Water buffaloes, neurasthenic

Dr Ronald's report predicting five million cases of HIV infection next year (Figures don't lie... (cont)) referred (as many such reports do) to the iceberg phenomenon. We often used to hear that for every case found, 'the WHO calculated' that there were 100 or 1000 (choose your figure) others that made up the unseen part of the iceberg under the water. I'm doubtful the WHO ever made such an assertion, but it became an urban myth.

Ignoring the figures (and the fact that icebergs are quite rare in the tropics), the metaphor of the tip and the submerged part is in fact totally inappropriate. The problem with it is that the full size of an iceberg never becomes apparent. As the ice melts, the iceberg gets smaller (never larger, like an epidemic), but the ratio of tip to submerged part remains the same. Thus the metaphor suggests that, as more and more cases are identified, so there are more and more cases unidentified.

With improved surveillance (active or passive), the proportion of unidentified cases is reduced. Epidemiologists then say that the size of the submerged portion decreases compared to the tip (see iceberg phenomenon). This is clearly wrong-headed, since such can never happen to an iceberg. And this wrong-headedness leads the press (and activists who should know better) to scream about an exploding epidemic each time the number of identified cases goes up.

I was reminded of this during my morning walk around the small farms at the back of the Halim airbase (the Sunter Valley is not quite as attractive as the Thames Valley, but this morning's walk was pleasant, with clear views of Mount Gede and Mount Salak to the south). I pass a number of muddy pools, and as usual this morning, several water buffaloes were enjoying their morning bath.

Some time ago, Doc Suharto (late of the Education Department and the National AIDS Commission) proposed this as a replacement metaphor for the AIDS epidemic in Indonesia. We first see only the buffalo's snout above the water, and we have no idea if it is a small baby or a large adult. But slowly the beast raises itself out of the mud, and its size starts to become apparent, until finally we can see its full extent.

This clearly provides a much more appropriate metaphor, besides being easier for people here to understand. At the start of the epidemic. the buffalo is small, but as time passes grows larger. But as surveillance improves, the buffalo heaves itself out of the mud, and we begin to appreciate its full extent.

Wonder if there's any hope of getting epidemiologists to change their metaphor?

Babé