Introduction
Tuberculosis , a highly communicable mycobacterial disease of humans and animals, is caused by members of Mycobacterium tuberculosis complex (MTBC) (Malama et al., 2013; Pal et al., 2014). Although, recent studies indicated that M. tuberculosis has been isolated from cattle (Ameni et al., 2011) and M .bovis from humans infected with bovine tuberculosis (Zeweld, 2014), M. tuberculosis is specifically adapted to humans while  M. bovis  is most frequently isolated from domesticated cattle (Girmay et al., 2012; Pal et al., 2014). In spite of variation in host specificity, the members of MTBC are characterized by 99.9% or greater similarity at nucleotide level, and are virtually  identical at 16s rRNA sequence (Brosch et al., 2002).

Bovine tuberculosis is a contagious disease, which can affect most warm blooded animals, including human beings (Pal, 2007; Radostits et al., 2007; Pal et al., 2014). Organisms are excreted in the exhaled air, in sputum, feces (from both intestinal lesions and swallowed sputum from pulmonary lesions), milk, urine, vaginal and uterine discharges, and discharges from open peripheral lymph nodes of infected animals (Radostits et al., 2007). In cattle, exposure to this organism can result in a chronic disease that jeopardizes animal welfare and productivity, and in some countries leads to significant economic losses by causing ill health and mortality (Ewnetu et al., 2012). Moreover, human TB of animal origin caused by M. bovis is becoming increasingly evident in developing countries (Russel, 2003; Girmay et al., 2012; Mamo et al., 2013; Pal et al., 2014).

Bovine tuberculosis diseased animal loses 10 to 25% of their productive efficiency; direct losses due to the infection become evident by decrease in 10 to 18% milk and 15% reduction in meat production (Radostits et al., 1994). Apart from effects on animal production, it has also a significant public health importance (Müller et al., 2013). Currently, the disease in human is becoming increasingly important in developing countries, as humans and animals are sharing the same microenvironment and dwelling premises, especially in rural areas, and susceptibility of AIDS patients to tuberculosis (Shitaye et al., 2007; Pal et al., 2014). It is estimated that  M. bovis  causes 10 to 15% human cases of tuberculosis in countries where pasteurization of milk is rare and bovine tuberculosis is common (Ashford et al., 2001; Pal et al., 2014).

In developing countries like Ethiopia, the socio economic situation and low standard living area for both animals and humans are more contributing in TB transmission between human to human and human to cattle  or vice versa (Ameni et al., 2010; Ejeh et al., 2013). Human infection due to M. bovis is thought to be mainly through drinking of contaminated or unpasteurized raw milk and under cooked meat (Pal et al., 2014).. The high prevalence of TB in cattle, close contact of cattle and humans, the habit of raw milk and meat consumption, and the increasing prevalence of HIV may all increase the potential for transmission of M. bovis and other Mycobacterium between cattle and humans (Shitaye et al., 2007; Pal et al., 2014).

Bovine tuberculosis is an endemic disease of cattle in Ethiopia (Girmay et al., 2012), with a reported prevalence of 3.5–5.2 % in abattoir (mostly zebu), and 3.5–50% in crossbreed farms (Shitaye et al., 2007; Demelash et al. 2009; Regassa et al., 2010; Berg et al., 2011). Nevertheless, the available information is limited due to inadequate disease surveillance and lack of better diagnostic facilities (Cosivi et al., 1998; Asseged et al., 2000). In particular, information on genotypic characteristics of M. bovis, a strain affecting the cattle population in Ethiopia, is limited (Biffa et al., 2010). Such information is critical to monitor transmission and spread of the disease among cattle (Berg et al., 2011).

According to the  World  Health  Organization,  the  status  of  TB  in Gambella  Region was  the  highest  from  all  the  Ethiopian  Regions,  with  the notification  rate  (new  and  relapse)  261- 421/100, 000 (WHO, 2009).This was one of the basis to undertake the present study. Gambella regional state has large livestock populations. Despite, the large number of livestock population in the region, there is no information on BTB. Despite the fact that bovine tuberculosis is a public health threat and also leads to economic losses, in Ethiopia research on and control of animal tuberculosis has not received much attention like human tuberculosis (Chukwu et al., 2013).Thus the present study was designed to determine the prevalence of bovine tuberculosis in Gambella town municipal abattoir and identifying risk factors associated with bovine tuberculosis, to isolate and molecular characterization of mycobacterial isolates from slaughtered cattle and from human pulmonary TB patients and to investigate the potential risk factors for zoonotic transmission of mycobacterial infections.

