Deciphering an outbreak of drug-resistant Mycobacterium tuberculosis

First outbreak of multi-drug resistant tuberculosis (MDR-TB) in Denmark involving six Danish-born cases

BACKGROUND

Denmark is a tuberculosis (TB) and multi-drug resistant (MDR) TB low-incidence country at 5 and 0.05 cases per 100.000 population, respectively. Until 2018, transmission of MDR-TB was nonexistent except for few pairwise related family-cases. In this study we describe the first MDR-TB outbreak in Denmark.

METHODS

Based on genotyping of all Mycobacterium tuberculosis (Mtb) culture-positive cases in Denmark spanning three decades, six molecular- and epidemiologically linked Danish-born cases were identified as the first cluster of MDR-TB in Denmark. The primary case was diagnosed posthumously in 2010 followed by five epidemiologically linked cases from 2018 through 2019.

RESULTS AND CONCLUSION

Through a combination of routine Mtb genotyping and clinical epidemiological surveillance data, we identified the first Danish MDR-TB outbreak spanning 10 years and were able to disclose the specific transmission pathways in detail guiding the outbreak investigations. The occurrence of an MDR-TB outbreak in a resource rich TB low incidence setting like Denmark, highlights the importance of a collaborative control system combining classical contact tracing; timely identification of drug resistant TB through rapid diagnostics; and a close collaboration between clinicians, classical- and molecular epidemiologists for the benefit of TB control.

Nano-antimicrobials: A New Paradigm for Combating Mycobacterial Resistance

BACKGROUND

Mycobacterium group contains several pathogenic bacteria including M. tuberculosis where the emergence of multidrug-resistant tuberculosis (MDR-TB) and extensively drug-resistant tuberculosis (XDR-TB) is alarming for human and animal health around the world. The condition has further aggravated due to the speed of discovery of the newer drugs has been outpaced by the rate of resistance developed in microorganisms, thus requiring alternative combat strategies. For this purpose, nano-antimicrobials have emerged as a potential option.

OBJECTIVE

The current review is focused on providing a detailed account of nanocarriers like liposome, micelles, dendrimers, solid lipid NPs, niosomes, polymeric nanoparticles, nano-suspensions, nano-emulsion, mesoporous silica and alginate-based drug delivery systems along with the recent updates on developments regarding nanoparticle-based therapeutics, vaccines and diagnostic methods developed or under pipeline with their potential benefits and limitations to combat mycobacterial diseases for their successful eradication from the world in future.

RESULTS

Distinct morphology and the underlying mechanism of pathogenesis and resistance development in this group of organisms urge improved and novel methods for the early and efficient diagnosis, treatment and vaccination to eradicate the disease. Recent developments in nanotechnology have the potential to meet both the aspects: nano-materials are proven components of several efficient targeted drug delivery systems and the typical physicochemical properties of several nano-formulations have shown to possess distinct bacteriocidal properties. Along with the therapeutic aspects, nano-vaccines and theranostic applications of nano-formulations have grown in popularity in recent times as an effective alternative means to combat different microbial superbugs.

CONCLUSION

Nanomedicine holds a bright prospect to perform a key role in global tuberculosis elimination program.

MDR-TB Outbreak among HIV-Negative Tunisian Patients followed during 11 Years

Background

Multidrug-resistant tuberculosis (MDR-TB) outbreaks that evolve, from the outset, in a context strictly negative for HIV infection deserve special consideration since they reflect the true intrinsic epidemic potential of the causative strain. To our knowledge, the long-term evolution of such exceptional outbreaks and the treatment outcomes for the involved patients has never been reported hitherto. Here we provide a thorough description, over an 11-year period, of an MDR-TB outbreak that emerged and expanded in an HIV-negative context, Northern Tunisia.

