Possible windborne spread of bluetongue to Portugal, June-July 1956.

The possible sources for the epidemic of bluetongue in Portugal at the beginning of July 1956 were examined. Introduction through authorized importation of domestic or wild ruminants was not feasible, since no cattle, sheep or goats were imported and the wild ruminants were confined to Lisbon Zoo, which was too far from the initial outbreaks. Weather maps were examined to see if the wind could have carried infected Culicoides midges from North Africa. On 21 June 1956 infected midges in Morocco could have been taken offshore by southeast winds and then carried by south winds unusual at that time of year to the south coast of Portugal. The 200-300 km sea crossing would have taken some 10 h and been by day when air temperatures near the sea surface were about 18-20 C. Bluetongue had not been reported at that time in Moroccco, and the possibility of the presence of the virus in moroccan animals without clinical signs is discussed.