Animals infected with bTB shed bacteria
through bodily fluids including saliva, feces and milk. In humans and animals, M.
tuberculosis and M. bovis are
commonly spread through aerosol exposure (Waters and Palmer 2015). This leads to the
common assumption that any lesion restricted to the respiratory tract was
contracted through aerosols or lesions restricted to intestines were contracted
through ingestion of M. tuberculosis
or M. bovis. However,
studies have fed calves with milk infected with M. bovis and calves developed lesions in the lungs with no evidence
of infection in the intestines or lymph nodes (Edwards 1937; Jones et al. 2016). Castillo-Velázquez et al. (2013) found in their study that a single bacterium can cause infection through the
respiratory tract but a higher infectious dose is required to infect through
the digestive tract. This shedding allows for easy spread of disease to other
cattle, their calves and humans. For humans the
primary exposure to M. bovis is from
unpasteurized milk and other unpasteurized dairy products but it can also come
from contaminated meat (Edwards 1937;
Grange 2001; Waters and Palmer 2015).

The spread of bTB is mostly through direct
contact with infected animals (Khatri et al. 2012). Housing of animals in close quarters or crowding can increase
the spread of bTB to young animals (Waters and Palmer 2015). The European Food Safety Agency (EFSA) reported slaughterhouse meat inspection is a main method of
bTB surveillance in Europe. Inspection involves examination of the slaughtered
cattle for characteristic lesions and collecting microbial samples from the
lungs and lymph nodes (EFSA 2013). In Canada, the Canadian
Food Inspection Agency (CFIA) controls the spread of positive bTB animals
through strict quarantine and animal movement controls, humane destruction of
infected and exposed animals, testing of at-risk herds or herds on previously
infected properties. If animals are infected, they are ordered to be destroyed
owners will be compensated based on market value (CFIA 2017). After infected animals are disposed of,
CFIA requires all areas where infected animals may have come into contact with
must be cleaned and disinfected, in accordance with international standards (CFIA 2017).

In one Argentina study where cattle were
routinely inspected in their National Program of bTB control program they
isolated M.bovis in 5 of 178 cattle
randomly sampled that had no signs of visible lesions (Kantor et al. 1987). The EFSA (2013) concluded
that there is no significant evidence of transmission of M. bovis during handling or consumption of the meat since it is not
meat-borne by nature. Due to difficulties in designing experiments it is challenging
to determine if meat-borne illness can infect humans. Accidental incision of the
lymph nodes during slaughter can result in cross-contamination with other high-priority
diseases such as Salmonella spp. (EFSA 2013) and it is believed that
bovine meat safety may be compromised in a similar manner (EFSA 2013). Minimizing interaction
with lymph nodes by excluding contact and incision of the lymph nodes can
decrease the risk of cross-contamination of the meat (EFSA 2013) and increase public health safety. Humans that
work in close contact with infected cattle can inhale aerosols containing the
bacteria or through direct contact with infected animals (occupational hazard).
A less common pathway of infection is through the skin. For example, butchers handling contaminated beef may develop
local lesions called “Butcher’s Wart” (Grange 2001). However, there
are no documents of skin lesions developing on cattle. For most people living in high-income countries such as Canada, USA,
and in the EU the risk to humans is very low because of pasteurized milk and
government run programs to control bTB.

The greatest
challenge when trying to manage bTB in cattle is that there is a broad range of
wildlife reservoirs, which act as a vector for transmission to cattle (Brook et al. 2013). Spread of TB is not limited to
political borders as infected wildlife can wander across borders and live
animals, including cattle, are imported from endemic-countries (Tsao et al. 2014). M. bovis is at an increased risk of spreading due to loss of
wildlife habitats, and imports and exports of animals and animal products
(Pollock and Neill 2002; Smith 2012), which is
inspected by and cases are reported to CFIA in Canada. 

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