Methodology/Principal Findings

From October 2001 to June 2011, the MDR-TB outbreak involved 48 HIV-negative individuals that are mainly young (mean age 31.09 yrs; 89.6% male) and noninstitutionalized. Drug susceptibility testing coupled to mutational analysis revealed that initial transmission involved an isolate that was simultaneously resistant to isoniazid, rifampicin, ethambutol, and streptomycin. The causative Haarlem3-ST50 outbreak strain expanded mainly as an 11-banded IS6110 RFLP profile (77.1%), from which a 12-banded subclone evolved. After undergoing a 2-year treatment with second-line drugs, 22 (45.8%) patients were cured and 3 (6.2%) completed treatment, thus yielding an overall treatment success rate of 52.1%. Among the patients that experienced unfavorable treatment outcomes, 10 (20.8%) failed treatment, 3 (6.2%) were lost to follow-up, 5 (10.4%) died, and 5 (10.4%) could not be evaluated. Poor adherence to treatment was found to be the main independent predictor of unfavorable outcomes (HR: 9.15; 95% CI 1.72–48.73; P = 0.014). Intriguingly, the evolved 12-banded subclone proved significantly associated with unfavorable outcomes (HR: 4.90; 95% CI 1.04–23.04, P = 0.044). High rate of fatality and relapse was further demonstrated at the long-term, since 70% of those whose treatment failed have died, and 24% among those deemed successfully treated have relapsed.

Conclusions/Significance

Taken together, the data obtained in this study indicate that MDR-TB clinical isolates could become fit enough to cause large and severe outbreaks in an HIV-negative context. Such MDR-TB outbreaks are characterized by low treatment success rates and could evolve towards increased severity, thus calling for early detection of cases and the necessity to raise the bar of surveillance throughout and beyond the treatment period.

Factors associated with being lost to follow-up before completing tuberculosis treatment: analysis of surveillance data

Completion of treatment is key to tuberculosis control. Using national surveillance data we assessed factors associated with tuberculosis patients being lost to follow-up before completing treatment (‘lost’). Patients reported in England, Wales and Northern Ireland between 2001 and 2007 who were lost 12 months after beginning treatment were compared to those who completed, or were still on treatment, using univariable and multivariable logistic regression. Of 41 120 patients, men [adjusted odds ratio (aOR) 1·29; 95% confidence interval (CI) 1·23–1·35], 15- to 44-year-olds (P<0·001), and patients with pulmonary sputum smear-positive disease (aOR 1·25, 95% CI 1·12–1·45) were at higher risk of being lost. Those recently arrived in the UK were also at increased risk, particularly those of the White ethnic group (aOR 6·39, 95% CI 4·46–9·14). Finally, lost patients had a higher risk of drug resistance (aOR 1·41, 95% CI 1·17–1·69). Patients at risk of being lost require enhanced case management and novel case retention methods are needed to prevent this group contributing towards onward transmission.

Drug resistant Mycobacterium tuberculosis of the Beijing genotype does not spread in Sweden

BACKGROUND

Drug resistant (DR) and multi-drug resistant (MDR) tuberculosis (TB) is increasing worldwide. In some parts of the world 10% or more of new TB cases are MDR. The Beijing genotype is a distinct genetic lineage of Mycobacterium tuberculosis, which is distributed worldwide, and has caused large outbreaks of MDR-TB. It has been proposed that certain lineages of M. tuberculosis, such as the Beijing lineage, may have specific adaptive advantages. We have investigated the presence and transmission of DR Beijing strains in the Swedish population.

METHODOLOGY/PRINCIPAL FINDINGS

All DR M. tuberculosis complex isolates between 1994 and 2008 were studied. Isolates that were of Beijing genotype were investigated for specific resistance mutations and phylogenetic markers. Seventy (13%) of 536 DR strains were of Beijing genotype. The majority of the patients with Beijing strains were foreign born, and their country of origin reflects the countries where the Beijing genotype is most prevalent. Multidrug-resistance was significantly more common in Beijing strains than in non-Beijing strains. There was a correlation between the Beijing genotype and specific resistance mutations in the katG gene, the mabA-inhA-promotor and the rpoB gene. By a combined use of RD deletions, spoligotyping, IS1547, mutT gene polymorphism and Rv3135 gene analysis the Beijing strains could be divided into 11 genomic sublineages. Of the patients with Beijing strains 28 (41%) were found in altogether 10 clusters (2-5 per cluster), as defined by RFLP IS6110, while 52% of the patients with non-Beijing strains were in clusters. By 24 loci MIRU-VNTR 31 (45%) of the patients with Beijing strains were found in altogether 7 clusters (2-11 per cluster). Contact tracing established possible epidemiological linkage between only two patients with Beijing strains.

CONCLUSIONS/SIGNIFICANCE

Although extensive outbreaks with non-Beijing TB strains have occurred in Sweden, Beijing strains have not taken hold, in spite of the proximity to high prevalence countries such as Russia and the Baltic countries. The Beijing sublineages so far introduced in Sweden may not be adapted to spread in the Scandinavian population.

