Rates of hepatitis A higher in an El Niño year

Researchers at the Queensland University of Technology have examined the possible association between the Southern Oscillation Index and the occurrence of hepatitis A in Australia. The results indicate that the SOI is statistically significantly associated with the transmission of hepatitis A.

Data was obtained on the monthly counts of hepatitis A cases in Australia and the monthly SOI between 1 January 1991 and 31 December 2000 from the Commonwealth Department of Health and Ageing and the Australian Bureau of Meteorology, respectively.

Data on population sizes were obtained from the Australian Bureau of Statistics. Cross-correlations were used to compute a series of correlations between SOI and the incidence of hepatitis A over a range of time lags (defined as the time span between the SOI and the incidence of hepatitis A).

A decrease in the SOI (ie, warmer and drier conditions) was statistically significantly associated, at a lag of 1 month, with an increase in the monthly incidence of hepatitis A. Two El Niño events (1991–92 and 1997–98) were also clearly associated with an increased incidence of hepatitis A.

The results suggest that there was an increase of about 360 cases per year in Australia for an, on average, interquartile range decrease in the SOI. The residuals in the model fluctuated randomly around zero, and there was no apparent autocorrelation between residuals at different lag times (data are available from the corresponding author).

El Niño Southern Oscillation (ENSO) has been found to be related to various health outcomes, including waterborne disease, vectorborne disease, and natural disaster-related deaths (eg, floods, bushfires and cyclones).

The study adds further evidence of ENSO-related health effects. Infectious diseases are, in general, sensitive to climate variability, as climate can influence the development and transmissibility of pathogens, and can also affect people’s behaviour.

If the relationship between ENSO and hepatitis A is confirmed by other studies, these findings may facilitate the development of early warning systems for controlling and preventing this widespread communicable disease.

El Niño and La Niña are major temperature fluctuations in the Pacific Ocean. They are Pacific signatures of the global ENSO phenomenon (El Niño-Southern Oscillation). Their role in global warming or cooling is controversial.

El Niño was originally recognized by fishermen off the coast of South America as the appearance of unusually warm water in the Pacific Ocean. El Niño is the warming of the surface waters of the eastern equatorial Pacific Ocean that occurs at irregular intervals of 2-7 years, usually lasting 1-2 years. Along the west coast of South America, southerly winds promote the upwelling of cold, nutrient-rich water that sustains large fish populations, that sustain abundant sea birds, whose droppings support the fertilizer industry. Near the end of each calendar year, a warm current of nutrient-poor tropical water replaces the cold, nutrient-rich surface water. Because this condition often occurs around Christmas, it was named El Niño (Spanish for boy child, referring to the Child Christ). In most years the warming last only a few weeks or a month, after which the weather patterns return to normal and fishing improves. However, when El Niño conditions last for many months, more extensive ocean warming occurs and economic results can be disastrous. El Niño has been linked to wetter, colder winters in the United States; drier, hotter summers in South America and Europe; and drought in Africa.

ENSO is a set of interacting parts of a single global system of climate fluctuations that come about as a consequence of atmospheric circulation. ENSO is the most prominent known source of interannual variability in weather and climate around the world (~3 to 8 years), though not all areas are affected. The Southern Oscillation (SO) is a global-scale seesaw in atmospheric pressure between Indonesia/North Australia, and the southeast Pacific. Its measure is through the Southern Oscillation Index (SOI). Global ENSO has signatures in the Pacific, Atlantic and Indian Oceans. In the Pacific, during major warm events El Niño warming extends over much of the tropical Pacific and becomes clearly linked to the SOI intensity. While ENSO events are basically in phase between the Pacific and Indian Oceans, ENSO events in the Atlantic Ocean lag those in the Pacific by 12-to-18 months. Many of the countries most affected by ENSO events are developing countries within main continents (South America, Africa...), with economies that are largely dependent upon their agricultural and fishery sectors as a major source of food supply, employment, and foreign exchange. New capabilities to predict the onset of ENSO events in the three Oceans can have global socio-economical impacts. While ENSO is a global and natural part of the Earth's climate, whether its intensity or frequency may change as a result of global warming is an important concern. Low-frequency variability has been evidenced. Interdecadal modulation of ENSO might exist. see http://en.wikipedia.org

These findings are reported in this week's issue of the Medical Journal of Australia.