Global Warming, Global Warning

This isn't the first time a new fungus has found a good home in the Pacific Northwest. After all, Oregon and Washington are states where, it is said, a rolling stone actually does gather moss — and grows it, too. But the fungal invader Cryptococcus gattii has some health researchers worried: The organism may be getting a boost from global warming. C. gattii, once relatively elusive in the region, has been showing up in record numbers in humans and animals, along with the variable microclimates of the past few years. It now has been detected in trees, fresh water, the ocean, and air. It even has been cultivated from the surface of tires.FigureThe only good thing that can be said about the organism so far is that it may offer a way for understanding the emergence of infectious disease. “We believe the dispersal mechanisms of C. gattii could be applied as a model for other organisms,” noted the Canadian researchers in British Columbia who have documented its migration southward. (Emerg Infect Dis 2007;13[1]:51). Global travel and immigration are the two causes most frequently cited as sources of new domestic disease, but they may be assisted by another factor as well: a changing climate. Not every scientist agrees about that, however. This past February, the issue was debated at length during the annual meeting of the American Association for the Advancement of Science. And last year, a group from the U.S. Centers for Disease Control and Prevention published a public health response to climate change, suggesting the correlates of emerging infections and increasing global temperature and carbon dioxide emissions. The proposed action plan emphasized tracking and research. (Am J Public Health 2008;98[3]:435.) What isn't clear, however, is the extent to which global warming actually might encourage emerging disease. It is fairly well established that periodic shifts in weather support such opportunism. “In situations like El Niño, where there are rainy, warm conditions, we have gained some good information. There seems to be some ‘disease response’ to that,” observed Jonathan Patz, MD, MPH, a professor of environmental studies and population health sciences at the University of Wisconsin-Madison, where he directs a university-wide initiative on Global Environmental Health. Hantavirus outbreaks, for example, rose with the growth of rodent populations associated with El Niño, noted Dr. Patz, who is the senior investigator of a study showing that ecological shifts can have a serious effect, even in the short term. (Med Clin North Am 2008;92 [6]:1473.) Comparisons of mosquito populations in the Peruvian jungle before and after deforestation suggest the impact of climate change on disease potential, he said. In that part of the rain forest, a species of malaria-carrying Anopheles proliferated in the wake of cutting. More sunlight was able to penetrate the pools of water that are the insect's breeding grounds. “By cutting the canopy and opening up the forest, the habitat may have changed to one more favorable to this mosquito,” said Dr. Patz, also an adjunct associate professor in environmental health sciences at the Johns Hopkins Bloomberg School of Public Health and an affiliate scientist of the National Center for Atmospheric Research (NCAR). The same situation seems to be occurring in Africa, in the same insect of a different species, after slash-and-burn practices there, he pointed out. In addition, temperature increases lead to drier conditions, possibly even drought. When that happens, food, in particular cereal grain, “trends downward,” Dr. Patz said. The resulting crop-dependent population becomes malnourished, immunity diminishes, and infection flourishes. “We do know that West Nile virus, for example, seems [more] transmissible in drought or in hot temperatures,” he added. Clusters of Disease Several years ago, the Center for Health and the Global Environment at Harvard Medical School posited that extreme weather conditions are creating settings conducive to clusters of insect-, rodent- and waterborne diseases. (Microbes and Infection 2001;3[9]:747.) Some rodent populations were found to be escalating after unseasonably wet winters in the southwestern United States. A subsequent study by the U.S. Department of Agriculture found fairly high infection rates there of Giardia in fecal samples of beaver and nutria. (J Parasitol 2002; 88[6]:1254.) Other changes seem to have had similar effects on marine health. Warmer water temperatures along coastal parts of the United States have been associated with the emergence of Vibrio parahaemoliticus infections from shellfish. Several studies show that Vibrio species grow better in higher temperatures, and that their presence in oysters in northern coastal waters had previously been relatively rare. Though such results intrigue, they have not necessarily led to definitive links between disease and climate change. Take, for example, another series of findings in the Pacific Northwest. There has been an explosion among invasive riparian inhabitants, such as nutria and rats. But has it led to more rodent-borne disease? “That's a good question,” said Morris Jones, PhD, a clinical investigator at the David Grant USAF Medical Center in Travis, CA. Dr. Jones has researched a cluster of chronic gastrointestinal illness of unknown origin. Assay by polymerase chain reaction detected a single-celled parasite, Blastocystis hominis, in nine patients with persistent diarrhea. In six of those, he and colleagues identified two subtypes, which he plans to