Infectious disease mortality in two Outer Hebridean islands: 2. pulmonary tuberculosis

To examine mortality from pulmonary tuberculosis (PTB) in the Outer Hebridean islands of Harris and Barra, and to compare it with that in Scotland as a whole.

Ages and dates of death from PTB in Harris and Barra between 1855 and 1990 were ascertained from copies of death certificates. For Scotland, annual numbers of deaths in 5-year age-groups were extracted from Annual Reports of the Registrars General. Proportionate mortality ratios (PMRs) were calculated for each population in the 1-14 and 15 or over age-groups, and the 'detrended' PMRs were subjected to spectral analysis. For this purpose the data were divided into the pre-antibiotic (1855-1950) and antibiotic/chemo-therapeutic (1951-1990) periods.

PTB was predominantly a disease of adolescence and early adulthood, although ages at death increased towards the end of the period of study. While PMRs for Scotland decreased steadily after the mid-1870s, apart from increases during the 1940s, those for the two islands increased during the late 19th century, and did not decrease markedly until the 1930s. In the younger age-groups the pattern of mortality was similar to that of an acute infectious disease, while in the older age-group the pattern was largely endemic. Spectral analysis of pre-1951 data from both islands showed marked periodicities in mortality, mostly among the younger age-group. The introduction of effective therapy in the 1950s appeared to lengthen inter-epidemic intervals. There appeared to be no relationship in either island between mortality and socio-economic deprivation, as indicated by numbers of new poor registrations. In Harris, mortality was higher in colder and wetter years. This was not the case in Barra.

The findings are reasonably consistent with a hypothesis that PTB was relatively uncommon in the Outer Hebrides until the later 19th century, and that it behaved epidemiologically like an acute infectious disease. This was not the case with Scotland as a whole, where the disease appeared to be largely endemic.