This study found that mycobacteria were present in 39% of cultured lymph nodes from slaughtered pigs in Mubende district of Uganda. Sixty three per cent were from lymph nodes with macroscopic lesions compatible with mycobacteriosis and 31.4% were from those without lesions. This was higher than the 21% and 14% isolated from lymph nodes of slaughtered pigs in Nigeria and Switzerland, respectively
[
13,
14]. This difference could be attributed to pig management practices. Pigs in industrialised countries like Switzerland and Lithuania are mainly kept indoors, and the exposure rate will therefore probably be lower and in cases where it occurs it is reported to be due to mycobacteria in water, peat and animal bedding
[
18]. Unlike the pigs in Nigeria, majority of pigs in Mubende district were reared using free or semi free range management systems. The role of management system in NTM prevalence was further consolidated by the evidence from the regression analysis of data which showed a significantly higher level of mycobacteria in free range or tethered pigs compared to housed pigs (OR

=

3; P

=

0.034). Free ranging pigs have a higher risk of exposure to environmental mycobacteria owing to their food scavenging behaviour. They are not provided with supplementary feeds and as a result are often malnourished and infected by different microorganisms, parasites, and possibly predisposing them to mycobacterial infections. This is in accordance with findings from other studies
[
1,
23,
24].
This study revealed that pigs from homesteads with valley dams or water holes as sources of water harboured significantly more mycobacteria than those with boreholes as water sources (OR

=

2.0; 3.0 and P

=

0.049; 0.014) as shown in Table
. In a previous study carried out in the same environments, more mycobacteria from these open water sources were detected than from boreholes
[
23]. Given that valley dam water is harvested as storm water from the hilly terrains, it is therefore expected to be highly loaded with microbes including mycobacteria
[
1,
2]. Such water is often shared by humans, livestock and wildlife, and thus constitutes a primary source of numerous infections including NTM.
Previous studies
[
5,
10,
13,
14] have shown that
M. avium is the most commonly detected
Mycobacterium in lymph nodes of pigs. This is in agreement with the findings of the present study, where
M. avium accounted for 19% of the isolated species. It is however lower than the 39.8%
M. avium isolated from porcine lymph nodes in the Netherlands
[
8].
M. avium subsp.
hominisuis were more prevalent than
M. avium subsp.
avium in this study, which is in agreement with previous studies
[
9]. In this study,
M. avium subsp.
avium was isolated from 4.3% (4/93) of the lymph nodes with lesions, and 2.1% from lymph nodes without macroscopic lesions, consolidating the findings from recent studies
[
18]. Although this is the first documentation of
M. avium subsp.
avium and
M. avium subsp.
hominisuis in pigs from Uganda, the former is known to be pathogen for birds and its presence in pigs is reported to indicate contact with infected birds
[
25]. This indeed could be the case given the free range and tethering system used for rearing pigs in Mubende district. Stringent post mortem inspection remains a critical control method in reducing the exposure risk to human populations. Given the HIV/AIDS status of Uganda, and the fact that 50% of the AIDS patients are likely to develop infections due to
M. avium if their immune status is low
[
3], this finding is of public health interest.
Many different NTM other than
M. avium were isolated from the pigs in the present study, most of these species have previously been also isolated from humans and pigs elsewhere
[
23,
26]. For instance
M. simiae has previously been reported to cause pulmonary infection in HIV patients
[
4,
26,
27]. In Uganda,
M. simiae has recently been isolated from the environments inhabited by primates in Mubende district
[
23], and from febrile patients in Mulago referral hospital in Kampala
[
27].
M. chelonae is known to cause cutaneous infections especially in patients with AIDS
[
28,
29] and
M. terrae has recently been isolated from the environments in Mubende district
[
23], suggesting that these free range pigs acquire the infection from the environment.
M. terrae has been isolated from patients with urinary tract infections and chronic tenosynovitis
[
4].
M. senuense has also been isolated from Mubende district before, although its role in disease causation is not well investigated.
M. asiaticum was detected from pigs in the present study, but not from the environment in the same study area
[
23]. Its clinical significance has been well documented in Queensland Australia
[
30], where it was isolated from patients with extra pulmonary infections. In Queensland, the environment was reported to be the most probable source.
M. gordonae, previously isolated from Mubende environments
[
23], is known to cause infections especially in patients with an underlying predisposition or immunosuppression such as AIDS, steroids therapy or patients undergoing peritoneal dialysis
[
4]. Petroleum by-product oxidising mycobacteria,
M. paraffinicum[
31] was also isolated from pigs.
This study found a higher prevalence of NTM in pigs from Madudu and Kiyuni and lower prevalence in Myanzi, Kiganda and Bukuya. This is could be due to the fact that Madudu and Kiyuni inherently have a high pig population thus apparently increasing the number of pigs exposed to the infection giving higher apparent prevalence of NTM. This is qualified by the large variation (VE

<

=0.86) between than with sub counties as shown by the regression model (Table
).
In rural settings of Uganda, illegal slaughtering of pigs in the backyard and in unhygienic slaughterhouses is a common practice. The lack of strict regulations and enforcement mechanisms in meat inspection practice means that carcasses are usually washed with unhygienic water before being made ready for sale. This water is known to be heavily contaminated, particularly following the rainy season
[
23]. Therefore, it is possible that some of the mycobacteria isolated might be due to contamination rather than infection. However, their presence on the carcass may pose a health risk for meat consumers.