Seasonal variations in spread of arthropod-borne disease agents of man and animals: implications for control

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Abstract
The disease agents under discussion include viruses and protozoans that undergo a cycle in animals as hosts and in insects as vectors. Such agents are found in climatic zones ranging from tropical rains to cool. Outside the tropical rain forests there are periods during which conditions are unsuitable for the cycle of infection to continue, because the activity of the insects is inhibited at temperatures above 35 °C and below 15 °C. During this ‘overwintering’ period the virus may survive by persistence in the host, or in the adult insect, larva or egg, or the virus may be introduced or reintroduced into the area by movement or migration of the host or by carriage of infected insects on the wind. Examples are given of the introduction of disease or infection by insects carried on the wind. Such examples include (i) between the southerly and northerly limits of the Inter-Tropical Convergence Zone, the wind carriage or possible wind carriage of insects infected with bluetongue or yellow fever in West Africa and Sudan and with Rift Valley fever in Egypt, (ii) the wind carriage of midges infected with bluetongue in the eastern Mediterranean and (iii) the introduction of Japanese encephalitis to Japan on monsoon winds by infected mosquitoes. Analysis of the host-vector cycles of the agents in the particular zones enables appropriate control measures to be devised. In endemic areas this would involve importation and vaccination of introduced animals at the appropriate time. In epidemic and sporadic areas, the uncertainty of the timing of outbreaks has to be recognized and the times of breeding of animals and the need for vaccination of animals assessed. During an epidemic the questions of eliminating insect breeding sites, and dipping and spraying of animals have to be considered in relation to possible reintroduction of infection at a later date.