sequence. (Parasitology Research 2009;104[2]:341.) Some of the patients in Dr. Jones' and his colleagues' study previously had tested negative for the parasitic infection. Exposure, anecdotally, seemed to be a result of tainted drinking water from wells or places along hiking trails. The parasite made headlines in the Oregon college town of Corvallis, where it was blamed for unexplained gastrointestinal illness after several people tested positive for it. Two years ago, legislation was proposed to make it a reportable disease, but the effort failed. Why? Determining the prevalence of Blastocystis hominis would require evidence that there is a significant difference between symptomatic and asymptomatic cases, which has yet to be proven. And it would take federal funding to activate surveillance, according to state health authorities. Blastocystis hominis, which was first described in the early 1900s, was last seen as a source of recognizable infection in the 1980s, when HIV-infected patients who became immunocompromised showed gastrointestinal disturbances associated with it. Whether there is a universally pathogenic form that can affect generally healthy individuals remains to be determined by scientific studies, said Emilio Debess, DVM, the state public health veterinarian. Genetically different variants may eventually come to light through the use of new genetic technology, as has been the case with the Giardia parasite, he noted. Sequencing the genome should provide more information about the possible different forms of infection, including virulence, Dr. Jones concurred. Transmissible Disease Small and isolated clusters of outbreaks can be just that — unimportant in the long run. One of the lessons of the 1918 flu pandemic, which killed more than 25 million people in six months, however, seems to be that a second wave can turn deadly rapidly, ushering in a third wave that sweeps across populations, growing from a handful of cases to thousands within a few weeks. (The Greatest Benefit to Mankind: Humanity. New York: W.W. Norton & Co.; 1999.) The case of severe acute respiratory syndrome (SARS), for example, illustrates just how quickly an agent can spread due to international travel. SARS is believed to have spread globally from a single index case: one infected tourist in a Hong Kong hotel. But the transmission didn't sustain long-term alarm, presumably because it was contained relatively quickly, observed Mitchell Cohen, MD, a rear admiral who is the director of the Centers for Disease Control and Prevention's Coordinating Center for Infectious Diseases. “But we were very fortunate in that, with SARS, the disease was most likely to be spread late in the illness, when people were often very sick and already hospitalized,” he explained. Symptoms from some infections, such as flu, may not be as obvious during the early stages of the illness, but the disease may still be transmissible, he said. And rapid travel has made global transmission of emerging infections more possible. “A person who acquires hemorrhagic fever virus may be exposed in a [remote] place, but be back home in Europe before they become ill,” he noted. In fact, this is precisely how one such Marburg virus infection made a recent, intercontinental jump from Uganda to the Netherlands. Yet reports on everything from SARS to salmonella seem to become disease-of-the-month accounts. They seize headlines but fail to ignite interest for very long. “I think our [public] attention span seems to be fairly short,” Dr. Cohen observed. Part of the reason, he speculated, may lie in the generational changes taking place. Today's young adults didn't experience epidemics of vaccine-preventable infections, such as polio. “I remember being told not to swim in the local pool. I remember lining up to be vaccinated,” Dr. Cohen said. Now, however, some vaccine-averse parents have succeeded in eschewing immunization for their kids, which means that now there are some children susceptible to conditions that once were nearly abolished. The public health field may have become a victim of its own success, Dr. Cohen added. Is there dwindling focus because the response to emerging infections has been so good that it is difficult to envision a pandemic? He thinks that might be the case. If so, this illusion may be slowly shattered, as it was on a comparatively small scale with Cryptococcus gattii. The pathogen proved more canny than previously assumed, finding a niche even in wood chips used for landscaping, far from where it was thought to be indigenous. And West Nile virus was first linked to dead birds on the grounds of city zoos, a situation once thought highly implausible. (Global Infectious Disease Surveillance and Detection. Washington, D.C.: National Academies Press; 2007.) Despite the scant evidence so far in Oregon tying the parasite Blastocystis hominis to gastrointestinal illness, the Public Health Agency of Canada put out an advisory that the protozoan might be linked to an outbreak among daycare center children. (CCDR 2001;27[9]:1.) In a time in which international travel makes introductions of bacteria, viruses, and other organisms possible in places they have not been known to occur and during a period of climate change in which environmental constraints cannot be counted on to contain them, what will happen next? That is open to speculation, but Dr. Cohen warned that complacency itself may be one of the most serious threats to public health. Comments about this article? Write to EMN at[email protected].

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