1 Materials and Methods

Study area

The study was conducted in Gambella town municipal abattoir and Gambella hospital of Gambella regional state, southwest Ethiopia from December 2014 to May 2015. The Gambella People's Regional State is located south west Ethiopia between the geographical coordinates of 6⁰ 28'38" to 8⁰ 34' North Latitude and 33⁰ to 35⁰ 11’11" East Longitude, 766 km far from Addis Ababa which covers an area of about 34,063 km². The Region is bounded to the North, North East and East by Oromya National Regional State, to the South and Southeast by the Southern Nations and Nationalities People's Regional State and to the Southwest, West and Northwest by the Republic of south Sudan (Behailu et al., 2011). The mean annual temperature of the Region varies from 17.3℃ to 28.3℃ and absolute maximum temperature occurs in mid-March and is about 45℃ and the absolute minimum temperature occurs in December and is 10.3℃. The annual rainfall of the Region in the lower altitudes varies from 900-1,500 mm; at higher altitudes it ranges from 1,900-2,100 mm. The annual evapotranspiration in the Gambella reaches about 1,612mm and the maximum value occurs in March and is about 212 mm (Tilahune, 2012). Based on the Census conducted by the Central Statistical Agency of Ethiopia (CSA), the Gambela region has total population estimation of 406,000 (CSA, 2014), and livestock population of Gambella 253,389 cattle, 39,564 sheep and 83,897 goat (CSA, 2011).

Gambella town municipal abattoir

The abattoir, which is administered under Gambella town municipality, is the only source of inspected beef for all inhabitants of the town. The average number of animals slaughtered per day during the study period was about 25 with all 100% of the slaughtered animals being cattle. The overall abattoir sanitary environment is below the requirements of good hygiene practices (GHP) in slaughter houses. The internal and external facilities and sanitary conditions of the slaughter house were very poor. Neither place for disposal of condemned carcasses nor facilities for wastewater treatment exist and it is not friendly with the environment. The abattoir workers had no clothing, boot, apron and other accessories. Three assistant meat inspectors delivered services only during antemortem, and none of them carried out post mortem examination during the study period in such a ways the population is endangered of meat borne zoonosis and sanitation problems.

Study population and study design

According to the available logistics and time, a total of 500 apparently healthy animals slaughtered in the abattoir of Local Nuer, Horro, and Felata breed cattle were included as study population for the stated objectives, and the major sources of cattle for this abattoir were Gambella town and its surroundings, Mettu, Gore, Bure, Sibo, and Gumero. In addition, 50 acid fast bacilli (AFB) positive sputum samples from human TB patients attending the health facilities in Gambella town were included.

A cross sectional study with systematic random sampling was carried out in abattoir to examine the carcass and sample suspected TB lesions from slaughtered cattle at Gambella town municipal abattoir. Similar cross sectional study and purposive sampling was carried out to collect samples from all AFB positive TB patients attending Gambella Hospital. Both sputum and extra pulmonary TB samples mainly fine needle aspirate from suspected human case was taken in the course of the study period for isolation and molecular characterization of the causative agents.

Sampling, sample size determination and study methodologies

All animals coming to the slaughter house from different areas during the study period were considered for sampling. The sample size calculation was based on 50% prevalence assumption (since there was no study on bovine tuberculosis in the area), 95% CI and d=0.05 (Thrusfeild, 2005).

Where n= required sample size

P_exp. =expected prevalence

          d=Desired absolute precision (5%)

          Z= Normal distribution constant

Therefore, the sample size calculated was 384, but to increase the precision using thumb rule by 20% and the total animals supervised were 500.     

The sample size for the questionnaire survey used was 100 for livestock owners, and abattoir workers. For human case, a total of 50 acid fast positive patients were interviewed about their association with cattle, habit of consumption of meat and milk and other relevant information related to tuberculosis.

