December 01, 2012

 

 The CDC for wildlife

​National center at forefront of wildlife disease research

 
Posted on November 19, 2012
 

 

Veterinary pathologist Dr. Dan Schenkman necropsies a Canada goose with assistance from Stephanie Steinfeldt in one of the National Wildlife Health Center’s biosafety level 3 laboratories.
Photo by R. Scott Nolen
 
The buzzer in the biosafety level 3 laboratory announces the latest arrival of animal carcasses at the U.S. Geological Survey’s National Wildlife Health Center. Dr. Dan Schenkman, one of the center’s veterinary pathologists, necropsies a Canada goose suspected of succumbing to botulism. Elsewhere in the laboratory, a technician swabs the mouths of three mallards to test for avian influenza virus before bagging, tagging, and arraying the birds along a stainless steel table.

For more than three decades, wildlife personnel from around the country have relied on the center’s scientists to investigate wild animal deaths likely attributable to infectious diseases or environmental toxins. This USGS scientific program is at the forefront of wildlife health research and the de facto Centers for Disease Control and Prevention for free-ranging North American fauna.

Along with conducting animal disease surveillance and assessing disease impacts on wildlife populations, the National Wildlife Health Center provides domestic and international conservation agencies with training and guidance for reducing animal losses during an outbreak. “We have a very unique mission and that is to safeguard wildlife and ecosystem health,” said Dr. Jonathan Sleeman, director of the center since 2009.

Established in 1975 partly in re­sponse to an earlier outbreak of duck plague that killed more than 40,000 mallards in South Dakota, the NWHC is the first federal program dedicated to wildlife health research on a national level. The center’s main campus is tucked away off a small, tree-lined road in Madison, Wis., where some 1,200 animal carcasses and tissue samples are examined each year.

Over the years, NWHC researchers have shed light on scores of infectious animal pathogens, often with little fanfare. Most recently, the center characterized Geomyces destructans, the novel fungus behind the epizootic of white-nose syndrome killing millions of hibernating bats in eastern North America, in 2009.

Vector-borne disease researcher Dr. Erik Hofmeister says several questions about West Nile virus remain unanswered, such as factors contributing to the high number of human infections over the summer that took public health officials by surprise.
Photo by R. Scott Nolen
 
Other noteworthy achievements include the 1991 federal ban on the use of lead shot for waterfowl hunting—a result of an NWHC investigation of waterfowl deaths. More than 100,000 bird samples were tested there for the avian influenza virus H5N1 strain from 2006-2010. Pioneering research at the center’s field station in Honolulu has yielded greater insights into coral reef health and disease.

Additionally, field tests are currently under way for a new vaccine created by NWHC scientists to protect prairie dogs, the main prey for black-footed ferrets, against sylvatic plague. The recovery of the black-footed ferret, considered the most endangered mammal species in North America, is hampered by plague outbreaks that decimate prairie dog populations.

Prior to his appointment as center director, Dr. Sleeman was the Virginia state wildlife veterinarian and had previously spent two years as head of the Mountain Gorilla Veterinary Project in Africa. He says that during the early days of the center, scientists worked mostly with pathogens affecting only wildlife populations. That is no longer the case. Beginning with the West Nile epidemic in 1999, U.S. public health officials were confronted by three exotic zoonotic diseases within five years, the others being monkeypox and severe acute respiratory syndrome. And by the mid-2000s, the United States was among a host of countries preparing for a possible H5N1 avian flu pandemic.

Whereas 70 percent of new and emerging infectious diseases are of animal origin, Dr. Sleeman points out that most are found in wildlife. He believes a positive outcome of recent wildlife disease events is growing support for the idea that wildlife, ecosystem, and public well-being are interrelated. “There’s increasing recognition of the importance of healthy wildlife and healthy ecosystems to healthy humans. It’s part of the one-health concept,” Dr. Sleeman said.

