The virus was swift and lethal, claiming 162 lives in just three months. It left behind corpses covered in skin lesions and showing signs of severe pneumonia.
Had the victims been human, the 2011 outbreak would have dominated the 24-hour news cycle. But, since the dead and dying were harbor seals washing up on the shores of New England, the story didn’t capture the nation’s attention, let alone the world’s.
That sort of thinking, public health officials say, is precisely why people are at risk from a range of potentially deadly diseases. We ignore wildlife diseases at our own peril, because a new or reemerging virus capable of killing animals also could be the harbinger of a human pandemic, such as Ebola, HIV/AIDS, SARS and MERS.
All of those were zoonoses—diseases that initially spread from animals to people. In fact, pathogens infecting wildlife are twice as likely to jump over to humans as those without wildlife hosts.
And yet, new viruses from animals continue to catch us off guard. When humans are diagnosed with certain diseases, it’s mandatory to report them to the U.S. Centers for Disease Control and Prevention (CDC). Not so with wildlife diseases, where reporting continues to be done on an ad hoc basis, even as outbreaks become more frequent in response to environmental change.
“Since the 1990s, the number of new emerging diseases that we’ve seen is ever-increasing,” says Jonathan Sleeman, an epidemiologist and the director of the U.S. government’s National Wildlife Health Center. These diseases can spread fast and far, he says. “And the consequences are more profound, causing marked declines in wildlife populations and, in some cases, extinction.”
Scientists are developing methods for treating animals and preventing viruses from spreading to the point where they become a public health threat. But, Sleeman says, “once a disease gets into a wildlife population, it can be very difficult to manage or control it.”
That’s why, he says, we need a more robust surveillance system to stop outbreaks before they occur. For starters, he’d like to see a network of labs that specialize in diagnosing wildlife diseases, all following the same standard operating procedures.
And, he says, there needs to be better coordination among state and government agencies: “The CDC studies human diseases, the USDA studies domestic animal diseases and we [the National Wildlife Health Center] study wildlife diseases. But I think, in this day and age, we need to be looking at how to combine our expertise and resources and come up with effective interventions that protect the agricultural economy and public health.”
Viral cocktail shaker
Many of these outbreaks have one mammalian species in common: Homo sapiens. Climate change, for example, is believed to be the culprit for the emergence of Bluetongue virus across Europe. Warming weather allowed disease-carrying insects to spread as far north as the Netherlands, infecting farm animals and wildlife such as red deer and wild mountain sheep.
Other zoonotic diseases, such as the Schmallenberg virus, have been spread around the world by insects that have hitched a ride on the international trade in produce and cut flowers. And environmental contaminants have suppressed the immune systems of some animals.
But the primary driver for zoonoses is habitat loss, as forests are cut down to make room for suburban sprawl and plantations, while canals and dams divert water from wetlands.
“You increase the likelihood of diseases as well as the severity of the spread when animals are concentrated in smaller areas,” Sleeman says.
And, ongoing human encroachment upon natural land makes it more likely that pathogens will spill over from wildlife into humans and domestic animals. In Malaysia, for example, slash-and-burn deforestation forced fruit bats to scavenge for food in orchards, which were located near pig farms that provided manure as cheap fertilizer. It didn’t take long for a disease, Nipah virus, carried by bats to spread to the pigs and then to their human handlers.
This transmission from species to species—and back again—acts as a viral cocktail shaker, raising the odds that a disease that originates in wildlife will mutate into something more deadly and infectious to humans and animals alike.
As a result, the “2.0 versions” of a disease can reinfect the same wildlife that introduced the initial strain.
In the 1990s, what came to be called avian flu began as a benign virus carried by wild birds. It then spread to poultry farms, where the crowded conditions served as an incubator for the highly pathogenic H5N1 virus, which in 1997 began infecting people who handled contaminated bird carcasses. H5N1 then jumped back into the wild, where migratory birds became both victims and carriers of the disease in Asia.
Wild fowl have also been identified as the source of H5 avian flu strains that are devastating the U.S. poultry industry. Since last December, 47 million farm birds have been culled in 21 states. These new viruses are a mixture of strains from both Asia and North America.
The CDC says that the avian viruses burning their way through American poultry farms pose no threat to people. The same cannot be said of a strain of the avian H3N8 virus, which killed the New England harbor seals in 2011. Recent studies confirm that this strain mutated within the birds themselves, acquiring the ability to directly jump species by binding to receptors found in mammalian respiratory tracts. Researchers found no evidence that humans are immune to the strain. In fact, H3N8 was the likely cause of a pandemic in the 1880s.
Begin with bats
In the ongoing battle against wildlife zoonoses, one animal in particular merits further study.
Bats have been identified as a vector for spreading diseases worldwide—most recently, Ebola in Africa and MERS in the Middle East. They are a reservoir species, otherwise known as an asymptomatic host, which means they can become infected without getting sick. Bats carry 60 different viruses and counting. Yet, little is known about their immune systems—a task made more challenging by virtue of the fact that there are more than 1,200 different bat species.
Daniel Streicker, a researcher at the University of Glasgow who specializes in the epidemiology of bats, cautions that we’re not really certain if all bats carry viruses without getting sick.
“We'll catch a bat in the field, and it will seem fairly healthy, but it's rare that you actually do the follow-up work, like tracking an individual bat that is infected with something and seeing what its fate is.”
On the other hand, if some bats really are asymptomatic, “that could provide clues to how they manage to control these infections,” says Streicker. That, in turn, could lead to the development of new treatments for humans.
One of Streicker’s current projects is in the emerging field of developing wildlife vaccines—specifically, for Peruvian vampire bats that are spreading rabies. Researchers have developed an oral rabies vaccine that is placed in a gel and spread on a bat’s back. The bat is then put in a cage with others. Since vampire bats are social creatures, they groom one another and ingest the vaccine.
Under lab conditions, 70 to 100 percent of the caged bats were successfully vaccinated against rabies using this method. Streicker is leading a team that is consulting with the Peruvian government about the possibility of testing the technique on bat communities living in the wild.
Sleeman is likewise excited about the potential for developing wildlife vaccines, noting that a similar project is underway in the United States: a sylvatic plague vaccine for prairie dogs delivered orally by mixing it with peanut butter.
For the time being, however, Sleeman believes that, as we continue to encroach upon natural land, we need to better understand the habits of our animal neighbors. He points to a recent study that found that 83 percent of miners and tourists who contracted Marburg virus from bats in Ugandan caves did so during breeding seasons, when there are large numbers of juvenile bats who are more susceptible to contracting and spreading the virus.
“Knowing that, we could possibly prohibit people from using the caves during these high-risk times of the year,” says Sleeman. “This is the basic sort of intervention we need to be devising to prevent spillover, allowing humans and wildlife to coexist.”