Ante and postmortem examination

Physical  examination  of  the  animals  were  carried  out before  they  were  slaughtered.  Body  temperature,  pulse rate, respiratory rate, condition of superficial lymph nodes and  visible  mucus  membranes  were  examined  and recorded  for  individual  animals  to  be  slaughtered.  Breed, source or origin and sex were also recorded.  Age  was  estimated  as described  by Amstutz (1998)  and  body  condition  scoring (BCS) chart was prepared  based  on  the  description given by Nicholson and Nicholson (1986). Detailed postmortem examination  (inspection,  palpation  and  incision)  of  the carcass,  lungs, liver,  and kidneys together with mesenteric, hepatic lymph  nodes,  and  lymph  nodes  of  the  head  was  undertaken in accordance with the method developed by Ethiopian Meat Inspection and Quarantine Division of the Ministry of Agriculture (Hailemariam,1975; Ameni et al., 2007). Lymph nodes were incised into a size of 2 mm to facilitate the detection of tuberculous lesions from each animal. These include mandibular, retropharyngeal, bronchial, mediastinal, and mesenteric lymph nodes. The animal was classified as lesioned (infected) when tuberculous lesion was found, and if not, as non lesioned (not infected). The severity of gross lesions in individual lymph nodes and other organs were scored as follows; 0= no gross lesion, 1= small lesion at one focus, 2= small lesions at more than one focus and 3= extensive necrosis as developed by Ameni and co-investigators (2006). The cut surfaces were examined under bright light for the presence of abscess, cheesy mass, and tubercles (Corner et al., 1990). In the presence of suspected tuberculous lesions, tissue samples were collected in sterile universal bottles containing 0.85% normal saline for culture kept at −20℃ in the refrigerator. The samples were transported under cold chain by ice box with packed ice to Akililu Lema Institute of Pathobiology, Addis Ababa for culture and further processing in three week basis.

Isolation of mycobacteria

Tissues with suspected lesions were collected and subjected to bacteriological culture examination. The tissue specimen or sputum collected from AFB and gene Xpert positive patients for culture were collected individually in to sterile universal bottles in normal saline and then labeled and kept frozen (−20℃) at Gambella regional hospital before being transported to Aklilu Lema Institute of Pathobiology, Addis Ababa.

The specimens were labeled and pooled together, kept in universal bottle containers, and then transported in ice pack box to Aklilu Lemma Institute of Pathobiology, Addis Ababa, within three week basis by airplane. There the samples were processed for isolation of M. tuberculosis complex according to the standard methods (Ameni et al., 2007).

Identification and characterization of mycobacteria

Initial identification of mycobacterial species from animal tissue was based on the rate of growth, pigment production, and colony morphology as described in OIE (2009). When visible colonies were observed, Ziehl Neelsen staining was performed to confirm the presence of acid-fast bacilli. AFB positive isolates were prepared by mixing two loops full of colonies in 200 mL distilled water, heat-killed at 80℃ for 1 hour using water bath, and stored at −20℃ until molecular characterization was perform and were subjected to PCR based on amplification of a multicopy DNA target sequence for identification of M. bovis and M. tuberculosis (Debebe et al., 2013).

RD deletion typing

For RD9 deletion typing of culture positives of sputum; RD9 intR: CTG GAC CTC  GAT  GAC CAC  TC, RD9 flankF:  GTG  TAG  GTC AGC  CCC  ATCC  and  RD9 flankR:  GCC  CAA  CAG  CTC GAC  ATC  primers to  check  for  the  presence  of  RD9  locus was used; The   HotStarTaq    Master  Mix  system  from  Qiagen  was  used  for  PCR,  with  primers  described  previously (Ameni et al., 2013).

The primers used were RD4F 5´­-CTCGTCGAAGGCCACTA AG­-3´. The mixture was heated in a Thermal Cycler (Applied Bio-systems; Gene AMP 9700) for 15 minutes at 95℃ and then subjected to 35 cycles of one minute duration at 95℃, one minute at 55℃, one minute at 72℃ and 10 minutes at 72℃. The presence of RD4 (RD4 is intact in M. tuberculosis, M. africanum) gives a product size of 335 bp (RD4 intF + RD4flankR), and its absence (M. bovis) gives a product size of 446 bp (RD4flankF + RD4flankR).

