'Contagion' Connections: How Links Among Humans, Animals And The Environment May Be Spawning A New Class Of Infectious Diseases
The first in a series investigating the complex linkages between human, animal and environmental health: The Infection Loop.
Bronx Zoo pathologist Tracey McNamara started to see lifeless crows dotting her zoo's grounds in mid-August 1999. After Labor Day weekend that year, she recalls, "All hell broke loose."
Even as crows kept falling from the sky, Chilean flamingos, laughing gulls and a snowy owl, among other captive species, also suddenly began dying. "Many of these birds were healthy at breakfast and dead by dinnertime," says McNamara, now a professor of pathology at Western University of Health Sciences in Pomona, Calif.
Among the casualties was the zoo’s mascot, a bald eagle named Clementine. A necropsy showed the worst brain inflammation in a bird that McNamara had seen in her 18-year career, and she fretted that her surgical mask wasn’t enough protection against whatever had killed Clementine.
"I got a sinking feeling in my stomach, the hair stood up on the back of my neck and I went home and wrote my will," she says.
McNamara became convinced that the surge in bird deaths was tied to the growing reports of New Yorkers sickened or dying with similar signs of muscle weakness and confusion. The Centers for Disease Control and Prevention reported that mosquito-borne St. Louis encephalitis was the killer, but she was skeptical, since that virus doesn't kill birds.
In fact, New York was seeing the first cases of West Nile virus ever reported in the Western hemisphere. Ultimately, 62 people were hospitalized in New York for West Nile that year and seven died. The CDC later admitted it erred by dismissing the potential link between the human and animal illnesses.
Twelve years later, are we preventing, preparing or monitoring possibly epidemic new diseases in adequate fashion? According to specialists who track seemingly exotic public health threats, the answer is no. Our proximity to migrating animals, rodents and livestock, combined with environmental upheaval, has created conditions that make animal-borne epidemics more likely –- a theme the new film "Contagion" embraces with enough zeal to throw Gwyneth Paltrow into a fit of lethal convulsions.
Animals carry a number of viruses, usually without consequence to themselves, but those same viruses can prove deadly to another species. Humans have simply yet to cross paths with most of these pathogens.
"In the future, we're going to come across viruses that have been around for millions of years in obscure animals," says Peter Daszak, president of EcoHealth Alliance, a New York-based organization of scientists dedicated to conserving biodiversity.
While science can typically track down creatures that are hosts to threatening viruses, such human factors as population growth, income inequality, environmental degradation, climate change and even global travel may all play a much more decisive role in unleashing outbreaks of deadly and hard-to-control diseases.
"Microbes are out there and they are paying attention," says James Hughes, a professor of medicine and public health at Emory University, who spent about three decades with the CDC. "They are pretty good probes for weaknesses in the public health system."
Just look around, analysts warn. As deforestation and development shrinks the margins between civilization and the untrammeled regions globally, diseases will have more opportunities for transmission to humans.
Intensifying agricultural production can also facilitate epidemics, which is why the United States made Daszak's watchlist for countries that are likely to be home to emerging infectious diseases. Combined with the overuse of antibiotics, tightly penned livestock such as chickens and cows can also play a role in jumpstarting outbreaks (as happened recently with both salmonella and E. coli threats).
A multidisciplinary movement called "One Health" has emerged as means of raising public awareness around threats of contagion and for developing ways of combating the problem. At its core, the movement seeks more recognition of the connections between the health of the environment, animals and human beings.
The movement's ecologists, veterinarians and doctors focus on a range of public health risks that stretch across their respective disciplines, including food- and water-borne diseases as well as other infections that are zoonotic (meaning they originate in animals before jumping to humans).
One Health advocates have plenty of work to do. Three of four newly emerging infectious diseases are zoonotic, and while there are roughly 2,000 known animal viruses, there are an estimated 1 million out there.
As a whole, infectious diseases kill some 15 million humans each year.
Ian Lipkin, a professor of epidemiology and pathology at Columbia University nicknamed the "microbe hunter," has come to know many of these diseases through his microscope. Lipkin helped McNamara prove that the same virus was behind both the bird and human outbreaks that became apparent at the Bronx Zoo in 1999.
"I'm convinced that was a bellwether event," Lipkin says of the West Nile virus outbreak. "A federal agency was embarrassed and vowed it would never happen again. One Health finally had legs."
At the end of "Contagion," a film on which Lipkin consulted, a short sequence of clips acts as a prequel to a One Health nightmare scenario: A bulldozer clears a patch of trees for a new piggery, into which a displaced and diseased bat drops a chunk of banana, which is gobbled by a pig that later lands in the hands of a chef.
"This is a classic example of an emerging infectious disease," says Lipkin. "The chef doesn't wash his hands, infects Gwyneth Paltrow, and we go from there," with a global pandemic soon following.
'VIRUS MIXING VESSELS'
The fictitious MEV-1 virus of "Contagion" was modeled after the real-life Nipah virus, which first swept through Malaysia's pig farms in 1998 and '99. Bats infected pigs, farmers got sick and millions of swine were slaughtered.
In fact, most of the biological factors that triggered the movie's pandemic have already happened, scientists say. The only element that remains fiction is the "incredible transmission among people," according to Rick Ostfeld, a disease ecologist at the Cary Institute of Ecosystem Studies in New York, who added that this last step is "not beyond the realm of possibility."
Hughes, the Emory professor, agrees.
"We've been relatively lucky that bird flu does not easily transfer from person to person," he notes. "But with an opportunistic mutation or two, an avian virus could be more easily transmitted to humans and then between humans."
A select group of animals pose the greatest threats of passing on a disease to humans, including one that could become contagious. These creatures -- whether wild, domestic or livestock -- tend to be those close to us, both in terms of physical proximity and genetics.
"The closer a species is related to us, the greater the chance that a pathogen it carries can infect us," says the EcoHealth Alliance's Daszak. "You're not going to die from a lizard virus, but you could from a mammalian virus."
Common culprits include ubiquitous rodents, backyard birds and primates, the latter blamed for the introduction of HIV. Bats pose yet another threat.
"Bats are the stuff Hollywood movies are made of," says Jennifer McQuiston, an epidemiologist at the CDC in Atlanta. "The fact that they can fly and migrate across great distances might mean they are exposed to more things that they can then bring back to naïve populations."
By destroying bat habitats, humans effectively encourage the winged mammals to search for surrogate sources of food –- such as fruit orchards -– that are located closer to where large clusters of people live. Bats are an effective carrier for Nipah because they don't suffer ill effects from the disease.
"A bat with Nipah doesn’t look ill at all," says Daszak. "But the virus is 70 percent lethal to people."