Temperatures and environmental conditions are changing, causing the spread of disease to shift. How those changes and shifts will play out, however, is the subject of debate. It’s impossible to build a computer model that perfectly mimics the real world and can thus predict, say, where mid-latitude regions will become warm enough for tropical diseases to thrive or wet enough to enhance the spread of water-borne pathogens. But research does suggest that—similar to shifts in animal and plant distributions as climate changes—some places will see rates of certain diseases drop, while others will see an increase or introduction of those diseases.
Shifting patterns of disease do not apply only by latitude, however. Just as how the distribution of desert cacti is slowly creeping into Arizona's hills or how lowland insects are moving into mountains in Borneo as climate warms, diseases can also broaden their distributions by reaching higher and higher elevations. And according to a new study published by American, British, Ethiopian and Colombian researchers in Science, it’s already happening.
The authors of the study turned their attention specifically to malaria, which infects an estimated 300 million people each year. Malaria might be particularly susceptible to changes in distribution due to warmer temperatures, they explain, because the Anopheles mosquitoes that carry the malaria parasite can only live in warm environments.
The researchers focused on the highlands of western Colombia (50 to 2,700 meters) and central Ethiopia (1,600 to 2,500 meters), which historically have been cool year-round but have experienced a flux of warmer and cooler seasons in recent years. To see how malaria might or might not have been affected by those climate variations, they compared records of malaria incidence from 1990 to 2005 in Colombia, and from 1993 to 2005 in Ethiopia, with temperature data from each of those years.
In warmer years, they found, malaria incidence did indeed occur at significantly higher elevations than in the cooler years. In Ethiopia’s Debre Zeit region, for example, an increase in 1ºC corresponded to an average of more than 2,100 additional cases during the transmission season, from September to December.
"This is indisputable evidence of a climate effect," said Mercedes Pascual, a theoretical ecologist at the University of Michigan and co-author of the study, in a statement.
She and her colleagues predict that these results would also apply to other countries and regions that suffer from malaria, although studies will have to be undertaken in those places to confirm that assumption. "The main implication is that with warmer temperatures, we expect to see a higher number of people exposed to the risk of malaria in tropical highland areas like these," Pascual added.
A permanent 1ºC temperature change in Ethiopia could mean three million more malaria cases per year in people under 15-years old alone, the authors estimate. Around 43 percent of the country's population currently lives in rural areas historically protected from malaria due their elevations of 1,600 to 2,400 meters, but which now fall within the potential danger zone for hosting the disease as climate warms.
"Our latest research suggests that with progressive global warming, malaria will creep up the mountains and spread to new high-altitude areas,” said Menno Bouma, a clinical lecturer at the London School of Hygiene & Tropical Medicine and co-author of the study. “And because these populations lack protective immunity, they will be particularly vulnerable to severe morbidity and mortality."
Malaria’s shifting distribution is certainly a cause for alarm. According to the United Nations, the disease causes around 2 million deaths annually—most of which are children—and acts as a significant burden to countries, keeping poor regions poor by reducing worker productivity and thus economic growth.
The study authors point out that their research is a heads-up about what will likely become an even greater problem in the future. They note that nonprofits, governments, and other groups interested in curbing the spread of malaria will need to establish intervention methods in places where they were previously not needed before, including at higher altitudes. Mapping where malaria may strike under different regimes of climate change "should further contribute to the early warning of epidemics and assist global malaria elimination,” they write.