The use of environmental factors as adjuncts to traditional tuberculosis contact investigation.

SETTING

A 25-year-old university student was diagnosed with cavitary pulmonary and laryngeal tuberculosis following symptoms of underlying cough of 6 months' duration.

OBJECTIVES

To estimate the hourly risk of infection (HRI) and examine the role of environmental factors, including room size and ventilation, in modulating this risk.

METHODS

Contact investigation.

RESULTS

Of 1100 contacts identified, 78.3% (n = 896) received a tuberculin skin test (TST), of whom 27.5% had a positive result. Among 634 Canadian-born contacts tested, 22.7% had a positive TST. The independent risk factors for a positive TST among Canadian-born university students were: > 35 h spent with the index case (adjusted OR 6.6, 95% CI 1.0-44.9) and smaller classroom size (aOR 5.0, 95% CI 1.4-10.0). In the first school term, the HRI among Canadian-born student contacts was 0.9%; in the second term, it was 1.6%.

CONCLUSION

There are inherent limitations in generalising findings from an outbreak investigation, due to the considerable variation in the infectiousness of cases. Nevertheless, in situations where the index case has a high degree of infectiousness, and there are numerous contacts with low expected prevalence of infection, the HRI can, together with ventilation measurements, be useful in guiding the extent of contact investigation needed.