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Sunday, March 25, 2007

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Why rainforests are "hot zones" for diseases?

Forrest M. Mims III has suggested that when there is mass burning in an area that produces large amounts of smoke, the resulting decline in UV-B light levels may permit higher levels of pathogenic airborne bacteria and viruses to build up, with possible human health implications.

An environment in which UV-B levels are permanently low is the rainforest. Some years ago, I measured UV-B levels in a rainforest in North Queensland. I found that, as expected, the levels of ultraviolet light on the floor under the rainforest canopy were very low.

A number of serious viral diseases have emerged from the rainforests of Africa in recent years:

"Among the extraordinary biodiversity of rainforests are a multitude of viruses and other diseases that have not yet come in contact with humans. Viruses, which previously remained hidden in remote rainforests suddenly have access to large human populations. The appearance of such "rainforest diseases" as HIV/AIDS, Ebola and Marburg suggests that these could result from tropical deforestation. The rainforests hold such viruses in check, but without the forests to 'lock' them in, they would be free to infect humans and other species."

But why are such viruses endemic to rainforests? It seems possible that the lack of ultraviolet light in the rainforest could allow them to thrive in that environment. Are these pathogens airborne? Marburg virus may be. Ebola may be, at least between monkeys and perhaps in humans.

Here is a note by Mims on UV-B as a controlling factor on influenza viruses. However these people, writing about foot-and-mouth disease virus in Australia, tend to discount a role for ultraviolet in limiting the spread of this virus at least.

Julian

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