Abstract
To determine the prevalence and molecular characteristics of drug-resistant tuberculosis in Hunan province, drug susceptibility testing and spoligotyping methods were performed among 171 M. tuberculosis isolates. In addition, the mutated characteristics of 12 loci, including katG, inhA, rpoB, rpsL, nucleotides 388 to 1084 of the rrs gene [rrs(388–1084)], embB, pncA, tlyA, eis, nucleotides 1158 to 1674 of the rrs gene [rrs(1158–1674)], gyrA, and gyrB, among drug-resistant isolates were also analyzed by DNA sequencing. Our results indicated that the prevalences of isoniazid (INH), rifampin (RIF), streptomycin (SM), ethambutol (EMB), pyrazinamide (PZA), capreomycin (CAP), kanamycin (KAN), amikacin (AKM), and ofloxacin (OFX) resistance in Hunan province were 35.7%, 26.9%, 20.5%, 9.9% 15.2%, 2.3%, 1.8%, 1.2%, and 10.5%, respectively. The previously treated patients presented significantly increased risks for developing drug resistance. The majority of M. tuberculosis isolates belonged to the Beijing family. Almost all the drug resistance results demonstrated no association with genotype. The most frequent mutations of drug-resistant isolates were katG codon 315 (katG315), inhA15, rpoB531, rpoB526, rpoB516, rpsL43, rrs514, embB306, pncA96, rrs1401, gyrA94, and gyrA90. These results contribute to the knowledge of the prevalence of drug resistance in Hunan province and also expand the molecular characteristics of drug resistance in China.
INTRODUCTION
The emergence of drug-resistant tuberculosis (TB), particularly multidrug-resistant tuberculosis (MDR-TB) and extensively drug-resistant tuberculosis (XDR-TB), has been identified as one of the major obstacles to effective TB control in many countries (1). In China, which is one of the 22 high-TB-burden countries, the prevalence of drug-resistant TB is also a serious problem. The latest data from the national baseline survey on TB indicate that the frequencies of drug-resistant TB, MDR-TB, and XDR-TB among pulmonary TB patients in China were 38.25%, 8.32%, and 0.68% (2), respectively. The magnitude and pattern of drug resistance varied greatly with the region because of the huge size of the country, the diverse population density, and the unbalanced economic development in China (3).
Drug-resistant TB is usually associated with inadequate anti-TB treatment or direct transmission of drug-resistant strains from one individual to another. Knowledge of the clinical characteristics and molecular characteristics of drug-resistant TB is very helpful in their rapid diagnosis and containment. However, previous studies demonstrated that such information varied in different geographical areas (1, 4–7).
Hunan province, with a total population of 69.251 million, is a high-TB-burden area in south-central China. According to recently updated epidemiological data, the prevalence of smear-positive pulmonary TB in this region was estimated at 94 cases per 100,000 inhabitants in 2010 (data not published), higher than the average level of China (66 cases per 100,000 inhabitants; http://www.gov.cn/gzdt/2011-03/21/content_1828718.htm). Unfortunately, thus far, the prevalence and molecular characteristics of drug-resistant TB of Hunan province remain unclear and should be explored to facilitate control of the TB epidemic in this region and throughout China.
Therefore, in the present study, we investigated the genotypes and current levels of resistance to the TB drugs isoniazid (INH), rifampin (RIF), streptomycin (SM), ethambutol (EMB), pyrazinamide (PZA), capreomycin (CAP), kanamycin (KAN), amikacin (AKM), and ofloxacin (OFX) among patients with pulmonary TB in Hunan. Moreover, we analyzed the association of 12 genetic loci, including katG and inhA (for resistance to INH), rpoB (RIF), rpsL and nucleotides 388 to 1084 of the rrs gene [rrs(388–1084)] (SM), embB (EMB), pncA (PZA), tlyA (CAP), eis (KAN or AKM), nucleotides 1158 to 1674 of the rrs gene [rrs(1158–1674)] (CAP, KAN, or AKM), and gyrA and gyrB (OFX), with drug resistance among drug-resistant isolates from Hunan.
MATERIALS AND METHODS
Ethical approval.
The study obtained approval from the Ethics Committee of National Institute for Communicable Disease Control and Prevention, Chinese Center for Disease Control and Prevention. The patients with TB were included in the present research only after we received informed written consent from themselves or from their parents/guardians in cases in which the patient was a child (<18 years of age). An assent was also obtained from participants between 14 and 18 years of age.
