The incidence of asthma amongst Tristan da Cunha islanders was recorded over a period of 16 months. Thirty-two per cent of the islanders gave a history of asthma, and 22% had one or more attacks during the survey period. Three-quarters of the affected islanders had mild asthma. Both the incidence and severity of asthma on Tristan have declined since 1944.
Over half the asthmatics were allergic to house dust extracts on skin testing; so was a large proportion of non-asthmatic islanders. Large numbers of house dust mites are present in Tristan dust, but airborne allergens are scarce. Total TgE levels were greater than normal in the serum of twenty-three of sixty-seven islanders investigated. Allergy was thought to be the main cause of asthma in about a third of the active asthmatics.
The high incidence of asthma on Tristan derives from two or three asthmatics amongst the fifteen ancestors of this highly inbred community. Nevertheless, asthma on Tristan, as elsewhere, seems to have a multifactorial aetiology.