Chronic wasting disease, Bryan Richards explains, is difficult to manage, because free-ranging cervids can be infected and capable of spreading the disease for nearly a year before having clinical signs of the neurodegenerative illness.
Photo by R. Scott Nolen
 
While the Department of Agriculture and Centers for Disease Control and Prevention have protected and advanced the health of the country’s domestic animals and public for decades, few saw a need to do the same for wildlife. But in the aftermath of the 1973 outbreak of exotic duck viral enteritis in South Dakota’s Lake Andes National Wildlife Refuge that killed more than 40 percent of the refuge’s mallards, it was clear that attitude had to change.

The U.S. Fish and Wildlife Service called on Milt Friend, PhD, at the time the chief of pesticide and wildlife ecology investigations at the FWS Denver Wildlife Research Center. A year before the duck plague epizootic, Dr. Friend had written a proposal for a national wildlife health program. In January 1975, he and two colleagues launched the National Fish and Wildlife Health Laboratory on the University of Wisconsin-Madison campus. A short time later, Dr. Friend was named center director, a position he held for 23 years. The name of the program has since changed, and oversight of the facility was transferred to the U.S. Geological Survey in 1996.

Currently, Dr. Friend volunteers at the National Wildlife Health Center and works as an adjunct professor at the University of Wisconsin, where he teaches animal health and biomedical sciences. Given the heightened concerns about bioterrorism and emerging infectious diseases, Dr. Friend believes the center is more relevant now than ever. “Initially, we were a migratory bird laboratory. Now, we’re a broad-reaching wildlife disease laboratory and research center,” he said.

“There is no real economic incentive to proactively deal with wildlife disease. It gets dealt with during a crisis because of public pressure, but ultimately, there is no highly integrated infrastructure for dealing with disease, period.”
Milt Friend, PhD, first director of the National Wildlife Health Center
 

Dr. Friend says historically, agencies have lacked the resources and manpower to effectively monitor and manage diseases in free-ranging animals on their own. What distinguishes the National Wildlife Health Center within the wildlife conservation community is its large size, number of specialists, geographic scope of activities, and network of national and international collaborators, he explained. The center has an annual budget ranging from $10 million to $12 million and a 90-member staff with 10 veterinarians, making it the leading employer of wildlife veterinarians.

“This is the most comprehensive facility that currently exists with the specific purpose of addressing wildlife health issues for the benefit of free-ranging wildlife populations and society in general,” Dr. Friend said.

“Throughout the history of the center, it has been at the forefront of identifying previously unreported diseases of North American wildlife. We’ve seen things nobody knew were out there. That early detection has fostered an array of actions that have helped suppress those diseases from spreading unabated.

LeAnn White, PhD, part of the National Wildlife Health Center field investigations team, measures the tarsus of a double-crested cormorant in Minnesota this past July.
Courtesy of U.S. Geological Survey
 
“When you talk about wildlife in this country, it’s a tragedy of the commons,” Dr. Friend said, employing an economics term describing the depletion of a shared resource by individuals each acting in their own self-interest, despite the knowledge that doing so is contrary to their own best interests in the long run.

“There is no real economic incentive to proactively deal with wildlife disease. It gets dealt with during a crisis because of public pressure, but ultimately, there is no highly integrated infrastructure for dealing with disease, period.”

The recent spike in West Nile viral infections underscores Dr. Friend’s point. Government agencies instituted surveillance programs when the exotic arbovirus was first identified in New York City and began spreading across the country. But within a few years, as the number of human cases declined, vigilance against the disease waned. Then this summer, the CDC announced an unexpected surge of West Nile virus–related illnesses and deaths. By early October, more than 4,500 cases had been reported to the agency, including 183 fatalities.

Dr. Erik Hofmeister has studied West Nile virus for the past eight years as a veterinary medical officer with the National Wildlife Health Center. Although the virus has been active in the United States for more than a decade, he says much remains unknown about its pathology and epidemiology. Scientists are uncertain why, for example, West Nile disease is so deadly in corvids and raptors but not in chickens and other gallinaceous bird species. 

On account of the virus’s lethality in avian species, West Nile virus surveillance is a core function of the National Wildlife Health Center’s field station in Hawaii. To date, the virus has not reached the island state, home to several rare bird species. Wildlife ecologists predict that such an event would have a catastro­phic effect on Hawaii’s wild avian population.