Mycobacterium tuberculosis isolates.
This study was carried out between December 2009 and August 2010 at the Hunan Chest Hospital located in the capital of Hunan province, which serves as the sole specialized TB hospital in Hunan. A total of 171 inpatients with clinically diagnosed pulmonary tuberculosis and positive cultures identified as M. tuberculosis complex (MTC) were interviewed and enrolled during the study period. Strain isolation and identification were performed at the provincial reference laboratory of Hunan Chest Hospital. Only one isolate per patient was collected and tested.
DST.
The drug susceptibility testing (DST) was performed using a Bactec MGIT 960 system in a national tuberculosis reference laboratory (NTRL). The critical concentrations for the DST were 0.1 μg/ml for INH, 1.0 μg/ml for RIF, 1.0 μg/ml for SM, 5.0 μg/ml for EMB, 100.0 μg/ml for PZA, 2.5 μg/ml for CAP, 2.5 μg/ml for KAN, 1.0 μg/ml for AKM, and 2.0 μg/ml for OFX (8).
Data collection and definitions.
The demographic and clinical information of enrolled patients, including gender, age, occupation, address, complication, and TB treatment history, was obtained from the inpatient's records.
MDR-TB was defined as resistance to the two first-line drugs, INH and RIF. XDR-TB was defined as resistance to INH and RIF plus OFX and at least one of the three injectable second-line drugs (CAP, KAN, or AKM). Pre-XDR TB was defined as TB with resistance to INH and RIF and either OFX or a second-line injectable agent but not both.
Migrants were defined as individuals from other provinces of China or another country who moved to Hunan. Residents were defined as persons with a registered permanent residence in Hunan. New or previously treated TB cases were defined as previously described (9).
DNA extraction.
Genomic DNAs were prepared by the cetyltrimethylammonium bromide (CTAB) method as described previously (10) and stored at −20°C for further use.
Spoligotyping and data analysis.
Spoligotyping was performed using 43 covalently bound oligonucleotides derived from the spacer sequences of M. tuberculosis H37Rv and Mycobacterium bovis BCG P3 as previously described by Kamerbeek et al. (11). The results were entered in binary format into an Excel spreadsheet and compared with data in the SpolDB4 spoligotyping database (http://www.pasteur-guadeloupe.fr:8081/SITVITDemo/index.jsp).
PCR amplification and sequencing.
Expected fragments were amplified using primers listed in Table S1 in the supplemental material. Each 30-μl PCR mixture contained 15 μl 2× Taq master mix (TaKaRa), 1 μl of the forward and reverse 5 μM primers, 12 μl distilled H2O, and 1 μl of genomic DNA. The reaction conditions consisted of a denaturation step of 5 min at 95°C, followed by 35 cycles of 30 s at 94°C, 30 s at 60°C, and 45 s at 72°C, and a final extension step of 5 min at 72°C. PCR products were sent for sequencing. All sequence data were manipulated with BioEdit version 7.05.3 and were compared to the published sequences (GenBank accession number NC_000962).
Resolution of discrepant results.
For the results that were discrepant between the DST and DNA sequencing, retesting was performed twice using both methods. If the retesting results were in conflict with the original data, a third round of testing was completed, with the final value representing two of the three cycles.
Statistical analysis.
The data were analyzed with the use of SPSS software, 16.0 version. Frequencies, percentages, ranges, and confidence intervals (CI) were calculated as appropriate. A chi-square test or Fisher's exact test was used for intergroup comparison. A two-sided P value < 0.05 was considered statistically significant.
RESULTS
Demographic information.
Among the 171 isolates, 129 (75.4%) were from male patients and 42 (24.6%) from female patients. The ages of the patients ranged from 15 to 88 years (mean ± standard error [SE], 44.8 ± 1.3). The majority (96.5% [165/171]) of them were Hunan residents, and 56.7% (97/171) were newly diagnosed cases.
Drug susceptibility patterns.