Mycobacterium tuberculosis genotypic diversity in pyrazinamide-resistant isolates of Iran

Pyrazinamide (PZA) is an important first-line drug used for the short-course treatment of tuberculosis in combination with isoniazid and rifampin. It has been reported that mutations in pncA gene correlate well with PZA resistance depending on the geographic area. On the other hand, different genotypes of Mycobacterium tuberculosis show different affinities to acquire resistance-related mutations. To determine the relative significance of various mutations in the pncA gene in Iranian PZA-resistant M. tuberculosis isolates and to analyze the association of different genotypes of M. tuberculosis with PZA resistance, 34 PZA-resistant M. tuberculosis isolates were analyzed for their pncA mutations using direct sequencing. These isolates were genotyped by IS6110 fingerprinting and spoligotyping methods. Mutations in the pncA gene were identified in 24 of 34 of these isolates (70.58%). No mutations were found in 10 PZA-resistant isolates, which implied that alternative mechanisms of resistance existed in these strains. PZA resistance was strongly (41.2%) associated with multidrug-resistant tuberculosis. Genotyping revealed the Central Asian (CAS) and East-African Indian families as the most prevalent families between PZA-monoresistant isolates versus the Beijing and Haarlem families which were the most frequent families between PZA including multidrug-resistant isolates.

Evidence for local transmission and reactivation of tuberculosis in the Toronto Somali community

To further understand the molecular and clinical epidemiology of tuberculosis in the Toronto Somali community, molecular fingerprinting using IS6110 restriction fragment length polymorphism typing or spoligotyping was performed on M. tuberculosis isolates obtained from Somali-Canadians who developed active disease from 1997 to 2001. Molecular fingerprints were further compared with those obtained from Somalis residing in Denmark. 142 Somali TB patients were reported, for whom, 80 isolates were fingerprinted. 25% of isolates were clustered. Three clusters involving 2 patients each were identified out of the17 isolates that underwent spoligotyping. Of the 63 isolates typed by the IS6110 method, 6 clusters (4 of 2 patients and 2 of 3 patients) were identified. 57% of these isolates were found to be identical to Danish isolates. Our study suggests that a combination of reactivation and recent transmission are responsible for the high incidence rates of tuberculosis in this community. We recommend that ongoing surveillance and treatment programmes be directed towards this community.

Molecular epidemiology of drug-resistant tuberculosis in Sweden

Drug-resistant tuberculosis (TB), including the more severe forms of multidrug- and extensively drug-resistant forms, is an increasing public health concern globally. In Sweden the majority of patients with TB are immigrants from countries with a high incidence of TB including the drug-resistant forms. In this study, the spread of resistant TB in Sweden was investigated by molecular fingerprinting. Isolates resistant to at least one of the drugs, isoniazid, rifampicin, ethambutol or streptomycin, from 400 patients collected between 1994 and 2005, were studied by restriction fragment length polymorphism (RFLP) and by spoligotyping. Thirty-five clusters of patients infected with strains with identical RFLP and spoligotyping patterns (2-96 patients per cluster), comprising a total of 203 patients, were found. One large outbreak of isoniazid resistant tuberculosis was identified, involving 96 patients, mainly from the Horn of Africa. To identify chains of transmission, molecular epidemiological characterization of TB isolates should, if possible, be performed on isolates from all new TB patients.

Impact of immigration on the molecular epidemiology of Mycobacterium tuberculosis in a low-incidence country

RATIONALE

Programs to prevent the incidence rate of tuberculosis (TB) from increasing in many low-incidence countries are challenged by international travel and immigration from high-burden countries.

OBJECTIVES

The current study aimed to determine the effect of such immigration on the genetic diversity of Mycobacterium tuberculosis isolates in an entire nation's population during 1994-2005.

METHODS

A total of 3,131 patients were notified with TB during the 12-year period. Of these, 2,284 (73%) had TB verified by culture, and isolates from 2,173 (96%) of these were analyzed by IS6110 restriction fragment length polymorphism.

MEASUREMENTS AND MAIN RESULTS

Only 31% of the included strains were isolated from nonimmigrants, the remaining 69% were isolated from immigrants. Although the incidence increased throughout the period, the genetic diversity remained high. A total of 135 clusters were identified; the percentage of recent disease was reduced among nonimmigrants, and remained stable among the immigrants during the study period. Although 69% of the isolates originated from immigrants from high-incidence countries, the established TB control program in the receiving country was adequate for the prevention of disease transmission. On average per year, only 2 nonimmigrants and 13 immigrants developed disease as a result of infection within the country by imported M. tuberculosis.

