Infectious disease mortality in two Outer Hebridean islands: 1. measles, pertussis and influenza

Objectives: The purpose of the study was to examine changes in mortality from measles, pertussis (whooping cough) and influenza (all epidemic diseases) in Harris and Barra, two Outer Hebridean islands, from 1855 to 1990, and to compare the findings with those from Scotland as a whole over the same period. It was also intended to relate changes in mortality to those in social and economic factors.

Materials and methods: Ages and causes of death in Harris and Barra were ascertained from copies of death certificates held at the General Register Office, Edinburgh, and for Scotland as a whole from the Annual Reports of the Registrars General for Scotland. Data were standardized by calculating the Proportionate Mortality Ratio (PMR), the proportion of deaths due to a particular cause to all deaths over a given period. Spectral analysis was employed to examine the durations of epidemic cycles.

Results: Ages at death increased slightly over the period of the study. For measles and pertussis, other than for the former in Harris, there were significant relationships between numbers of deaths per decade, and numbers of new susceptibles, estimated as the numbers of births. Epidemics of measles and pertussis in the islands occurred at intervals, usually separated by years of no mortality. The highest PMRs were generally during the later decades of the 19th and first decade of the 20th centuries; this may have been related to the economic problems of agriculture and fishing, and to increasing population density. Influenza epidemics were more frequent than those of the other two diseases. For all three diseases in both islands, there were significant negative power relationships between epidemic size and frequency of occurrence; those in Harris were the stronger. The relationships between length and frequency were significant only in Harris. Generally, epidemic lengths seemed less variable than sizes, possibly because of the rather 'coarse' units of length (quarters) employed. Spectral analysis of the 'detrended' data for the period before the introduction of specific immunoprophylaxis revealed that for measles the main epidemic cycle in all three populations was between 7.3 and 7.8 years' duration. Barra and Scotland had additional 2.5- and 2-year cycles, respectively. For pertussis, Harris and Barra had main cycles of 7.4 years. Harris had an additional cycle of 3.2 years. Scotland had cycles of 4 and 2 years. For influenza, Harris had a main cycle length of 7.4 years, and a less-defined one of about 2.6 years. Barra had a main cycle of 6.9 years, and a subsidiary one of 2 years. Scotland has a single cycle of 8 years. Cubic regressions of the spectral densities on cycle frequencies showed large coefficients for Harris and Barra, but small ones for Scotland. Measles coefficients were closely similar in the two islands, but not those for pertussis.

Conclusions: The findings demonstrate the episodic occurrence of epidemics of these three diseases in the two islands, as against their continual presence in the much larger population of Scotland. They reveal also the decreasing importance of these causes of death in all three populations. The data from Harris and Barra suggest that measles is a more epidemiologically 'stable' disease than pertussis. Both islands appear to obey Hamer's law of 'mass action'. The relatively long intervals between epidemics in the islands may be due partly to their isolation, and partly to the slow accumulation of sufficient numbers of susceptibles to enable an epidemic to occur.