The National Wildlife Health Center is currently field testing an oral vaccine to protect prairie dogs, the main prey of the endangered blackfooted ferret (above), from sylvatic plague.
Courtesy of U.S. Geological Survey
 
Of the many vector-borne diseases Dr. Hofmeister has researched, West Nile is among the most fascinating. A new strain of the virus emerged in New York City between 2002 and 2003. This novel viral strain, he explained, was similar to the original in most ways, but one difference was significant. “What changed is the virus could more rapidly develop in mosquitoes. It became better adapted to our mosquitoes on this continent,” Dr. Hofmeister said. Soon enough, the new strain had entirely replaced the original West Nile virus strain flaring up nationwide.

Dr. Hofmeister believes no one could have foreseen the high number of West Nile cases this past summer, the result of a confluence of warm weather and changes in the ecosystem, among other possible factors. “Part of it may involve climate; part of it certainly might be the population of birds that became immune to West Nile has died off and new birds are hatched that are totally susceptible to the virus,” he reasoned. Studies show birds do develop West Nile virus antibodies that they pass along to offspring. These antibodies are short-lived, however, lasting as little as seven days in wild birds, providing no long-term immunity.

Despite this year’s reminder that West Nile virus remains a threat, Dr. Hofmeister isn’t optimistic about the chances of renewed vigilance against the disease agent. “I don’t think we’ll ever get back to where we were in the mid-2000s when the majority of states were doing either mosquito or bird surveillance,” he stated. “I certainly hope this past summer does rekindle surveillance efforts that might allow for greater awareness of West Nile virus activity.”

“We believe maintaining healthy wildlife populations is necessary for ecosystem integrity.”
Dr. Jonathan Sleeman, director, National Wildlife Health Center
 

National Wildlife Health Center activities aren’t limited to zoonotic diseases. “We believe maintaining healthy wildlife populations is necessary for ecosystem integrity,” Dr. Sleeman explained. For example, researchers there have spent years studying chronic wasting disease, a fatal transmissible spongiform encephalopathy infecting North American deer, elk, and moose. First identified at a Colorado research facility in 1967, the disease has since been documented in free-ranging and captive cervid populations in 15 states and two Canadian provinces.

Bryan Richards is the wildlife center’s chronic wasting disease project manager and has also spent the past year as acting chief of the disease investigation branch. Richards joined the center about two years after the disease was de-tected in free-ranging Wisconsin deer in 2002. Consensus within the scientific community is that an infectious prion causes chronic wasting disease, which is transmitted primarily through cervid feces and saliva.

CWD is especially challenging to manage as a disease, according to Richards. Its mean incubation period is 18 to 24 months, and infected deer can shed infectious prions for nearly a year before the onset of clinical illness. “You’ve got an animal out there that’s really more or less a Typhoid Mary, spreading disease without showing signs of illness,” he said. Complicating matters further is the ability of the prion that causes CWD to persist for years in the environment.

Richards cited the ongoing research of Christopher Johnson, PhD, a research biologist with the center who’s discovered that when these proteins bind to certain soil particles, their infectivity increases by up to 700 percent.
Most states conduct some level of chronic wasting disease surveillance, but wildlife experts say the disease is nearly impossible to control once it emerges, necessitating such preventive measures as banning imports of live cervids.

The chances of a person or domestic animal contracting CWD are “extremely remote,” Richards said. The possibility can’t be ruled out, however. “One could look at it like a game of chance,” he explained. “The odds (of infection) increase over time because of repeated exposure. That’s one of the downsides of having CWD in free-ranging herds: We’ve got this infectious agent out there that we can never say never to in terms of (infecting) people and domestic livestock.”

It’s unusual that a skin fungus can kill an animal, says Dr. Anne Ballmann, who is studying the pathology of Geomyces destructans, the novel fungus behind the white-nose syndrome epizootic killing millions of North American bats.
Photo by R. Scott Nolen
 
Other wildlife species might also be at risk of CWD. The carcasses of infected deer, elk, and moose are eaten by any number of animals, including rodents, which Richards says have been experimentally infected with the disease.

