The weather has a profound effect on the pattern of infectious diseases throughout the world. This effect may be either direct (affecting the ability of the infectious agent to survive and/or spread) or indirect (affecting the behaviour, susceptibility or survival of the host).

Seasonal epidemics

Tropical climates There are many infections that peak during particular seasons. Some good examples in tropical areas include: epidemic meningococcal disease, which occurs in sub-Saharan Africa during the dry season and comes to an abrupt end with the onset of the rains; cholera epidemics, which are seen following heavy rainfall. Heavy rains and higher than average temperatures in Africa in 1997/8 resulted in a large epidemic - 6% of deaths were from this infection; water shortage, resulting in high levels of diarrhoeal diseases due to poor hygiene; malaria epidemics, which follow heavy rainfall and higher than average temperatures (this is because the mosquito, which transmits malaria, is able to thrive in these conditions). The ability to forecast large epidemics could allow efficient targeting of limited resources in these countries. Although this is an area of intensive research, forecasting is not yet a reality.

Temperate climates

Closer to home, in temperate climates, respiratory virus infections (for example, influenza, parainfluenza, respiratory syncytial virus (RSV)) show marked seasonality, occurring in the winter months, whether in the northern or southern hemispheres. Alternatively, gastrointestinal infections and chickenpox peak in the summer months. But it is the winter respiratory virus activity that has the greatest impact on healthcare services.

Monitoring illness in the UK

The number of UK patients consulting their GPs with influenza-like illness (ILI) and bronchitis has been monitored since 1967. Factors such as host susceptibility to changing virus strains and levels of vaccination against influenza virus are important in determining the amount of illness observed, but the precise timing of peaks is currently unpredictable. How the virus persists through the summer months is not well understood, nor is the explosive and widespread nature of outbreaks when they occur in the winter. What is known is that, on average, 9,000 excess hospital admissions (in England only) are attributed to the influenza virus each winter.

The data collated since 1967 shows two annual winter peaks of GP consultations for bronchitis in different age groups. The first smaller peak - in the 0-4 year age group - usually occurs before Christmas and is associated with large increases in children's hospital admissions for bronchiolitis. The second, larger peak - in the over-65s age group - usually occurs one to two weeks into the New Year and is associated with large increases in elderly 'respiratory' admissions. It has been suggested that RSV infection circulating in the childhood population is transmitted to the elderly over the Christmas holiday period.

For more information, e-mail health@metoffice.gov.uk.

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