Spoligotyping

Spoligotyping was carried out using the commercially available kit according to the manufacturer’s instructions and as previously described by Kamerbeek and others (1997).

Questionnaire survey

The roles of various risk factors in the occurrence and spread of bovine TB among cattle, and between cattle and people, were assessed by a questionnaire. Structured questionnaire was distributed to TB patients, cattle owners, and abattoir workers to assess the perception of stakeholders on the occurrence of bovine tuberculosis, livestock constraints, socioeconomic status, herd composition, and awareness on the potential risk of zoonotic transmission of bovine tuberculosis.

Data management and analysis

Prevalence was calculated as the proportion of suspected lesion positive animals from the total number of animals visited (Thrusfield, 2005). Data related with age, sex, breed, origin and body condition of each animal was recorded on a data sheet during ante-mortem examination. Presence or absence of TB like lesions and affected tissues were recorded during postmortem examination. The recorded data was entered and stored in Microsoft Excel computer program and analyzed by STATA version 11 (STATA Corp. College station, TX). The variations between different factors were also analyzed using multi variable logistic regression and chi-square (χ2) was used for association of different risk factors. A p-value <0.05 was considered statistically significant, 95% confidence interval was considered and Odds ratio analysis was used (Table 1).

Table 1 Univariate and multivariable logistic regression analysis of tuberculous lesions with various host related risk factors in Gambella municipal abattoir. * Statistically significant

In molecular epidemiology study of isolates from human pulmonary tuberculosis patients  and animals tissues, the spoligotype patterns were converted into binary and octal formats and entered to the online spoligotype database, http://www.pasteur guadeloupe.fr:8081/SITVIT Demo/index.jsp to determine the shared international spoligotype (SIT) number, and the results were compared with already existing designations in the international spoligotyping database (SpolDB4.0 database). Those isolates with no designated SIT number were considered as new to the database. Two or more isolates with identical spoligotype pattern were considered as clustered, while those with single SIT were considered as non-clustered isolates. TB-lineage and family were determined using SPOTCLUST database, http://tbinsight.cs.rpi.edu/about_spotclust.html.

Ethical considerations

Ethical clearance was obtained from the Ethical Committee of Gambella regional health office (Reference number of 16/3776/7) and working permission was received from the hospital higher managers and the municipality.

2 Results

Prevalence of bovine tuberculosis

The overall prevalence of bovine tuberculosis in slaughtered cattle of Gambella municipal abattoir was 13.2% (66/500: 95% CI, 10.22-16.18) based on the occurrence of gross tuberculous lesions (Table 1). 

Gross pathology

Gross lesions were observed in the lymph nodes and lung of the slaughtered cattle and the majority of the lesions were considered typical of tuberculous lesions characterized by central round, oval, or irregular, often coalescing areas of caseous necrosis and mineralization (calcification) (Figure 1, Figure 2). Whenever gross lesions suggestive of TB were detected in any of the tissues, the tissue was classified as having lesions (Table 3).

The frequency and distribution of lesions according to organ level and anatomical site is indicated in Table 2.

Table 2 Distribution of lesions in different anatomical sites with their respective frequency of occurrence

Table 3 Mean pathology scoring of lesion from lung and lymph node of bovine tuberculosis in cattle slaughtered at Gambella municipal abattoir

Figure 1 Typical TB lesions of cattle slaughtered in Gambella town municipal abattoir. A: Granulomatous lesion from mediastinum; B=Caseous and granulomatous necrosis in lung; C&D= calcified and granulomatous lesion in mesenteric lymph nodes.

Figure 2 Mean severity of lesions in lung and different lymph nodes

Mycobacteriological culture

Out of 82 tissue samples examined, 14 (17.07%) showed growth on LJ medium, and out of 50 sputum samples and one FNA sample,  17 (34%) of sputum samples revealed growth on LJ media while the FNA sample did not yield any growth (Table 4).

Table 4 Bacteriological results on Lowenstein Jensen (LJ) medium

Molecular Characterization of Mycobacterial Isolates

Figure 3 to Figure 5 and the Table 5 were the Characterization analysis of Mycobacterial isolates.

