Climate Change and Infectious Disease: Is the Future Here?

Abstract

After kayaking on Vancouver Island’s eastern shore, a 45-year-old woman’s headaches and night sweats were little indication that she had been exposed to a rare and dangerous fungus while on the water. Her doctors, mystified as to the cause of the symptoms, didn’t recognize that the infection was Cryptococcus gattii—a species of pathogenic yeast—until shortly before her death in 2002.1 Most cases of C. gattii have been reported in the warmer climates of Australia, Asia, Africa, and Southern California. But at least two strains of the fungus are now affecting humans, pets, and wild animals in the U.S. Pacific Northwest.2 From January 2004 through July 2010, a total of 15 people died from C. gattii infection, according to the Centers for Disease Control and Prevention (CDC), and 60 human cases of the illness were reported in Oregon, Washington, California, and Idaho.3 Despite the alarming sound of the disease, public health officials in Oregon have urged residents not to consider the spread of C. gattii as a health emergency—although the fungus is present in the wild, few people have become seriously ill with this infection, and even fewer have died, says Oregon state epidemiologist Katrina Hedberg. Nevertheless, the mere presence of this foreign species so far from its home raises questions. Reports suggest the fungus may have been exported from its native habitat on commercially valuable trees such as eucalyptus and ornamental Ficus species.4 “We don’t know exactly why the [Pacific Northwest] outbreak emerged,” says Edmond Byrnes, a postdoctoral fellow at the Johns Hopkins University School of Medicine who has studied the fungus.5 “One of the more current hypotheses is that climate change is one of the factors that should be considered.” Byrnes explains that C. gattii may be able to establish itself in the Pacific Northwest region because of milder winters with daily average temperatures above freezing. C. gattii is one of several pathogens whose spread is hypothesized to be linked to climate change. Like many aspects of climate change, the connection with infectious disease involves controversy. Some scientists argue that improved climate models may give a false impression that climate change is driving a spread in infectious diseases; others point to human activity and other factors as far more important determinants than climate.6,7,8,9 But health practitioners know that a myriad of factors affect the spread of any disease, and many of these—including human migration, poverty, water and air quality, land use decisions, ecological change, the strength of the local public health system, and even access to air conditioning—are themselves intertwined with climate change.10 Moreover, says Jonathan Patz, a professor of environmental studies and population health sciences at the University of Wisconsin–Madison, “So many infectious diseases are sensitive to climate that if the majority of the climatologists around the world are telling us that climate is changing and will continue to change under the scenario of global warming, then disease incidence will change.” Several scientific studies and reports suggest these proposed future effects of climate change are in fact already occurring.11