A recent study conducted by the Health Protection Agency has found that higher temperatures and increases in humidity are associated with an increase in cases of Legionnaires’ disease.

The study is one of the first of its kind in Europe and was presented at the Health Protection Agency’s annual conference at Warwick University. The study looks at the association between humidity, temperature and the peak in Legionnaires’ cases over a period of several years including 2006, when there were unusually high numbers. In 2006 there were 551 reported cases and 52 deaths. This compared with 389 reported cases the previous year.

Kate Ricketts, a scientist specialising in Legionnaires’ disease at the Agency’s Centre for Infections, said: “In this study there appears to be an increase in cases following warm, humid periods; this was especially pronounced during the summer of 2006. The study suggests that the number of cases may be associated with the weather. It remains to be seen what impact the weather will have in the future. There may be important implications for public health authorities if climate change leads to warmer temperatures.”

The infection is known to show seasonal variation, with an increase in cases typically seen during August and September. The peak in 2006 was thought to be linked to the unusually warm temperatures followed by wet periods that were experienced that year. Additionally, whilst there were a small number of outbreaks, the majority of cases were not linked. This makes it more likely that the increase was associated with climatic factors.

Temperature is important for the bacteria’s survival and multiplication, which occurs between 20°C and 45°C; an optimum temperature can cause a rapid increase in numbers of the organism. As transmission of the bacterium occurs by aerosol, humidity is an important factor in its survival. The longer cells remain viable in an aerosol, the greater the opportunity for susceptible individuals to inhale the bacteria.

The HPA plans further research on the possible effects on cases due to climate change. If climate change leads to an increase in temperature and humidity there may be an increase in cases in succeeding years and a need to take this into account in public health planning. Studies are also being carried out in other European countries, highlighting the importance of the association between climatic factors and incidence of this disease.