A model of tuberculosis clustering in low incidence countries reveals more transmission in the United Kingdom than the Netherlands between 2010 and 2015.

Ellen Brooks-Pollock,Colin Campbell,Maeve K Lalor,Dick Van Soolingen,Leon Danon,Jennifer A Davidson,Andrew M T Pollock,Hester Korthals Altes,Roger Dimitri Kouyos

doi:10.1371/journal.pcbi.1007687

Abstract

Tuberculosis (TB) remains a public health threat in low TB incidence countries, through a combination of reactivated disease and onward transmission. Using surveillance data from the United Kingdom (UK) and the Netherlands (NL), we demonstrate a simple and predictable relationship between the probability of observing a cluster and its size (the number of cases with a single genotype). We demonstrate that the full range of observed cluster sizes can be described using a modified branching process model with the individual reproduction number following a Poisson lognormal distribution. We estimate that, on average, between 2010 and 2015, a TB case generated 0.41 (95% CrI 0.30,0.60) secondary cases in the UK, and 0.24 (0.14,0.48) secondary cases in the NL. A majority of cases did not generate any secondary cases. Recent transmission accounted for 39% (26%,60%) of UK cases and 23%(13%,37%) of NL cases. We predict that reducing UK transmission rates to those observed in the NL would result in 538(266,818) fewer cases annually in the UK. In conclusion, while TB in low incidence countries is strongly associated with reactivated infections, we demonstrate that recent transmission remains sufficient to warrant policies aimed at limiting local TB spread.

Highlights

Tuberculosis (TB) is a chronic infectious disease and a major global public health threat
Multiple tuberculosis (TB) cases infected with a single strain are known as a TB cluster
In the United Kingdom (UK) for example, TB clusters vary in size from two cases up to over 200 cases

Summary

Introduction

Tuberculosis (TB) is a chronic infectious disease and a major global public health threat. In many low TB incidence countries, a high proportion of cases occur in persons born abroad, and control measures such as migrant screening have been introduced to limit imported infection and reduce treatment costs[2,3]. It is often not known whether foreign-born individuals were exposed to TB before or after immigrating [4,5], which affects the impact of such interventions. Distinguishable strains are used to estimate the fraction of cases that are not due to recent transmission, but due instead to the reactivation of existing infections or cases infected elsewhere[14]

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Conclusion