From the evidence above, it can be seen that both in the USA and Canada, the incidence of many of the vector-borne diseases has been mounting and infections have often spread to geographical areas from which they have not previously been reported. It is important to understand the reasons why this is occurring as a basis for controlling these infections and attempting to prevent their further spread. In addition, new diseases are emerging, sometimes after having been introduced from elsewhere as has been the case with West Nile virus. In other cases, the emerging diseases may have been long-endemic and only recently recognized. The emergence may also be a consequence of changing ecological conditions. There are also instances of new clinical symptoms or conditions which have been reported or an increased virulence among infections long endemic.

The effect of ecological changes

Obviously, substantial urban and rural ecological changes have occurred in the USA and Canada, particularly since the mid-part of the twentieth century. These changes have included increases in human population densities in cities and, more recently, significant population movements to suburban areas resulting in greater exposure to vector populations. As in Europe, increased leisure time and the manner in which it is being used has increased exposure to vectors, especially to ticks. In most instances, the appearance of new diseases and syndromes and the resurgence of old can be associated with ecological changes that have favoured increased vector densities and exposure to them.