Statistical analyses
The farm was defined as positive if at least one swine was detected as positive. Individual prevalence was calculated as the ratio of the number of positive swine to the total number of swine. Similarly, herd prevalence was calculated as the number of positive farms to the total number of farms examined. The prevalence and the 95% confidence intervals were determined using the binomial distribution with Microsoft Excel 2016 (Microsoft Corporation, USA). OR and Fisher’s exact test in SPSS 25 (International Business Machines Corporation, America) were used to determine the difference in positive and negative results between groups. p < 0.05 was considered as significantly different, and p < 0.01 as extremely significantly different.
Results
In total, 1379 serum samples from 45 herds were collected from February to June 2021 in Gansu province. On average, 98.5 serum samples (ranging from 90 to 150) were selected randomly from each city (Table 1). The overall STB prevalence at the individual level was 0.22% (95% CI: 0.04%, 0.63%) and varied from 0.00% to 2.20% in different cities (Table 1) (Figure 2). The prevalence at the individual level was 2.20% (95% CI: 0.27%, 7.71%) in Zhangye city, and that in Wuwei city was 1.11% (95% CI: 0.03%, 6.04%).
The overall STB prevalence at the herd level was 4.44% (95% CI: 0.54%, 15.15%) and varied from 0.00% to 33.33% in different cities (Table 1). Both Wuwei city and Zhangye city had one positive herd with a prevalence of 33.33% (95% CI: 0.84%, 90.57%) at the herd level.
The STB prevalence in intensively raised three-crossbred pigs (0.23%, 95% CI: 0.05%, 0.68%) was higher than that in free-range raised Tibetan pigs (0.00%, 95% CI: 0.00%, 3.85%), but the difference between them was not significant (p = 0.81). In addition, the STB prevalence in Guangxi province (1.67%, 3/180, 95% CI: 0.35%, 4.79%) was higher than that in Gansu province, and the difference between them was significant (p < 0.05) with odds ratio (OR) of 7.77 (95% CI: 1.56, 38.81).
Discussion
Although STB has not been subjected to the official investigation, there is a strong body of evidence that TB can be actively present in the pig population. Either in Spanish wild boars (Naranjo et al. 2008) or Nebrodi Black Pigs in Sicily (Di Marco et al., 2012), the high rates of reported granulomatous lesions related to TB suggest that suids can act as maintenance hosts. Furthermore, a novel strain of M. bovis was isolated from Nebrodi Black Pigs, arousing the concern of transmission of M. bovis genotypes in the porcine species (Amato et al., 2018; Di Marco et al., 2012). This implies that the control of STB should be mandatory if the presence of pigs poses an epidemiological risk.
The carcass inspection at the slaughterhouse, which is regulated by legislation, is not the best approach for STB prevention and control. It has been reported that the isolation of M. bovis does not necessarily correspond to visually detectable lesions at the abattoir (Garcia-Jimenez et al., 2013; Martin-Hernando et al., 2007). Besides, the organs from which M. bovis is most frequently isolated in swine are mandibular lymph nodes and tonsils, but these organs tend not to be examined during swine carcasses official inspection (Gortazar et al., 2011). Although, identification of specific members of MTBC is desired to provide proper treatment or control measures in STB. But it is quite difficult currently and time-consuming, and may produce false-negative results. Since swine appear to produce humoral responses soon after M. bovis infection, and these humoral responses are maintained during the development of disease, antibody diagnostic technique is suitable for STB prevention and control (Garrido et al., 2011; Roos et al., 2016). PPD-B-ELISA is an ideal choice to identify the risk of STB transmission due to its advantages of simple, easy, rapid, and high-throughput screening. In addition, the 6 positive sera and 20 randomly collected negative sera were tested with PPD-A-ELISA and the results were negative, which means PPD-B-ELISA had an excellent analytical specificity and the seropositive swine were infected with MTBC other than environmental mycobacteria.
To our knowledge, this is the first cross-sectional survey of STB in China. Our data indicated that the STB prevalence in Gansu province, China was 0.22% (95% CI: 0.04%, 0.63%), which was lower than that in Egypt (8.90%) (Mohamed et al., 2009), Ethiopia (5.83%) (Arega et al., 2013), Uganda 3.1% (Muwonge et al., 2010), and Spain (2.3%) (Cano-Terriza et al., 2018). STB prevalence in Gansu province, China was the same as that in Czech Republic (0.22%) which has been included amongst the states declared free from bovine tuberculosis within the European Union on February 31, 2004 (Pavlik et al., 2005). The lower STB prevalence in Gansu province might be associated with the preventive measures adopted by China since the Ministry of Agriculture and Rural Affairs of the People’s Republic of China launched the control and eradication project on bovine TB in 2012 (Shufang et al., 2015). In June 2017, the Ministry of Agriculture and Rural Affairs of the People’s Republic of China issued the National Guidelines for Controlling bovine TB (2017–2020), which adopted comprehensive prevention and control measures such as quarantine culling, risk assessment, movement control, and strengthening health management (The Ministry of Agriculture and Rural Affairs of the People’s Republic of China., 2017). The World Organization for Animal Health, the World Health Organization, the Food and Agriculture Organization, and the International Union Against Tuberculosis and Lung Disease also jointly launched the first-ever roadmap to tackle zoonotic TB in 2017.
The STB prevalence in Guangxi province, China (1.67%, 95% CI: 0.35%, 4.79%) was significantly higher than that in Gansu province, China (p < 0.05) with OR of 7.77 (95% CI: 1.56, 38.81). The relatively higher bTB prevalence in Guangxi province might be due to the fact that the cold and humid environment in the winter in this region was more suitable for MTBC survival (Fine et al., 2011; Santos et al., 2015; Tanner et al., 1999). Wild or free-range swine usually exhibit higher STB prevalence than intensively raised swine. However, in this study, the STB prevalence in intensively raised pigs (0.23%, 95% CI: 0.05%, 0.68%) was higher than that in free-range Tibetan pigs (0.00%, 95% CI: 0.00%, 3.85%), but the difference between them was not significant (p = 0.81). The reason for this might be the limited serum samples of Tibetan pigs used in this study. Future investigations are suggested to collect more serum samples, especially from free-range Tibetan pigs to elucidate the epidemiological status of STB.