Kashgar has always been one of the highest TB-burden regions in China. In 2020, the reported TB incidence in Kashgar was 250.74 per 100,000 people, which was 4.3 times as that of the national average for the same time period [2]. Here, we firstly used WGS combined with high-throughput quantitative MIC measurements to determine the species composition, lineage distribution and drug-resistance profile of tuberculosis across the whole Kashgar prefecture.
In recent decades, increasing incidences and prevalences of non-tuberculous mycobacteria (NTM) and Nocard's bacillus have been reported [17], which are widely distributed in the environment and can cause clinical symptoms similar to pulmonary tuberculosis and cause positive results in acid-fast staining, but the drug regimen of them is greatly different from that of TB [18]. Our results showed that as high as 4.75% of the strains (NTM or other acid-fast staining positive species) were misidentified as Mtb in Kashgar region based on smear result. The high prevalence of NTM or nocardia or gordonia among symptomatic participants found in this population-based study demonstrates that acid-fast positive bacteria other than Mtb has also been a public health problem in this region. As smears are commonly used to diagnose TB and to initiate treatment in Kashgar region, symptomatic NTM or nocardia or gordonia cases seeking care in health facilities could be misdiagnosed and then treated as TB cases. Without adequate clinical response, these cases may subsequently be misdiagnosed as TB treatment failures. Thus, false MTB-positives may in fact be clinical NTM or nocardia or gordonia and if the treatment outcomes do not improve, the patient may mistakenly be classified as DR-TB. Such patient misidentification may negatively impact the health status of the individual and consequently pose additional cost to the health system. Therefore, precise molecular-based species identification methods were urgently needed.
Due to the special geographical location, the MTBC in Kashgar prefecture owe special lineage composition. 1702 cases of MTBC clinical strains were composed of L1-L4 and La1. Lineage 2 was still dominant in Kashgar prefecture but its proportion was relatively low compared with other region of China (45.83% VS 73.9%) [19]. Lineage 3 in Kashgar prefecture accounted for 27.14%, much higher than that of other regions of China, as Kashgar prefecture borders Pakistan (Lineage 3 was dominant; 70.40%) [20], and the border crossing (Khunjerab Port) is located there. It is speculated that the frequent movement of people between the two places caused the spread of Lineage 3 strains to this region [21]. Furthermore, the transmission probability may increase with the growth of population migration, trade, and cultural exchange between the two countries. La1 (M. bovis) account for 0.24% of all lineage and its proportion was much higher than that of other region in China [19, 22], which may be due to the pastoral livelihood in Kashgar, where TB in cattle is prevalent in virtually all major livestock-producing region, for the zoonotic M. bovis TB was the most often transmitted strain to humans by the consumption of M. bovis–contaminated dairy products that caused lymphatic TB, eventually becoming pulmonary TB [23]. Different Mtb lineage is clinically, virulency, and radiologically diversity [24, 25], lineage accurate identification can be most efficiently by using WGS.
It is notable that the prevalence rates of drug resistance in Kashgar prefecture were very low compared with other region of China [26, 27], the prevalence of MDR tuberculosis was only 2.4%. Drug-resistant TB arises for two reasons: first, the selection of de novo resistance during treatment of the index case, and second, the transmission of drug-resistant TB. Kendall et al. have demonstrated current estimates of MDR-TB prevalence among TB notifications are most consistent with the hypothesis that over 80% of incident MDR-TB in present-day epidemic settings results from transmission of MDR-TB [28]. We speculate that the main reason for this situation was that all confirmed active TB patients were given inpatient isolation treatment until completely cured in Kashgar prefecture, and thus the incidence of drug-resistant tuberculosis was reduced due to inappropriate treatment as well as the transmission of drug-resistant tuberculosis, resulting in this measure being extended throughout the whole country. In addition, we find new anti-TB drugs (bedaquinoline and delamanid) and repurposed drugs (clofazimine and linezolidine) resistant rate was very low as well, which indicates application of a short-course drug-resistant tuberculosis treatment regimen containing new drugs in Kashgar prefecture is promising.
Although Kashgar has low drug resistance rate, it remains at risk of an epidemic of drug-resistant TB. This survey not only provides the prevalence of drug-resistant TB but also presents the MIC distributions of 13 drugs, we can find a significant proportion of strains had MIC values at ECOFF and sub-ECOFF resistance level, these sub-threshold elevations in MIC may nevertheless be clinically meaningful, as the combination of significant interpatient pharmacokinetic variability and elevated MICs predisposes Mtb strains to develop resistance, risking more treatment failure and worse patient outcomes [29]. In addition, due to the limitation of laboratory conditions and the lack of skilled personnel, drug sensitivity test cannot be carried out in the whole Kashgar prefecture to detect drug-resistant tuberculosis in time. The existing drug-resistant TB is prone to epidemics easily through transmission. Using WGS combined with quantitative MIC measurements, our results showed that WGS is highly accurate at predicting phenotypic drug resistance except isoniazid in Kashgar prefecture, in consideration of high concordance rates of isoniazid prediction demonstrated by previous studies [14], we speculate that there may be a new isoniazid resistance mechanism of Mtb isolates in Kashgar area, which needs further exploration. Mtb strains that are resistant to new and repurposed anti-TB agents are rare to date, especially in low TB incidence countries. Consequently, there are insufficient phenotypic DST data globally to confidently interpret mutations associated with resistance to these agents as evidenced by the current WHO catalogue of mutations for Mtb, and the accuracy of new and repurposed anti-TB drugs will also improve as data accumulates. Thus WGS with the ability to accurately predict drug resistance could give us a more accurate picture of local resistance profile and provide the basis for the formulation of individual patient precision therapeutic regimen. Moreover, the cost of WGS is already lower than phenotypic testing and is progressively decreasing. WGS is also easy to operate and is less instrument-dependent than phenotypic testing. For these reasons, it could be particularly valuable in settings with weak laboratory capacity and a shortage of skilled people.
Our study had some limitations. First, we did not perform drug susceptibility testing on isolated NTM, Nocardia and Gordonia, and thus we cannot acquire their drug resistance characteristics and compared them with local Mtb strains. Second, different resistant mutations can confer different resistant level, but due to the small number of resistant strains, we did not analyze the resistant level of different mutations. Additionally, due to incomplete information collection and low number of drug-resistant isolates, we did not perform drug-resistant related risk factor analysis.
In conclusion, accurate strain identification, lineage determination and drug resistance detection are very important for TB treatment and control. Our findings prove that WGS can have an important role in species and lineage identification, and drug resistant tuberculosis diagnosis, especially in consideration of the limitation of laboratory equipment conditions and the lack of skilled personnel in high TB-burden regions. Additionally, from this study, we have a baseline of tuberculosis drug-resistance mutations in the whole Kashgar prefecture. According to the change in drug-resistance trends in this region, the targeted strategies for drug-resistant TB can be formulated.