VECTOR-BORNE DISEASES AND THE BASIC REPRODUCTION NUMBER - A CASE-STUDY OF AFRICAN HORSE SICKNESS

The basic reproduction number, R(0), can be used to determine factors important in the ability of a disease to invade or persist. We show ho w this number can be derived or estimated for vector-borne diseases wi th different complicating factors. African horse sickness is a viral d isease transmitted mainly by the midge Culicoides imicola. We use this as an example of such a vector-transmitted disease where latent perio ds, seasonality in vector populations, and multiple host types may be important. The effect of vector population dynamics which are dependen t on either host or vector density are also addressed. If density-depe ndent constraints on vector population density are less severe, R(0) i s more sensitive to vector mortality and the virus development rate. H ost-dependent vector dynamics change the relationship between R(0) and host population size. Seasonality can either increase or decrease the estimate of R(0), depending on the lag between the peak of the midge population and the infective host population. The relative abundance o f two host types is a factor in the ability of a disease to invade, bu t the strength of this factor depends on the differences between the h osts in recovery from infection, mortality and transmission. Removal o f a reservoir host may increase R(0).