Vector-borne diseases: the basic reproduction number R0 and risk maps

Abstract

This thesis deals with the derivation of the basic reproduction number (R0) for vector-borne diseases, in the context of studying the effect of climate change on the risk of emergence diseases. Vector-borne diseases are transmitted from an infected individual to another individual by vectors, usually arthropods. Striking examples of emergence of vector-borne diseases are the introduction and spread of West Nile virus in the New World, the bluetongue epidemic in the Netherlands and surrounding countries, and the recent outbreak of chikungunya in Italy. The (re-)emergence of vector-borne diseases has often been linked to climate change, as the survival and development rates of vectors and the development rates of pathogens are often highly sensitive to temperature and other climatic factors. Within the framework of a large multidisciplinary research project, we developed epidemiological models and/or risk maps for various diseases, including West Nile virus, Lyme disease, tick-borne encephalitis, bluetongue and canine leishmaniasis. The common underlying methodology in all studies is the use of the next-generation methodology to derive an expression for the basic reproduction number (R0). R0 is defined as the average number of secondary cases caused by one typical infectious individual placed in a population consisting entirely of susceptible individuals. R0 is a measure of the success of invasion of a pathogen into a population of susceptible individuals; if an average case infects more than one other individual, a chain reaction starts and a large outbreak is possible, whereas if a case causes on average less than one new case, the introduction will fail. The next-generation matrix is a method used to overcome problems that arise from the fact that vector-borne diseases involve different types of infected individuals: at least one vector species and one host species. The dominant eigenvalue of a next-generation matrix equals R0. Here, the next-generation method is applied to various vector-borne diseases. In Chapters 2, 3 and 4, the focus is on the correct derivation of an expression of R0 and on quantifying the contribution of different transmission routes to R0; this was done for West Nile virus and for several tick-borne diseases. Another important aim of this project is the development of a methodology to construct risk maps, with which the effect of changes in climate or land use on the risk of vector-borne disease can be studied. In Chapters 5 and 6, the construction of R0 maps is demonstrated for bluetongue in The Netherlands and for canine leishmaniasis in a region in France. An R0 map gives the value of R0 for each area or point on the map, indicating the risk that an introduction in this area will lead to an outbreak. To construct these maps, all relevant biological, epidemiological and environmental information for each disease had to be brought together in a Geographical Information System. This is now possible, what is now needed is proper validation and more standardized, long-term field work and lab work to inform such systems. After all, a model is as good as the data that it based upon.