CONCLUSIONS

Twelve years of M. tuberculosis importation as a result of immigration from high-incidence countries had little influence on the transmission of this pathogen in the receiving low-incidence country. To prevent future increase of transmission of TB, the current control strategies of low-incidence countries are adequate but must be maintained.

Drug resistance of Mycobacterium tuberculosis in the Sunamganj District of Bangladesh

Spread of drug-resistant tuberculosis (TB) threatens TB-control programmes, and all countries need to monitor the patterns and trends of anti-TB drug resistance. Such data assess the quality of control programmes and help forecast future trends of drug resistance. It will also help to establish guidelines for TB therapy. The aim of the current study was to describe the rate of drug-resistant Mycobacterium tuberculosis in the Sunamganj District of Bangladesh. Bacterial isolates were collected from sputum smear positive (ss+) patients who attended the National TB Programme from November 2003 to December 2004. A total of 95 isolates was tested for susceptibility to streptomycin (SM), isoniazid (INH), rifampicin (RMP) and ethambutol (EMB) at the National Reference Laboratory for Mycobacteria at the Norwegian Institute of Public Health (NIPH), Oslo. The total resistance among new cases to any drug was 31%. For SM it was 18%, INH 23%, RMP 2%, EMB 10% and 2% were multidrug-resistant (MDR). The National Tuberculosis Programme (NTP) in Sunamganj is still effective, although the high resistance to INH is alarming. An increased risk of treatment failure has been demonstrated in areas with high levels of INH resistance, and a high proportion of INH resistant cases may develop resistance to RMP during treatment.

Key issues in multidrug-resistant tuberculosis

Tuberculosis is responsible for 2 million deaths worldwide and 8 million new cases are reported globally every year. Multidrug-resistant tuberculosis (MDR-TB) is an emerging and difficult public health problem worldwide. In the presence of resistance to key first-line antituberculous agents, treatment with less effective and more toxic second-line agents must be instituted. Consequently, patients remain infectious for a longer period and require prolonged courses of treatment. There may be a role for surgery in selected cases. Care must be taken in terms of isolation procedure and infection control in MDR-TB. Although the diagnosis is made microbiologically, there are certain factors that predispose to the emergence of MDR-TB, notably a history of previous treatment for TB, particularly if that treatment was inadequate or incomplete. Prescription errors made by physicians also contribute, such as adding a single drug to a failing anti-TB regimen. The use of DNA amplification techniques, for example polymerase chain reaction has resulted in the rapid diagnosis of MDR-TB compared with traditional solid culture media. Treatment of MDR-TB usually involves five drugs to which microbiologically, the organism has been shown to demonstrate susceptibility, and one of these drugs should be an injectable agent. There is a need for greater research into developing more effective antituberculous medications and immunotherapy may play an adjunctive role in future management.

rpoB gene sequencing and spoligotyping of multidrug-resistant Mycobacterium tuberculosis isolates from India

Multi drug-resistant Mycobacterium tuberculosis (MDR TB) has been well studied in outbreaks in settings of low endemicity in developed countries. However, the characteristics of MDR TB in the community with high endemicity such as India have not been well investigated. Mutations in the 81-bp rifampicin resistance-determining region of the rpoB gene were analyzed by DNA sequencing of 187 M. tuberculosis clinical isolates (149 resistant and 38 sensitive) from different parts of India. 146-Point mutations and two insertions were found in 146 of 149 resistant isolates in seven codons. The most common mutations were in codons 531 (59%), 526 (22%), and 516 (11.5%). Mutations were not found in three (2%) of the resistant isolates. N-terminal sequencing in these isolates showed no mutation at codon V176. None of the drug-susceptible isolates showed any mutation in the 437-bp rpoB gene segment sequenced. Genotypic analysis revealed a total of 80 different spoligotypes. A unique pattern was found in 65 (43.6%) isolates, whereas 84 (56.4%) were in 15 clusters. Comparison with an international spoligotype database showed ST26, Delhi type (18.1%), ST1, Beijing type (9.4%), and ST11 (5.4%), as the most common. The majority of isolates in the Beijing genotype (13/14) were associated with mutation 531TTG and similar drug-resistance patterns while other major clusters showed that the nature and frequency of occurrence of mutations in the rpoB gene were independent of spoligopatterns.

