Bovine Tuberculosis in Uganda

BTB is a chronic infectious disease caused by M. bovis that mostly affects ungulates (OIE 2009). Nearly all warm-blooded animals can be infected by M. bovis, following which they develop clinical signs of BTB.

22.3.1 Historical Overview

The history of any mycobacterial disease is dependent on how pathogens and different disease conditions evolved over time in response to changing population structures and the environment. Human TB, leprosy, and Buruli ulcer dominated the history of diseases caused by the genus Mycobacterium in Uganda (Daniel 1998, 2006). The control of these diseases effectively started in the 1960s after Uganda’s independence, but efforts were halted by the civil unrest that plagued the country from 1966 to 1986 (Ministry of Health 1992). Studies conducted during these turbulent times estimated the average prevalence of BTB to be 22% in cattle (Nyakahuma and Kimezire 1995). For a disease known to have been present in Uganda since the early 1900s (Opuda-Asibo 1995), it is only recently that the holes in our knowledge are slowly being patched by comprehensive epidemiological and socio-anthropological studies. The majority of these studies have shown that BTB is most prevalent in the UCC (Oloya et al. 2007a; Inangolet et al. 2008; Muwonge et al. 2012a; Asiimwe et al. 2009; Nasaka 2014).

22.3.2 Diagnostic Challenges

Making an accurate diagnosis is an important component of BTB control and management in animals, but, as is the case with human TB (Mwinga 2005; Sharma et al.

• Currently, not a single test fulfills all the criteria necessary to identify all BTB-infected animals (Strain et al. 2011),

• The effectiveness of a diagnostic test depends on the ability (dependent on the objectives) of the test to correctly identify animals at different stages of the disease (Nielsen et al. 2011), i.e., exposed, infected, infectious (present an immediate threat), and animals with clinical disease.

• Finally, the intervention that will follow and resources available influence the choice of test.

Unlike in developed countries where the life span of animals is shorter due to a younger slaughter age, in Ugandan cattle, the life span is substantially longer. This means that there is a higher likelihood of detecting infected animals at slaughter as a longer life span favors the progression of a chronic disease and the likelihood of becoming infected. In such settings, animals are expected to remain infectious for a longer duration (Strain et al. 2011; Shittu et al. 2013).

If Uganda were to adopt a comprehensive BTB control policy, the choice of tools would have to take these features into consideration. Since Uganda does not have a structured BTB control policy, the diagnostic and prevalence data of BTB presented in this section are results of academic research projects and are not based on statutorily sanctioned BTB surveillance programs and diagnostic techniques.

22.3.3 Ante-mortal and Postmortem Inspection for Bovine Tuberculosis in Livestock

Ante-mortal diagnosis of BTB is difficult due to the lack of specific diagnostic clinical signs. Due to an absence of BTB surveillance programs using the intradermal tuberculin test, most M. bovis-infected cattle in Uganda will live their entire life without being tested for BTB.

At slaughter, BTB presents with nodular, granulomatous tubercles, predomi­nantly localized in anatomical sites reflecting the route of transmission (OIE

2009). In pigs, lesions have predominately been observed in the abdominal lymph nodes (Muwonge et al. 2012a), while in cattle they occur in both pulmonary and extrapulmonary lymph nodes (Muwonge et al. 2012a; Asiimwe et al. 2009). Recent unpublished data also indicated that goats had extrapulmonary tuberculous lesions especially in the mesenteric lymph nodes from which M. bovis was isolated (Nalapa et al. 2015, unpublished data).

One of the greatest shortcomings of postmortem inspection is its low sensitivity for the detection of gross BTB (tuberculous) lesions, making it impossible to detect a significant number of animals infected with and containing lesions of BTB (En0e et al. 2003; Murray 1986). According to similar studies conducted in Uganda, postmortem inspection for BTB had a sensitivity of only 37% (Muwonge et al. 2012a, b).

22.3.4 Tuberculin Test

Screening for BTB is both qualitative and quantitative. In Uganda, screening has been based on the comparative intradermal test (CCT) using 0.1 ml of purified bovine and avian tuberculin (purified protein derivative or PPD) injections, at a concentration of 20,000 UCT/ml. After 72 hours, an increase of >4 mm in skin thickness at the M. bovis tuberculin injection site, and if in excess of the increase in the thickness of the skin at the site of the M. avium tuberculin injection, is considered positive for BTB.

The lack of the ability to control TB due to M. bovis in Uganda is a hole caused by the lack of sufficient nation-wide data to provide a reliable estimate of the extent of the problem to guide the decision-making process. In the last decade, only ten reports have been published on the occurrence of tuberculosis in Uganda, most providing information about the prevalence of BTB in districts in the UCC. According to these reports, the prevalence of BTB in cattle, based on the tuberculin skin test, ranges from 1.3% to 51.6% at herd level and 0.6%-13% at slaughter (Table 22.1) (Inangolet et al. 2008). It is critical to quantify the problem countrywide to allow an equitable allocation of resources for its control.

22.4