The use of maximum likelihood methods to estimate the risk of tuberculous infection and disease in a Canadian First Nations population

Background Tuberculosis (TB) notification rates among First Nations people in British Columbia, Canada, are higher than those among non-First Nations people, although rates are declining more rapidly in the First Nations population. The epidemiology of tuberculous infection and disease during the period 1926-2000 in this population was investigated. Methods The trend in the annual risk of infection (ARI) since 1926 was estimated using tuberculous meningitis mortality statistics and skin testing data. Risks of progression from infection to disease were estimated by fitting model predictions of disease incidence to TB notifications, using maximum likelihood methods. Infectious TB notifications were matched with ARI estimates to obtain the number of transmissions per infectious case over time. Results We estimate that the ARI decreased from more than 10% during the prechemotherapy era to less than 0.1% by 2000. The risks of primary, reactivation, and exogenous re-infection disease among adults aged 25-44 years were 22%, 0.1%, and 6%, respectively. The number of transmissions per infectious case decreased from 16 to 2 from the early 1970s to the late 1990s. Conclusions This study shows that the risk of infection among British Columbia First Nations people is decreasing, while the relative contribution of reactivation to disease incidence is increasing. Once infected, First Nations people may have a higher risk of developing disease than other populations.