In this study, 40.9% (95% CI, 33.6% to 48.3%) of the TB cases were resistant to at least one of the five first-line drugs (INH, RIF, SM, EMB, and PZA); 11.1% (95% CI, 6.4% to 15.8%) were resistant to at least one of the four second-line drugs (CAP, KAN, AKM, and OFX) (Table 1). The proportions of MDR, pre-XDR, and XDR isolates were 25.2% (95% CI, 18.6% to 31.7%), 8.7% (95% CI, 4.5% to 13.0%), and 1.8%, respectively (Table 1). In addition, the proportions of drug-resistant TB among new cases and previously treated cases are summarized in Table 1.
TABLE 1.
Drug susceptibility patterns of 171 clinical M. tuberculosis isolates
| Susceptibility or resistance category or drug(s) | Total patients (n = 171) |
New patients (n = 97) |
Previously treated patients (n = 74) |
|||
|---|---|---|---|---|---|---|
| No. | % (95% CI) | No. | % (95% CI) | No. | % (95% CI) | |
| Overall susceptibility to first-line drugs | 101 | 59.1 (52.0–66.4) | 78 | 80.4 (72.5–88.3) | 23 | 31.1 (20.5–41.6) |
| Overall first-line drug resistance | 70 | 40.9 (33.6–48.3) | 19 | 19.6 (11.7–27.5) | 51 | 68.9 (58.4–79.5) |
| INH | 61 | 35.7 (28.5–42.9) | 13 | 13.4 (6.6–20.2) | 48 | 64.9 (54.0–75.7) |
| RIF | 46 | 26.9 (20.3–33.6) | 7 | 7.2 (2.1–12.4) | 39 | 52.7 (41.3–64.1) |
| SM | 35 | 20.5 (14.4–26.5) | 13 | 13.4 (6.6–20.2) | 22 | 29.7 (19.3–40.1) |
| EMB | 17 | 9.9 (5.5–14.4) | 2 | 2.1a | 15 | 20.3 (11.1–29.4) |
| PZA | 26 | 15.2 (9.8–20.6) | 3 | 3.1a | 23 | 31.1 (20.5–41.6) |
| Overall MDR | 43 | 25.2 (18.6–31.7) | 6 | 6.2 (1.4–11.0) | 37 | 50.0 (38.6–61.4) |
| INH + RIF | 10 | 5.9 (2.3–9.4) | 1 | 1.0a | 9 | 12.1 (4.7–19.6) |
| INH + RIF + SM | 7 | 4.1 (1.1–7.1) | 2 | 2.1a | 5 | 6.8a |
| INH + RIF + EMB | 1 | 0.6a | 0 | 0 | 1 | 1.4a |
| INH + RIF + PZA | 7 | 4.1 (1.1–7.1) | 0 | 0 | 7 | 9.5 (2.8–16.1) |
| INH + RIF + SM + EMB | 3 | 1.8a | 0 | 0 | 3 | 4.1a |
| INH + RIF + SM + PZA | 4 | 2.3a | 1 | 1.0a | 3 | 4.1a |
| INH + RIF + EMB + PZA | 4 | 2.3a | 0 | 0 | 4 | 2.3a |
| INH + RIF + SM + EMB + PZA | 7 | 4.1 (1.1–7.1) | 2 | 2.1a | 5 | 5.4a |
| Susceptibility to all second-line drugs | 152 | 88.9 (84.2–93.6) | 95 | 98.0a | 57 | 77.0 (67.4–86.6) |
| Overall second-line drug resistance | 19 | 11.1 (6.4–15.8) | 2 | 2.1a | 17 | 22.9 (13.4–32.6) |
| CAP | 4 | 2.3a | 0 | 0 | 4 | 5.4a |
| KAN | 3 | 1.8a | 0 | 0 | 3 | 4.1a |
| AKM | 2 | 1.2a | 0 | 0 | 2 | 2.7a |
| OFX | 18 | 10.5 (5.9–15.1) | 2 | 2.1a | 16 | 21.6 (12.2–31.0) |
| Overall pre-XDR | 15 | 8.7 (4.5–13.0) | 1 | 1.0a | 14 | 14.9 (6.8–23.0) |
| CAP | 4 | 2.3a | 0 | 0 | 4 | 1.4a |
| KAN | 3 | 1.8a | 0 | 0 | 3 | 1.4a |
| AKM | 2 | 1.2a | 0 | 0 | 2 | 0 |
| OFX | 14 | 8.2 (4.1–12.3) | 1 | 1.0a | 13 | 13.5 (5.7–21.3) |
| Overall XDR | 3 | 1.8a | 0 | 0 | 3 | 4.1a |
| CAP + OFX | 1 | 0.6a | 0 | 0 | 1 | 1.4a |
| CAP + KAN + AKM + OFX | 2 | 1.2a | 0 | 0 | 2 | 2.7a |
95% CI was not determined.