Evaluation of multiple genetic markers for typing drug-resistant Mycobacterium tuberculosis strains from Poland

In the present study, 77 drug-resistant Mycobacterium tuberculosis strains isolated in Poland in 2000 were characterized by the mycobacterial interspersed repetitive unit-variable number tandem repeat (MIRU-VNTR) typing and our novel method based on PCR amplification of DNA regions between IS6110 and 16-bp GC-rich frequent repeats (designated IS6110-Mtb1/Mtb2 PCR). The results were compared with previous data of the more commonly used methods, IS6110 restriction fragment length polymorphism (RFLP) and spoligotyping. The discriminatory power of IS6110-Mtb1/Mtb2 method was only slightly lower than that of IS6110 RFLP, whereas MIRU-VNTR typing was the least discriminative among the 4 methods used. Clustering of strains by using results of IS6110-Mtb1/Mtb2 PCR correlated well with RFLP-defined clusters, further confirming epidemiologic relationships among patients. These results indicate that the novel genotyping method could be an attractive alternative for other PCR-based typing procedures, such as spoligotyping and MIRU-VNTR typing. Also, it seems to be a valuable adjunct to the reference IS6110 RFLP method for studying the genetic diversity of drug-resistant M. tuberculosis strains in Poland.

Épidémiologie moléculaire de la tuberculose en Guadeloupe de 1994 à 2000

In Guadeloupe, the incidence of tuberculosis decreased between 1994 and 2000. The rate of resistance to at least one antibiotic remained constant at 11%, whereas the rate of multiple-drug resistance increased from 0.9 to 2.4% in 2000. The proportion of patients of foreign origin (mainly from Haiti and the Dominican Republic) increased whereas the number of French patients decreased. These results show that the epidemiology of tuberculosis in Guadeloupe is similar to industrialized countries as older people, foreigners from countries where TB is endemic, and HIV+ patients are at a higher risk to declare tuberculosis disease. Molecular typing realized by spoligotyping showed the importance of previous successive colonizations and migrations as characterized by the presence of major phylogenetic families originating essentially from Northern Europe (Haarlem), Latin America and Mediterranean (LAM) and from Anglo-Saxon countries (X). The sub-typing of clustered strains by IS6110-RFLP and by a PCR method based on the variable number of tandem DNA repeats (VNTR), highlighted 29 clusters, corresponding to 44.8% of clustered strains, and allowed to estimate the rate of recent transmission at 32.2%. The epidemiologic data associated with fingerprinting results underlined the importance of reactivation cases among older people, a significant number of imported TB cases without evident links, and casual contacts.

Molecular analysis of isoniazid and rifampin resistance in Mycobacterium tuberculosis isolates recovered from Barcelona

We studied the presence of mutations in the whole katG gene and specific regions of the oxyR-ahpC and mabA-inhA regulatory region in 61 Mycobacterium tuberculosis isoniazid-resistant isolates. An 81-bp region of the rpoB gene was also sequenced in 17 rifampin-resistant strains. Alterations in the katG gene were detected in 55% of the isolates. Mutation in codon 315 was the most prevalent (32%). Strains showed a high level of resistance, and most maintained a substantial catalase-peroxidase activity. Three strains with an isoniazid MIC of >or=32 microg/ml lacked catalase-peroxidase activity. Two of them had deletions in the catalytic domain of the KatG protein. One strain with deletion and three strains with mutations in the C-terminal domain showed low-level resistance and conserved the catalase-peroxidase activity. Mutations in the mabA-inhA regulatory region were identified in 32% of the isolates. All had low-level resistance, and the vast majority conserved catalase-peroxidase activity. Seventeen percent of the isoniazid-resistant isolates had no detectable alterations at the studied loci. Resistance to rifampin was associated with mutations in the 81-bp of the rpoB gene in all cases. IS6110 analysis indicated that recent transmission contributed substantially to the emergence of isoniazid- resistant tuberculosis in Barcelona through short transmission chains. A rapid genotypic assay, including the 315-katG codon and the -15 nucleotide of the mabA-inhA regulatory region, may cover 62% of isoniazid- resistant strains in Barcelona. In contrast, the targeting of the 81-bp region of rpoB would detect all our rifampin-resistant isolates.

