Objectives: (i) To determine the prevalence of multiple sclerosis (MS) in New South Wales and South Australia; (ii) to compare these prevalences with those in other areas of Australia and to determine the relationship between prevalence and latitude; (iii) to examine the relative contribution of genetic and environmental factors in the aetiology of the disease in Australia; and (iv) to ascertain whether there had been a change in the frequency of the disease since 1961.
Results: The crude prevalence of MS in New South Wales on prevalence day (National Census Day, 30 June 1981) was 37.2/100,000 and the age-standardised prevalence 36.6/100,000. The female:male ratio was 2.3:1. The crude prevalence in South Australia was 29.4/100,000 and the age-standardised prevalence 28.8/100,000. The female:male ratio was 2.4:1. No Aborigines or Torres Strait Islanders with MS were identified. There was a significant increase in the prevalence with increasing south latitude in Australia, MS being about seven times more frequent in Hobart than in tropical Queensland, but no genetic differences were found in the surveyed population in different parts of Australia. A significant increase in the prevalence of MS occurred in most areas of Australia between 1961 and 1981, but this may not reflect a true increase in incidence.
Conclusion: The increasing prevalence with increasing south latitude cannot readily be explained by genetic susceptibility, and suggests that environmental factors are important for expression of the disease.