiarid regions of Africa, close contact between animals occur in, night shelters, watering points, vaccination centers, marketing yards, and at dipping tanks while in intensive production close contact occurs during milking and in watering and feeding troughs [36]. Ingestion of contaminated feed and water is generally considered to be a secondary, less important route of transmission but in countries where untreated manure is commonly used as a fertilizer in farms, such manure can become a source of infection to animals through pasture and vegetation contamination [36, 37]. The oral route is also particularly important in calves nursing from infected cows.

Other rare routes of infection include cutaneous, genital during coitus, congenital through placental or umbilical infection, and transmission through udder infections [47]. Contact between domestic and wild animals through pasture contamination is a risk factor. Domestic species reported to be reservoirs and spill-over Diseases Caused by Bacteria in Cattle: Tuberculosis DOI: http://dx.doi.org/10.5772/intechopen.82051

hosts include sheep and goats. The low prevalence of tuberculosis in these species in the African region, however, indicates that they may not be significant in transmission of disease to cattle [48, 49].

Human to animal transmission through aerosols is well documented and patients with pulmonary tuberculosis pose danger to animals [50]. Humans with urogenital tuberculosis represent a source of infection for animals through contamination of pastures with urine. In Ethiopia, the traditional practice of spiting chewed tobacco into mouths of livestock as anti-parasitic treatment is a potential source of infection with M. tuberculosis [3].

Male animals were more significantly affected by than female animals while Bos indicus (zebu) have been found to be more resistant than Bos Taurus (Exotic breeds). At the herd level, herd size increases infection due to increased exposure and introduction of new animals into a herd is a risk factor [51].

The primary source of infection by MOTTs is presumably the environment [16], and although, the specific source of individual infections may not be easily identified, and the route of infection may be deduced from the localization of granulomas.
