Estimating the basic reproduction number for the 2015 Nyimba district bubonic plague outbreak

Plague is a re-emerging flea-borne infectious disease of global importance and in recent years Zambia has periodically experienced increased incidence of bubonic plague outbreaks. However, there are currently no studies in the country that provide a quantitative assessment of the ability of the disease to spread during these outbreaks. This limits our understanding of the epidemiology of the disease especially for planning and implementing quantifiable and cost-effective control measures. To fill this gap, the basic reproduction number, Ro, for bubonic plague was estimated in this study. Ro is the average number of secondary infections arising from a single infectious individual during their infectious period in an entirely susceptible population. It gives a quantitative measure of the transmissibility of an infectious disease in the population and it is used to estimate the expected magnitude and extent of spread for an infectious disease outbreak. More importantly, Ro is used to guide the magnitude of control measures that will be required to control the disease. Secondary epidemic data from the most recent 2015 Nyimba district bubonic plague outbreak in Zambia was analyzed. Ro was estimated as a function of the average epidemic doubling time based on the initial exponential growth rate of the outbreak and the average infectious period for bubonic plague. Ro was estimated to range between 1.5599 [95% CI: 1.382 - 1.7378] and 1.9332 [95% CI: 1.6366 - 2.2297], with average 1.7465 [95% CI: 1.5093 - 1.9838]. Further, an SIR deterministic mathematical model was derived for this infection and this estimated Ro to be about 1.4 to 1.5, which was within the range estimated above considering the 95% confidence interval. This estimated Ro for bubonic plague is an indication that each bubonic plague case can typically give rise to almost two new cases during these outbreaks. This Ro estimate can now be used to quantitatively analyze and develop measurable interventions against future plague outbreaks in Zambia. For example, based on the average Ro estimate in this study, a minimum mass treatment rate of about 43% would be enough to prevent the disease spread in such high risk populations in the country.