Factors linked to drug-resistant tuberculosis.
Table 2 shows the results of analysis of risk factors for MDR and drug-resistant (but not MDR) tuberculosis among all the 171 patients. The previously treated patients presented significantly increased risks for developing drug resistance (but not MDR), with an odds ratio (OR) of 3.35 (95% CI, 1.40 to 8.03; P = 0.01). Furthermore, the risk increased highly significantly in MDR cases, with an OR of 6.17 (95% CI, 1.98 to 19.23; P = 0.00).
TABLE 2.
Factors associated with drug-resistant tuberculosisa
| Factor | No. (%) of isolates |
Drug-resistant but not MDR TB vs pan-susceptible TB |
MDR TB vs drug-resistant but not MDR TB |
||||
|---|---|---|---|---|---|---|---|
| Pan-susceptible TB | Drug-resistant but not MDR TB | MDR TB | Odds ratio (95% CI) | P value | Odds ratio (95% CI) | P value | |
| Sex | |||||||
| Male | 76 | 20 | 33 | Reference | Reference | ||
| Female | 24 | 8 | 10 | 1.27 (0.50–3.24) | 0.62 | 0.76 (0.26–2.24) | 0.62 |
| Age group | |||||||
| <30 yrs | 27 | 8 | 6 | Reference | Reference | ||
| 30–59 yrs | 48 | 15 | 32 | 1.06 (0.40–2.81) | 0.91 | 2.84 (0.84–9.67) | 0.09 |
| ≥60 yrs | 25 | 5 | 5 | 0.68 (0.20–2.34) | 0.53 | 1.33 (0.26–681) | 1.00 |
| Occupation | |||||||
| Farmer | 53 | 19 | 29 | Reference | Reference | ||
| Others | 47 | 9 | 14 | 0.53 (0.22–1.29) | 0.16 | 1.02 (0.37–2.82) | 0.97 |
| Treatment | |||||||
| New cases | 77 | 14 | 6 | Reference | Reference | ||
| Previously treated cases | 23 | 14 | 37 | 3.35 (1.40–8.03) | 0.01* | 6.17 (1.98–19.23) | 0.00** |
CI, confidence interval. *, P < 0.05 (significant); **, P < 0.01 (highly significant).
Genotyping results.
Among 171 TB isolates, 126 (73.7%) belonged to the Beijing family, while 45 (26.3%) belonged to the non-Beijing family (see Table S2 in the supplemental material). Isolates classified into the non-Beijing family included 34 isolates (19.9%) from the T family, 4 (2.3%) from the U family, 1 (0.6%) from the CAS1-DELHI family, and 1 (0.6%) from the MANU2 family, with the remaining 5 being from the Orphan family (2.9%).
Drug susceptibility profiles of different genotypes.
The drug susceptibility profiles of isolates belonging to the Beijing family and the non-Beijing family are compared in Table S3 in the supplemental material. The distribution of resistances to any drug was observed more frequently among Beijing family members than among non-Beijing family members, but there was no significant difference except for SM (P < 0.05).
Detection of drug resistance-associated mutations by DNA sequencing.
DNA sequencing identified 78.7% (48/61) INH-resistant isolates, 87.0% (40/46) RIF-resistant isolates, 88.6% (31/35) SM-resistant isolates, 76.5% (13/17) EMB-resistant isolates, 84.6% (22/26) PZA-resistant isolates, 75.0% (3/4) CAP-resistant isolates, 100.0% (3/3) KAN- or (2/2) AKM-resistant isolates, and 77.8% (14/18) OFX-resistant isolates. All the mutated characteristics of drug-resistant isolates are summarized in Table S4 to Table S10 in the supplemental material. The major mutations were katG codon 315 (katG315) (67.2%), inhA15 (9.8%), rpoB531 (47.8%), rpoB526 (21.7%) and rpoB516 (8.7%), rpsL43 (71.4%), rrs514 (11.5%), embB306 (70.6%), pncA96 (11.5%), rrs514401 (75.0% or 100.0%), and gyrA94 (55.6%) and gyrA90 (22.2%) (Table 3). In addition, no mutations were found within the target regions of tlyA, eis, and gyrB among the corresponding drug-resistant isolates.