Figure 3 Electrophoretic separation of PCR products by RD9 deletion typing of 11 mycobacteria isolates from human TB patients. 1~11: Samples from TB patients; 12: Mycobacterium bovis control; 13: Distilled water negative control; 14: Mycobacterium tuberculosis control, 15: 100 bp Ladder Marker

Figure 4 Electrophoretic separation of PCR products by RD4 deletion typing of 8 mycobacteria isolates from tissue sampled culture. M: Ladder (100bp); 11: Mycobacterium tuberculosis control; 10: Distilled water negative control; 9: Mycobacterium bovis control ; 1~8: samples from tissue culture positives

Figure 5 Spoligotype patterns of mycobacterial isolates recovered from sputum of human patient and tuberculosis lesions in cattle. The filled boxes (blacks) represent the presence of spacers, and the empty boxes indicate the absence of spacers

Table 5 Lineages of the isolates

BTB Awareness and risk factor Assessment

The analysis of BTB Awareness and risk factor Assessment were seem in Table 6.

Table 6 Client’s (farmers, abattoir workers, TB patients) awareness of bovine tuberculosis and its mode of transmission

3 Discussion

Tuberculosis remains a major global health problem causing high morbidity and mortality among millions of people each year (Pal, 2007; WHO, 2014). Tuberculosis caused by M. bovis is clinically indistinguishable from tuberculosis caused by M. tuberculosis, and globally the proportion of human tuberculosis caused by M. bovis is estimated to 3.1% of all forms of which 2.1% are of pulmonary, and 9.4%  are of extra pulmonary forms (Cosivi et al., 1998; Pal et al., 2014).

Ethiopia is one of the countries with highest number of livestock resource in Africa, and animal tuberculosis is known to be endemic and wide spread in the country. However, in spite of high prevalence both human and animal tuberculosis in the country, the emphasis given on bovine tuberculosis to the Gambella region is very little and so far no research were carried out on BTB in Gambella region. Infection of cattle with M. bovis constitutes a human health hazard as well as an animal welfare problem. Furthermore, the economic implications in terms of trade restrictions and productivity losses have direct and indirect implications for human health and the food supply (Zeweld, 2014).

In the present study, an attempt was made to determine the prevalence of bovine tuberculosis in Gambella town municipal abattoir, and identifying risk factors associated with bovine tuberculosis, to isolate and molecular characterization of mycobacterial isolates from slaughtered cattle, and from human TB patients and to investigate the potential risk factors for zoonotic transmission of mycobacterial infections from animal to human and vice versa. 

Based on detailed postmortem inspection, the prevalence of BTB in slaughtered cattle was found to be 13.2%, which is moderately high and this observation was comparable with other pervious research reports carried out on cattle originated from extensive and pastoral production system of Ethiopia: 11.50% by Abdurohaman (2009) in Butajira, and 11% by Mamo et al. (2013) in Afar, but less than a result from 19.8% record from cattle slaughter in rural Tanzania (Cleaveland et al., 2007). The results of the present study showed higher prevalence than the findings of other investigators. Biffa and others (2010) reported 4.2% prevalence in cattle slaughtered at in Yabello municipal abattoir, and 4.5% prevalence was recorded at Hosaana abattoir by Teklu and co-workers (2004). In addition, the results were also higher than previously reported by other researchers in Northern and Central parts of the country (Nemomsa et al. (2014) (9%); Zeru et al. (2013) (6.4%); Romha et al. (2013) (5.8%). This difference in prevalence of tuberculous lesions could be due to the difference in origin or types of production system and breed of animals, which were slaughtered in the abattoirs.

In respective of small sample size due to wondering of the Felata breed from place to place, association of breed with prevalence of BTB showed a statistically high significant difference among different local breed of cattle, (P = 0.000) animals, which might be related to the genetic difference of the breeds, Other previous studies also showed different breeds could result in difference in susceptibility to BTB infections (Ameni et al., 2007). 

