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.