A hypothesis for explaining single outbreaks (like the Black Death in European cities) of vector-borne infections

We propose a mechanism by which single outbreaks of vector-borne infections can happen even when the value of the basic reproduction number, R(o), of the infection is below one. With this hypothesis we have shown that dynamical models simulations demonstrate that the arrival of a relatively small (with respect to the host population) number of infected vectors can trigger a short-lived epidemic but with a huge number of cases. These episodes are characterized by a sudden outbreak in a previously virgin area that last from weeks to a few months, and then disappear without leaving vestiges. The hypothesis proposed in this paper to explain those single outbreaks of vector-borne infections, even when total basic reproduction number, Ro, is less than one (which explain the fact that those infections fail to establish themselves at endemic levels), is that the vector-to-host component of Ro is greater than one and that a sufficient amount of infected vectors are imported to the vulnerable area, triggering the outbreak. We tested the hypothesis by performing numerical simulations that reproduce the observed outbreaks of chikungunya in Italy in 2007 and the plague in Florence in 1348. The theory proposed provides an explanation for isolated outbreaks of vector-borne infections, ways to calculate the size of those outbreaks from the number of infected vectors arriving in the affected areas. Given the ever-increasing worldwide transportation network, providing a high degree of mobility from endemic to virgin areas, the proposed mechanism may have important implications for public health planning. (C) 2009 Elsevier Ltd. All rights reserved.

Introduction

In 1691, eighteen years after its original publication, Samuel Pufendorf’s De officio hominis et civis appeared in English translation in London, bearing the title The Whole Duty of Man, According to the Law of Nature. This translation, by Andrew Tooke (1673–1732), professor of geometry at Gresham College, passed largely unaltered through two subsequent editions, in 1698 and 1705, before significant revision and augmentation in the fourth edition of 1716. Unchanged, this text was then reissued as the fifth and final edition of 1735, which is here republished for the first time since.1 Five editions, spanning almost half a century, bear testimony to the English appetite for Pufendorf’s ideas.