TABLE 3.
Most frequently identified mutations within 12 drug-resistance-associated loci among drug-resistant M. tuberculosis isolates
| Drug (no. of drug-resistant isolates) | Locus | Mutated position | No. (%) of isolates | Relative frequency (%)a |
|---|---|---|---|---|
| INH (61) | katG | 315 | 41 | 67.2 |
| inhA | −15 | 6 | 9.8 | |
| RIF (46) | rpoB | 531 | 22 | 47.8 |
| 526 | 10 | 21.7 | ||
| 516 | 4 | 8.7 | ||
| SM (35) | rpsL | 43 | 25 | 71.4 |
| rrs(388–1084) | 514 | 4 | 11.5 | |
| EMB (17) | embB | 306 | 12 | 70.6 |
| PZA (26) | pncA | 96 | 3 | 11.5 |
| CAP (4) | rrs(1158–1674) | 1401 | 3 | 75.0 |
| tlyA | None | NAb | NA | |
| KAN (3) | rrs(1158–1674) | 1401 | 3 | 100.0 |
| eis | None | NA | NA | |
| AKM (2) | rrs(1158–1674) | 1401 | 2 | 100.0 |
| eis | None | NA | NA | |
| OFX (18) | gyrA | 94 | 10 | 55.6 |
| 90 | 4 | 22.2 |
Relative-frequency data represent comparisons to the total number of isolates resistant to the drug of interest.
NA, not applicable.
DISCUSSION
The present study demonstrated that 41.5% (71/171) of patients with pulmonary TB had drug-resistant disease, suggesting a serious epidemic of drug-resistant tuberculosis among patients with pulmonary TB in Hunan. The proportions of MDR- and XDR-TB among patients were 25.2% and 1.8%, nearly three times the proportions presented in the data from a national baseline survey in 2007 (2). The reason for this obvious difference was probably that the isolates of present study were obtained from the specialized hospital instead of from an epidemiological survey, resulting in a distinctly higher proportion of previously treated patients (43.3%) than in the data from the national baseline survey (22.7%). The proportions of drug-resistant TB in previously treated patients were considerably higher than those in new patients (2) and were confirmed by this study. A higher risk of drug resistance was also found among previously treated patients. Hence, some appropriate strategies must be implemented to increase continuity of treatment and reduce the rate of treatment default. Notably, a lower incidence of drug resistance in new patients was observed in this study than in the national baseline survey, implying that drug resistance in new cases varied greatly with the region and that the transmission of drug-resistant TB in Hunan is less extensive than in some other regions.
The percentages of second-line injectable drug resistance for CAP, KAN, and AKM were 2.3%, 1.8%, 1.2%, respectively, significantly lower than that of OFX (10.5%). These really low resistance levels were detected also in MDR-TB (9.3% for CAP, 7.0% for KAN, and 4.7% for AKM). All these results point out that the second-line injectable drug is excellent and represents the potential of a successful treatment against TB or MDR-TB. The high rate of OFX resistance in TB isolates, especially in MDR-TB isolates (32.6%), indicated that OFX has been used extensively in Hunan during the past years to treat drug-resistant TB patients and retreatment patients. In addition, among patients with MDR-TB, 34.9% (15/43) were pre-XDR TB, placing them only one step away from having XDR-TB.
The Beijing family was the most dominant genotype in Hunan province, which is in accordance with the findings from most areas of China (12–14). In addition to the Beijing family, other families, including T, H, MANU2, U, CAS1-DELHI, and Orphan, were also identified. Notably, only 1 strain belonged to the CAS1-DELHI family. This strain was isolated from a patient who was a student from Pakistan, a country with a predominance of examples of the CAS family (including CAS1, CAS subfamilies, and Orphan Pak clusters) (15).
Some publications showed that the Beijing family was associated with drug resistance (15, 16). However, lesser associations were reported also in other geographic settings (17, 18). Our study suggested that the association with drug resistance was significant only for SM, indicating that the Beijing family is less likely to be associated with the most drug-resistant strains in Hunan area.