There is a statistically significant difference in the prevalence of the disease (P = 0.000) between BCS, the prevalence being the highest in poor body condition (32.6 %) as compared to medium (12.7% ), fatty (good) animals (6.3 %), which is in agreement with study conducted by Nemomsa and others (2014). This could be related to the weak protective immune response in poor body conditioned animals as compared to good one that may result extensive lesions, and wasting of the body condition as well as chronic nature of the disease. The present result is consistent with previous reports, which indicated that animals with good BCS have relatively good immunological response to the infectious agent than animals with medium BCS (Radostits et al., 2007).

In this study, gross tuberculosis lesions were found most frequently in the lymph nodes of thoracic cavity (50%), mesenteric lymph node (25.6%), followed by lymph nodes of the head (24.4%). This finding is significantly different from previous studies done in Ethiopia (Tamiru et al., 2013) where 70 and 70.7% TB lesions were reported in lungs and associated lymph nodes, respectively. However, the distribution of TB lesions in the current study is significantly similar with reports from Mexico (Ndukum et al., 2010) where 49.2% of lesions involved the thoracic lymph nodes. The results, therefore, indicate that the primary route of infection was through the respiratory route, which can also spread to other parts of the body (Radostits et al., 2007; Pal et al., 2014).

In this study, culture positivity in primary culture media was found low and confirmed in 23.49% (31/133), slightly lower than the findings of Ameni and co-investigators (2007) who recorded 56% culture positivity. This low isolation rate of mycobacteria may have resulted from reduced sensitivity of culture arising from prolonged storage at field sites and the freeze-thaw cycles that occurred during transportation and contamination of tissue samples (OIE, 2009). Furthermore, the presence of caseous and/or calcified lesions, and even lesions resembling tuberculous lesions may not always found to be of mycobacterial origin; they can be caused by any other intracellular organisms or parasites, or viable mycobacteria may not be present in calcified lesions (Corner, 1994).

In the present study, interestingly, M. tuberculosis strain SIT523 was isolated and characterized with spoligotyping from cattle mediastinal lymph node tissue, and the result implies the occurrence of reverse zoonosis in the study area where human strains could be transmitted to cattle.  The transmission to cattle could be through different routes including ingestion of feed contaminated with infected sputum and/or urine from M. tuberculosis infected farmers. Humans suffering from active TB are the most probable source of M. tuberculosis in animals, with infection spread via sputum, and rarely urine or feces (Stee and Thoenle, 1995) or respiratory route as in rural area of Ethiopia, grazing cattle are commonly brought into the farmer’s households at night where they may become infected via aerosol transmission from humans (Ameni et al., 2013). Previous studies in Ethiopia  had confirmed transmission of M. tuberculosis from farmers to their cattle, goat, and camel ( Ameni et al., 2011;  Gumi et al., 2012; Mamo et al., 2012) supporting the result of our study. Even though human to cattle transmission of M. tuberculosis has been reported, it is generally held that disease in cattle due to M. tuberculosis is less severe than that caused by M. bovis ,and the identification of M. tuberculosis in cattle by itself is intriguing (Tsegaye et al., 2010). Hence, the identification of M. tuberculosis from cattle tissues requires further investigation.

In molecular characterization of isolates from human tuberculosis patients, M. tuberculosis   was the predominant species causing TB in human and the genetic diversity of the isolate on the spoligopattern was 45.45%, which was higher than  previous reports in other part of Ethiopia where  39% of spoligotype based genetic diversity where reported in Afar TB patients (Mamo et al., 2013). The difference might be related to difference on geographic and sociocultural difference among the studied population which might affect the transmission pattern of the organism. The most common spoligotype identified from TB patient was the SIT 289 is in agreement with previous study (Ameni et al., 2011), which also reported the same SIT289 strains in pulmonary TB patients of central Ethiopia. In the present study, the predominant lineage was unknown according to TB-insight database analysis. Similar, unknown lineage had been previously reported form patients from Northwestern Ethiopia (Belay et al., 2014), and this indicates the need for further studies.

