Transmission Characteristics Of W-Beijing Genotype Mycobacterium Tuberculosis In Part Area Of Eastern China

Tuberculosis (TB) is a major threat to human health and social stability, and China is one of high TB burden countries. W-Beijing genotype strains are a genetically related group of Mycobacterium tuberculosis (M.TB), which is widespread in many regions of the world. This kind of strains may be associated with drug-resistant TB and cause outbreaks of TB.Traditional epidemiological methods are limited in tracking the infection source and clarifying the route of transmission, while the molecular technologies have advantages to tackle the above two epidemiological questions. Compared to the traditional epidemiological link, molecular epidemiology is more likely to be accurate to predict the epidemic of disease and has the ability to confirm the mode of transmission. This study, conducted in Funing county, Deqing county and Yinzhou district, aims to describe the prevalence of W-Beijing genotype strains in eastern China and its impact factors, evaluate the proportion of recent transmission through identifying the VNTR-MIRU genotypes, and determine the transmission pattern of TB in eastern China.Methodology and main results:1. Risk factors of the epidemic of W-Beijing genotype strainsThe participants of the study were the TB patients that registered in one of the three counties between June,2009and December,2010. Sputum samples of subjects were collected and cultured by local center for disease control and prevention (CDCs). A total of426M.TB strains were collected from the three sites, and we finally obtained346cases that had complete information for data analysis. We use DTM-PCR to differentiate W-Beijing family strains with modern W-Beijing strains. Of the346M.TB strains,90%(311) were identified as W-Beijing genotype strains, due to deletions of RD105. The proportions of W-Beijing family strains were87-96%(124) in the three counties. The W-Beijing family strains were further divided by RD181, and294(77%) isolates belonged to modern W-Beijing family. 2. DNA sequencing results of the resistance related genesDirect sequencing was applied to investigate the mutation in katG gene to INH and rpoB gene to REF. A total of7.77%(24/309) of M.TB strains had mutations at codon351of katG gene, and4.83%(7/145) of M.TB strains had mutations at codon531of rpoB gene. The rate of mutation was no statistical difference in katG and rpoB respectively. Among the M.TB strains that had mutations, only one that mutated in katG gene was a clustered strain, and the proportion of clustering of mutated strains was4%. However, rpoB gene mutations were all presented as unique strains.3. Genotypes of the M.TB strains and risk factors of clusteringUsing the7loci VNTR-MIRU, there409different genotypes, including395unique strains and14clusters that together included31isolates; the genotypes in clusters ranged in size from2to3patients withinW-Beijing members. As a result, the proportion of cases due to recent transmission was estimated at4%, and the proportion of clustering was7%. Of the14clusters, there were13clusters (29isolates) in FN, with only one cluster (2isolates) in DQ,while no cluster was identified in YZ. The proportions of clustering were estimated to be15%and2%in FN and DQ respectively, and the difference was significant (x2=30.435, P<0.001).Among all W-Beij ing strains, in general, ancestral strains were more likely to be clustered (10%) compared to modern strains (7%, χ2=0.588, P=0.443), although no statistical difference was found. There were clusters that were mixed with the strains from ancestral and modern genotypes.The clustering rate of floating population was significantly lower than that of local residents (aOR=0.107,95%CI:0.013-0.856). No statistically significant differences were found between history of BCG vaccination, previous treatment history or sputum smear result. We identified a mix cluster that involved both a local resident and a migrant case. However, no epidemiological link was found among the patients in clusters.4. Discriminatory power of the VNTR-MIRUA total of199M.TB strains from Funing county was genotyped by15loci VNTR-MIRU. The seven loci showed high discriminatory power, and VNTR3820showed the highest genetic polymorphism and discriminatory power (HGI=0.8597), while Mtub21showed the lowest (HGI=0.6050). However, the discriminatory power was largely different among the15VNTR-MIRU loci, with the smallest one as0.0804(ETR C), the biggest one as0.8597(VNTR3820). The HGI of the combination of15loci was0.9994for both M.TB strains and W-Beijing family isolates in Funing, while that of7loci was0.9990and0.9988respectively. There were190different genotypes in199strains, representing7clusters and183unique isolates. MIRU-15was more likely to be clustered (the proportion of clustering of MIRU-15vs. MIRU-7:8%vs.15%,x2=4.234, P=0.040). The discriminatory power was improved by combination of VNTR-MIRU loci. However, too many loci could not increase HGI remarkably.Conclusions:1. W-Beijing family isolates were the predominant prevalent strains in eastern China, with no statistical difference was observed in the patients with different socio-demographic or clinical characteristics.2. Among W-Beijing genotype M.TB isolates, modern strains had a higher proportion than that of ancestral strains. The epidemic of modern strains was not related to demographic or clinical characteristics, and they always accounted for a large part of prevalence, indicating that they were likely to cause infection among people and had strong pathogenicity.3. The small proportion of clustering in this study indicated that the incidence of TB may be the result of reactivation of previously infection, while the proportion of recent transmission accounted for a low percentage of the incidence. There was no epidemiological relation among patients in clusters, which indicated that the causal contact may contribute a lot to the transmission of TB in eastern China.4. The registered residence was one of risk factors of clustering, and the local people had higher risk of clustering, compared to the floating people. It was suggested that the recent transmission would occur in local population more often than in floating people. The transmission between local residents and floating population may exist.5. We recommend new combination of7loci, including VNTR3820, Qub11a, Qub18, Qub11b, MIRU10, MIRU26and MIRU31, to apply in M.TB genotyping researches in eastern China.