MOSQUITOES AND HUMAN DISEASE IN CANADA

Abstract

AbstractIn the early 19th century, malaria was indigenous in Upper Canada (Ontario), but indigenous infection has not been seen in Canada since about 1909. For many years mosquitoes were regarded principally as harmless pests of man and domestic animals until the recognition of a number of mosquito-borne viruses. At present, mosquitoes in Canada are known to be involved in the maintenance or spread of at least seven arboviruses. One of these viruses, western equine encephalomyelitis, occurs in epidemics in man and horses at irregular intervals in the western provinces. The extent to which the other arboviruses might cause disease in man or domestic animals in Canada is unknown at present but California encephalitis virus infection rates can be high in human residents of central Saskatchewan, central Alberta, and southern British Columbia. On the basis of observations made in the United States, St. Louis encephalitis virus could pose a threat to public health in Canada whereas the Cache Valley virus might involve livestock. Even if man and livestock are not involved at present in epidemics of disease due to these viruses, the presence of the viruses and their vectors should be of concern. Epidemics could arise either from amplification of causal viruses already present in indigenous foci or by amplification of viruses from new enzootic foci recently established by extension of the geographic ranges of the viruses. Evidence indicating which category each of the known viruses in Saskatchewan belongs to is briefly reviewed. Japanese B encephalitis virus has not yet been seen in Canada but the environmental prerequisites for an epidemic or epizootic due to this virus appear to be present and awaiting the arrival of the virus.