Do the tropics have a flu season?

by revere, cross-posted from Effect Measure

The scientific literature is full of specialized papers that on their face would seem to be of little interest. Here's a title like that: "Prevalence and seasonality of influenza-like illness in children, Nicaragua, 2005-2007" (Gordon et al., Emerging Infectious Diseases 2009 Mar). Over 4000 Nicaraguan children, aged 2 to 11 years old and living in the capital of Managua were followed for 2 years, April 2005 to April 2007 and observed for development of ILI (influenza-like illness). We know a lot about influenza in major industrialized countries in the northern and southern temperate zones, but very little about the epidemiology of seasonal influenza in tropical regions. Is the pattern of the disease in these populations the same as in temperate climes? Is there a lot of flu or just a low level? Is it still seasonal influenza? The US and Europe have recently set up surveillance systems that help answer these questions but most countries don't have those resources.

The question of seasonality is especially interesting because we still don't understand why flu exhibits such a strong seasonal pattern (many hypotheses have been advanced but there is still no agreement). Managua is warm, with a fairly constant temperature between 28 and 32 degrees Celsius. In temperate climates flu season takes place mostly with temperatures below 20 degrees Celsius and low absolute humidity. But in the two years of this study flu peaked in June and July each year, the middle of the rainy season (May to November). Furthermore, this paper also showed a peak in the November - December months during one (2006) but not the other of the two years.

Recent animal studies have suggested cold and dry air is the key to influenza seasonality, but this pattern is not consistent with that finding. The relationship with rainy seasons has been observed before, so the data are far from convincing in view of the epidemiology, scanty as it is. The other interesting finding of this study is that there appears to be a substantial burden of influenza amongst Nicaraguan children.

In other words, it is not the case that influenza in this tropical country is relatively low level and spread throughout the year. In the two years of data in this study there is a lot of it and it shows a characteristic seasonality, peaking in the middle of the rainy season (northern summer). We need to look more closely at other tropical countries, and as this paper shows, it isn't easy, by any means. I haven't gone through the details of the study design but it was logistically expensive and time consuming and required an extensive field operation and willing in-country collaborators.But studies like this give us information about how flu is, or is not, transmitted.

Sometimes uninteresting sounding titles are quite interesting.