INFECTIOUS disease is on the march. Dengue fever, Japanese encephalitis, yellow fever and other diseases that were once under control are now resurgent. Others such as West Nile virus and Rift Valley fever are expanding their range. And still more, such as sabia virus, have appeared in humans for the first time. A major underlying cause is suspected to be climate change.
Now an analysis of more than 300 human diseases has found that the distribution of pathogens worldwide is strongly linked to climate, and that climate change could lead to a massive increase in human vector-borne disease.
There are ominous signs this is already happening. For example, the spread of West Nile virus across the US has been strongly linked to a changing climate. The initial outbreak in New York in 1999 was preceded by a three-week drought, which provided ideal conditions for the virus鈥檚 vector, the mosquito Culex pipiens, to breed in organically rich sludge in dried-up drains. And the virus鈥檚 march into 44 US states and five Canadian provinces mostly occurred during the 2002 drought. Other major epidemics in Romania and Israel followed dry periods.
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Numerous other epidemics have been tied to climate. Hantavirus outbreaks in the US and South America occur when heavy rain follows a drought. The dry period kills off large numbers of predators such as owls and snakes, so when the rains come the numbers of hantavirus-carrying rodents explode.
Likewise, Lyme disease is extending its range in North America because warmer winters are helping the ticks that carry it to survive. Outbreaks of mosquito-borne Ross river virus in Australia have been linked to rainfall patterns.
鈥淚t is extreme climatic events that are driving these epidemics,鈥� Paul Epstein of the Centre for Health and the Global Environment at Harvard Medical School in Boston told 麻豆传媒. He says extreme weather is expected to increase as global climate warms (see 麻豆传媒, 29 June 2002, p 22).
Now a team led by Jean-Fran莽ois Gu茅gan at the Institute for Development Research in Montpellier, France, has examined data on 332 human pathogens in 224 countries and overseas territories. The team鈥檚 analysis, which includes bacteria, viruses, fungi, protozoa and helminth worms, asked why countries support certain pathogens and not others.
The model factored in geographic features such as patterns of land use, demographic factors such as human population size and density, and each country鈥檚 GDP. Climate and latitude came out as being the most important predictors of the number of pathogens present, with pathogen diversity greatest in the tropics. In particular, variability in rainfall was the climatic factor most closely linked to pathogen diversity, backing the idea that alternating wet and dry periods are a key factor in disease transmission (Public Library of Science Biology, DOI:10.1371/journal.pbio.0020141).
Increases in air travel, urbanisation, intensive agriculture and resistance to control measures are also likely to have played a part in the spread of human pathogens. 鈥淲ealth and socio-economic factors are of course important,鈥� says Gu茅gan. 鈥淏ut climate is more important to explain the pattern we observe.鈥�
Not surprisingly, the analysis suggests that vector-borne diseases and those with wild animal hosts, follow the pattern most closely. By contrast, the spread of viral diseases transmitted directly from human to human, such as measles, was not linked to latitude.
Paul Ready, an expert in insect vectors at the Natural History Museum in London, says the analysis gives an interesting overview of the distribution of disease now. But he is not convinced that it shows a link to climate change. The pattern could simply reflect our origins as an African primate. As we have migrated out of the tropics, our diseases may not have had time to adapt to conditions at higher latitudes, he says. 鈥淐limate alone is not going to tell you enough.鈥�
Gu茅gan accepts that this could be true, but with numerous studies of individual diseases pointing to a role for climate his hunch is that our primate origins play only a minor role.
The work has also allowed the team to gauge the diversity of pathogen species in all host animals in the tropics, which is hard to estimate by other means. Based on the increasing human pathogen diversity as latitude decreases, they estimate that total tropical pathogen diversity is 20 to 40 times higher than previously thought. The team suggests there could be at least 10 times as many species on the planet as previous estimates.
Epstein describes the study as a 鈥渕asterful job鈥�. If the pattern holds for agricultural pests and livestock diseases, he says, the negative economic consequences of global warming could be more serious than even the gloomiest predictions.