Molecular epidemiology of Mycobacterium tuberculosis in western Sweden

The genetic diversity of Mycobacterium tuberculosis isolates among patients from Sweden was determined by a combination of two PCR-based techniques (spoligotyping and variable number of tandem repeats analysis). It resulted in a clustering of 23.6% of the isolates and a rate of recent transmission of 14.1%. The clustered isolates mainly belonged to the Haarlem family (23.2%), followed by the Beijing (9.8%), Latin American and Mediterranean (LAM; 8%), and East African-Indian (EAI; 6.2%) families. A comparison of the spoligotypes with those in the international spoligotyping database showed that 62.5% of the clustered isolates and 36.6% of all isolates typed were grouped into six major shared types. A comparison of the spoligotypes with those in databases for Scandinavian countries showed that 33% of the isolates belonged to an ill-defined T family, followed by the EAI (22%), Haarlem (20%), LAM (11%), Central Asian (5%), X (5%), and Beijing (4%) families. Both the highest number of cases and the proportion of clustered cases were observed in patients ages 15 to 39 years. Nearly 10% of the isolates were resistant to one or more drugs (essentially limited to isoniazid monoresistance). However, none of the strains were multidrug resistant. Data on the geographic origins of the patients showed that more than two-thirds of the clustered patients with tuberculosis were foreign-born individuals or refugees. These results are explained on the basis of both the historical links within specific countries and recently imported cases of tuberculosis into Sweden.

Primary Drug Resistance and Molecular Epidemiology ofMycobacterium tuberculosisIsolates from Patients in a Population with High Tuberculosis Incidence in Turkey

To determine the rate of primary drug resistance and compare the fingerprint pattern diversity of the resistant and sensitive Mycobacterium tuberculosis isolates, antituberculosis susceptibility testing and restriction fragment length polymorphism (RFLP) analysis were performed on 88 M. tuberculosis isolates of the patients who were diagnosed as new tuberculosis cases in 2000. Primary resistance to isoniazid, rifampicin, ethambutol, and streptomycin were determined by the BACTEC method. IS6110 and pTBN12 were used as molecular markers. The frequency of resistance to at least one drug was 32.95%, whereas 10.23% of the isolates were resistant to more than one drug. Single-drug resistance to isoniazid, streptomycin, ethambutol, and rifampicin was found in 9 (10.22%), 7 (7.95%), 4 (4.54%), and 0 (0.0%) strains, respectively. Two M. tuberculosis strains (2.26%) showed multiple drug resistance. The combination of two fingerprinting procedures on a total of 88 isolates identified 58 (65.9%) strains as unique and clustered 30 strains in 11 clusters (clustering = 34.1%). The clustering rate for resistant and sensitive isolates was 13.8% and 40.1%, respectively. In conclusion; drug susceptibility testing showed that the majority of the drug-resistant infections involved either isoniazid or streptomycin alone. In addition to the high tuberculosis incidence, elevated primary drug resistance and high clustering rate indicate problems in the present control programs. New control strategies supported by molecular typing might be more effective to reduce tuberculosis.

Continued low rates of transmission of Mycobacterium tuberculosis in Norway

In this study, we determined the genetic diversity of Mycobacterium tuberculosis isolated in Norway from 1999 to 2001. The results were compared to those for strains isolated from 1994 to 1998. A total of 818 patients were diagnosed with tuberculosis (TB) during the last 3-year period. Of these cases, 576 (70%) were verified by culturing, and strains from 551 patients (96%) were analyzed by the IS6110 restriction fragment length polymorphism (RFLP) method. We excluded 13 strains (2.4%) from the analyses, since they were found to represent false-positive samples. A total of 67 strains (12%) that carried fewer than five copies of IS6110 were analyzed by spoligotyping. The strains were from 157 patients (29%) of Norwegian origin and 381 patients (71%) of foreign origin. The rate of diversity among all of the strains was 90%, while in 1994 to 1998 it was 87%. Clusters were assumed to have arisen from recent transmission; the degree of such transmission was 10% in 1999 to 2001, while for the whole 8-year period (1994 to 2001), it was 11%. Of the 109 patients diagnosed as being part of a cluster in 1999 to 2001, 89 were infected with a strain that carried more than four copies of IS6110. Among these 89 patients, 52 (58%) were infected with a strain that had already been identified in 1994 to 1998. The results indicated that most cases of TB in Norway were due to the import of new strains rather than to transmission within the country. This finding demonstrates that screening of immigrants for TB upon arrival in Norway needs to be improved. Outbreaks, however, were caused mainly by strains that have been circulating in Norway for many years.