Setting: Denmark, a country with a low incidence of tuberculosis (TB).
Objective: To evaluate the value of the nation-wide DNA fingerprinting of Mycobacterium tuberculosis isolates performed in Denmark since 1992.
Design: Prospective study of consecutive patients with culture-verified TB from five large TB Departments in Denmark during a 7-month period in 1998. Results of IS6110 RFLP and spoligotyping were compared to those in the nation-wide Danish DNA-fingerprint database which covers approximately 95% of all culture-verified TB cases from 1992 onwards. Questionnaires asking about contact tracing and epidemiological links were sent to the patients' treating physicians.
Results: Of the 177 patients included in the study, 57 were Danes, one was from Iceland, 111 were immigrants, and eight were from Greenland. Responses to the questionnaires were obtained from 163 patients (92%). Four cases of unsuspected transmission were detected: one of nosocomial spread of TB, one of occupational acquisition of TB and two of transmission in an international school, leading to further contact tracing among 75 schoolchildren. These four cases were all the result of short-term contacts. In 22 cases, contact with one or more TB patient(s) was reported. In six of these, the DNA-fingerprint result revealed that the presumed contact could not be the source of infection, even though in two of the cases the known TB contact was from the household.
Conclusion: Nation-wide DNA fingerprinting of TB isolates provides information that could not have been obtained otherwise, and contribute to the understanding of TB transmission in Danish society. In some cases the results lead to further contact tracing. Short-term contact can apparently result in transmission of TB.