Rodents serve as the basis for food webs, and almost every animal’s eaten something that’s eaten a vole,” he said.
Chronic wasting disease also exacts a sociological cost. Richards noted studies suggest hunters and their families will begin considering changing their behaviors once the prevalence of CWD in a region gets to 30 to 40 percent. “Even though science suggests the likelihood of disease transmission to humans is low, if you’re hunting in an area that has high prevalence, maybe you’re going to go hunting somewhere else, so there’s definitely a stigma associated with high CWD prevalence,” he said.

Dr. Sleeman, the National Wildlife Health Center director, finds few emerging animal diseases as worrisome as white-nose syndrome. Since the fungus’s discovery in hibernating bats in caves in eastern New York in 2006, it has spread to 19 states and four Canadian provinces, killing an estimated 5 million bats. “It’s events like that that really concern me,” Dr. Sleeman said. “We get these devastating diseases that basically wipe out populations, that can either lead to extinction or a situation where they’re unlikely to recover to former numbers.”

Dr. Anne Ballmann joined the center in 2008 and within a few months was part of a field investigation team tackling white-nose syndrome. Not only was the outbreak an unprecedented disease event in bats, but also, the pathogen itself is unusual.

“It’s very rare that a skin fungus can actually cause mortality in any animal. It’d be like athlete’s foot killing you, which is just bizarre,” Dr. Ballmann said. She helped write the U.S. Fish and Wildlife Service national strategy to manage the disease and currently leads a government working group writing a plan coordinating diagnostic laboratories and testing standards for white-nose syndrome.

“We have good earth-monitoring systems, good climate-monitoring systems, good public health disease–monitoring systems, but we don’t have that same proactive systematic collection of data for wildlife health,” Dr. Jonathan Sleeman said.
Photo by R. Scott Nolen
 
Several hypotheses concerning how white-nose syndrome causes bat deaths are being tested at the moment, ac-cording to Dr. Ballmann. The fungus appears to capitalize on qualities unique to bat species that hibernate in caves, which account for about half of the 47 North American bat species. The membrane of a bat’s wings regulate the animal’s body temperature and play a part in water balance. A healthy bat wakes from its torpor approximately every 14 days to groom and drink. It is believed that white-nose fungus disrupts this process, causing infected bats to exhaust their limited fat reserves by waking more frequently. Another school of thought is that the fungus kills the bat by depleting its electrolytes in a way similar to the deadly chytrid fungus in amphibians. Still another theory is that white-nose fungus damages the bat’s wings, hindering its ability to hunt and causing the animal to starve.

“There’s a lot to learn,” Dr. Ballmann acknowledged. “Probably, multiple factors are at play in causing bat mortality.” In addition to identifying G destructans as the causative agent of white-nose syndrome, research at the National Wildlife Health Center has demonstrated that the fungus can persist in the environment. “If a site is infected, even if bats aren’t present, the fungus is still there and can be picked up by any naive bat, although we don’t yet know how long the fungus remains,” she said.

As Dr. Sleeman sees it, the emergence of white-nose fungus and the spike in West Nile disease caught the nation unawares and are indicative of a fundamental problem in the way the nation deals with wildlife diseases. “We have good earth-monitoring systems, good climate-monitoring systems, good public health disease–monitoring systems, but we don’t have that same proactive systematic collection of data for wildlife health. Until we do that, it’s going to be very hard to get a handle on what’s driving these diseases,” he observed.

As a remedy to this problem, the center presented its government partners earlier this year with a draft proposal for a collaborative approach addressing North American wildlife health issues. The strategy includes creating a comprehensive framework comprising a broad range of agencies and experts to mitigate the impacts of wildlife diseases and other stressors on wildlife, domestic animal, and human health.

Dr. Sleeman would ultimately like to evolve beyond monitoring and managing programs and into predictive modeling, for him the holy grail in wildlife disease work. With enough data to understand the environmental drivers behind these diseases, it may be possible to anticipate these events and even prevent them from occurring. “With the state of knowledge we have now, that’s not possible,” he said. “Until we have a long-term, comprehensive monitoring system that can then be combined with other data sets, only then will we develop those models to start to predict emerging disease.”