DNA sequencing of the hot spot regions in genetic loci demonstrated that the frequency of mutations occurring among INH-resistant isolates was 78.7%, similar to the results reported in Jiangxi (19) but lower than the data from Fujian and Shanghai, China. For the RIF-resistant isolates in China, substantial proportions (90.2% to 97.2%) of mutations within the rpoB gene were observed (19–25). Our results showed that 87.0% of the RIF-resistant isolates carried mutations in the rpoB gene. We presumed it probable that this was due to the limited number of RIF-resistant isolates (46 isolates) analyzed in our study. Furthermore, the mutated profiles of M. tuberculosis isolates from different areas might have somewhat geographically related differences.
The mutation frequencies in SM-, EMB-, PZA-, and OFX-resistant isolates were 88.6%, 76.5%, 84.6%, and 77.8%, respectively, which were in agreement with the findings from many investigations (19, 26–28). However, the mutation frequencies in CAP- and KAN/AKM-resistant isolates (75.0% and 100.0%) were inconsistent with previous reports (19, 27–30). One possible reason was that the number of CAP- and KAN/AKM-resistant isolates investigated in our study was very small.
There were still some resistant isolates harboring no mutation within the sequenced regions. Although retesting was performed using DST and DNA sequencing, the retesting results were unchanged. This implied that these isolates probably harbored mutations outside the sequenced area or that the resistance may be caused by other mechanisms, such as efflux pumps (31). In addition, the TB cultures might also have comprised a mixed population of resistant and susceptible bacteria (32), and thus, ordinary PCR-based DNA sequencing could not detect the low proportion of drug-resistant TB strains among the predominant wild-type TB strains, leading to a falsely low detection rate.
The most common mutations among these drug-resistant isolates were katG315, inhA15, rpoB531, rpoB526 and rpoB516, rpsL43, rrs514, embB306, rrs1401, and gyrA94 and gyrA90, as previously reported (19, 27–30, 33). For the pncA gene, although codon 96 was the location of major mutation, it was found in only 3 PZA-resistant isolates (11.5%). Next were codons 120, 141, 142, and 177, which were identified in 2 PZA-resistant isolates each. Other mutations consisted of 11 unique alterations distributed throughout the whole gene. These results supported the hypothesis that the mutations harbored by pncA are tremendously diverse and scattered along the whole gene (27, 34, 35). There was no mutation found within tlyA, eis, and gyrB conferring drug resistance in the present study, which was probably due to the limited number of resistant isolates corresponding to CAP, KAN/AKM, and OFX.
The limitations of the current study should be addressed. First, this study was carried out in only one specialized TB hospital in Hunan. Although this hospital is the largest specialized hospital for TB in the region, the results based on inpatients from only one hospital might not reflect the overall situation in the region. Then, the specialized hospital might have higher rates of serious TB cases than other hospitals in the region, probably leading to an overestimation of the incidence of drug-resistant tuberculosis. Next, the number of second-line drug-resistant isolates, especially for CAP-resistant (n = 4), KAN-resistant (n = 3), and AKM-resistant (n = 2) isolates, was relatively small in our research. This might have limited the detection of the variety of gene variations. Moreover, since the sequencing data for drug-susceptible isolates were not included in our study, the specificities of different resistance-related mutations were not evaluated. Therefore, additional studies that include a substantial panel of drug-resistant and -susceptible isolates will be required in the future.
In summary, our results indicated that the prevalences of INH, RIF, SM, EMB, PZA, CAP, KAN, AKM, and OFX resistance in Hunan province were 35.7%, 26.9%, 20.5%, 9.9%, 15.2%, 2.3%, 1.8%, 1.2%, and 10.5%, respectively. The majority of M. tuberculosis isolates belonged to the Beijing family. Almost all the drug resistance demonstrated no association with genotype. The most frequent mutations were katG315, inhA15, rpoB531, rpoB526 and rpoB516, rpsL43, rrs514, embB306, pncA96, rrs1401, and gyrA94 and gyrA90. These results were very helpful to generate the appropriate TB control policy and establish the rapid molecular diagnostic methods which will be implemented in Hunan province and throughout China.
Supplementary Material
ACKNOWLEDGMENTS
We declare that we have no conflicts of interest.
This study was supported by the projects of National Natural Science Foundation of China (grant no. 81201348) and the National Key Program of Mega Infectious Diseases (grant no. 2013ZX10003002-001). The funder had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Footnotes
Published ahead of print 14 April 2014
Supplemental material for this article may be found at http://dx.doi.org/10.1128/AAC.02426-14.
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