In the present study, the questionnaire  survey  of  the respondents showed  that  22 % of  them  were  aware  of  BTB  with  no  knowledge about  zoonosis  of  the  disease.  This disagrees with  report from Tamiru and co-workers (2013) 80.7% of  them  were  aware  of  BTB  with  low  level of knowledge about   zoonotic implication  of  the  disease. This result was comparable with the study on assessment  of  the knowledge  of  cattle  owners  about  BTB  in  Wuchale Jida district,  Ethiopia  showed  that  38.3%  (36  of  94)  of  the respondents  knew  that  cattle  can suffer  from tuberculosis,  and 30.8% (29 of 94) recognized that BTB is a zoonotic disease (Ameni et al., 2003).  Ameni and others (2007) indicated that lack of understanding regarding the zoonotic importance of BTB, food consumption behavior, and poor sanitary measures are the potential  risk factors of BTB to public health. The  proportion  of  BTB  contributes  to  total tuberculosis cases in humans depends on the prevalence of the disease in cattle, consumer habits, socio-economic conditions, and level  of  food  hygiene  (Ashford  et  al.,  2001), and  medical  prophylaxis  measures  in  practice  (Tigre  et al.,  2011).  According  to  the  results  of  the present  study,  45% consumed  unpasteurized  or  raw  milk.  Similarly,  studies conducted  in  different  parts  of  Ethiopia  indicated  the habits  of  raw  milk  consumption.  The finding of our study on the habit of raw milk consumption was lower than 85.7%, which was reported from Jimma town, Ethiopia by Tigre and others (2011). Study conducted in Wuchale Jida  district  indicated that 52.1%  (49  of  94)  households had habit of consuming raw milk (Ameni et al., 2003), which is significant when compared with  the  current findinds.  None  of the respondents in our study were found to be aware  about  the  transmission  of  the  disease  from  cattle to human  and  vice  versa. 

In our study, keeping cattle in close proximity to their house and calves in their house was a common practice of households. This, indeed, can facilitate transmission of the causative agent from animal to human or vice versa.  According  to  Bogale  (1999), conditions  such  as  customs  of  consuming  raw  milk, keeping cattle in close proximity to the owner house  and using cow dung for plastering wall or floor and as source of  energy  for  cooking  do  exacerbate  the  chance  of spreading tuberculosis as zoonosis in Ethiopia.

Conclusions

The result of the present study has shown that bovine tuberculosis was prevalent in cattle slaughtered at Gambella municipal Abattoir with moderately high prevalence (13.2%). This study also revealed that a high proportion of tuberculous lesions in the thoracic cavity lymph nodes indicate that the respiratory route was the major means of transmission. Isolation and molecular characterization of one M. tuberculosis isolate (SIT523 strain) from animal tissue sample suggested the occurrence of transmission of the agent between the communities and animals that implies reverse zoonosis. The high genetic diversity (45.5%) of the human M. tuberculosis isolates (SIT289, SIT134, SIT1634, SIT142, and the new one), and presence of clustering of the isolates might indicate the recent transmission pattern, and circulation of the agents in the study communities. Lack of awareness regarding BTB and its routes of transmission in the study population was high, and existence of habits of consumption of raw animal product, and sharing of the same microenvironment with their livestock could be potential risk factors for zoonotic transmission of the disease.

Further study should be conducted with larger sample size and geographic coverage to elucidate the role of M. tuberculosis complex in humans and animals. With the finding of promising results on molecular characterization using few samples, a broader study to investigate the molecular epidemiology in human and animal tuberculosis is essential. Public health awareness campaigns should be launched, and needed to raise community awareness about the risk of BTB transmission through consumption of raw/under cooked meat; and the zoonotic implications of BTB/, route of reverse zoonosis are of extreme importance for effective implementation of TB control measures. Establishment  of  collaboration  between  physician  and veterinarians  to  trace  back  positive  patients  to  get  profile of  their  cattle  in the slaughterhouses across the region so as to estimate the regional prevalence of BTB as well as identification and characterization of the M. tuberculosis complex, and evaluation of their pathogenicity in bovine is highly imperative.

Acknowledgements

This study was done under the Thematic Research Project: “Mycobacterial infection in selected rural communities of Ethiopia and their animals”, a collaborative research between Aklilu Lemma Institute of Pathobiology and College of Veterinary Medicine and Agriculture, and funded by the Addis Ababa University Research and Technology Transfer office. Thanks are also due to Dr.Vijay J.Jadhav,Veterinary College, Hissar, India for his excellent computer help.

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