West Nile Virus

In just four years, West Nile virus has become a staple of summer news in the U.S. Human deaths make headlines — but what happens to the survivors? Why your local zoo might hold some answers?

In the summer of 1999, a mysterious outbreak of human illness in New York City was blamed on St. Louis Encephalitis. At the same time, crows in the city were dying.

And at the Bronx Zoo, a Bald Eagle, flamingos, and cormorants died too. Tracey McNamara — chief pathologist at the zoo — was the first scientist to make a connection between the human and bird deaths. She discovered the true culprit — West Nile virus. She’s now studying animals that become infected but survive.

Tracey McNamara: Are they going to go on and die from heart disease at a much younger age than they would have had they not been exposed to West Nile virus? Are they going to become diabetic? I suspect that there may be what we call vertical transmission, that infected birds may be able to pass the infection to their chicks in the egg. Are we going to lose a whole generation of birds or not?

When animal survivors of West Nile virus later die of other causes, autopsies might reveal the kinds of complications that human survivors can expect. McNamara helped create a network of zoos that work with the Centers for Disease Control to test sick or dead zoo animals — possibly to give human health workers an early warning of an outbreak.


  • After West Nile Virus”, by Susan Milius, Science News, March 29, 2003, Vol. 163, p. 203

The most important thing that I couldn’t fit into this script was an acknowledgement of how dangerous West Nile virus can be to endangered species. For example, only two, immunologically na•ve Hawaiian crows live in the wild. In the continental United States, it appears common for a half of the local crow population to die during infection. It would be disastrous if the disease spread to the Hawaiian islands.

The network of zoos mentioned in today’s show was organized by Tracey McNamara, Dr. Dominic Travis of the Lincoln Park Zoo in Chicago, and Dr. Amy Glaser of Cornell University.

In the summer of 1999, an outbreak of human illness in New York City was blamed on St. Louis Encephalitis. People developed flu-like symptoms and in some cases, fatal brain inflammations. At the same time, crows all over the city were dying. Yet birds that get St. Louis Encephalitis aren’t supposed to show any symptoms. Meanwhile at the Bronx Zoo, a Bald Eagle, flamingoes, and cormorants died too. These animal deaths brought in Tracey McNamara, chief pathologist at the zoo, and the first scientist to make a connection between the human and bird deaths. Thanks to her nudge in the right direction, scientists eventually ruled out St. Louis Encephalitis in favor of a disease new to our continent — West Nile virus. This was the first known report of the disease in humans or animals in the Western Hemisphere.

There have now been over 200 species in the Western Hemsiphere that have become infected with West Nile virus.

Transcript of an interview with Dr. McNamara:

– Before the summer of ’99, I don’t think people really recognized what excellent biosurveillance was routinely practiced in the major metropolitan zoos and, I guess if you don’t work in a zoo it would never have occurred to you. But from our point of view, we have to be eternally vigilant against the threat of introduced disease to our captive collections. In a zoo setting, individual animals are very important. I mean we may be talking about endangered species or at least threatened species and keeping them healthy is our number one concern. But, in a zoo setting where we are always located in these beautiful parks, those lovely environments also bring with them local wildlife. And all of which can serve as a source of infection for our captive animals and that’s why in the summer of 1999 when crows started dying on the grounds of the zoo, we launched a very aggressive investigation as to what the causative agent might be, out of concern that it might be something that could spread to our bird collection…

– I think I’ve been credited with being the person who made the link between the human deaths and the bird deaths. And the bottom line was that, although an announcement had gone out from the health department and the CDC that people had been dying of St. Louis Encephalitis in New York City, and that announcement in and of itself was unusual because we had not had a mosquito borne disease outbreak in the human population since the 1800s and it was also unusual in that nothing was going on further south. And St Louis Encephalitis is a disease that we primarily see in the southern states.

– According to the text books birds are asymptomatic reservoirs, or carriers of St. Louis Encephalitis and it doesn’t kill them, but the fact of the matter was that I had an unprecedented die-off of crows. I mean, large numbers of crows that had attracted public attention, this was not something you could ignore, and when I evaluated the crows they had evidence of encephalitis, and inflammation of other organs…

– And when zoo birds started dying with lesions that were the same, as I had already seen in the crows, and, you know, the initial cases I saw were from the same location as where the human cases were occurring, I thought that the temporal and spatial relationship between the bird die off and the human cases of encephalitis was too striking to ignore. We later pursued a viral ideology for the bird deaths…

– Later on we found out that the human cases were in fact not St. Louis Encephalitis and that both the people and the birds had been dying of West Nile Virus, a virus that had never before been seen in the Western Hemisphere.

– It alarmed me because, the lesions I saw in the crows were what we would call hot. You know, pretty severe disease, and then when our flamingoes and cormorants, you know, and our bald eagle, our snowy owl, a whole bunch of birds across the whole spectrum started dying with lesions that were similar to what I had already seen, I was very concerned that what had been killing the crows had indeed spilled over to our birds.

– Unlike other people that were working just with people or just with crows, I had the advantage of working in zoo setting, where because we had a children’s zoo, we had domestic poultry, through deductive reasoning, not poultry disease,

– So basically having, you know, 17 years of experience in comparative pathology, I knew that what I was seeing at the time of necropsy or post mortem on these birds, and what I was seeing in the microscope, based – drawing on everything I knew as a comparative veterinary pathologist, I knew enough to know this was something I had never seen before, that it was something new in my experience, and I had a hunch, I was pretty sure, that it was the same thing that was killing the people.

– After the West Nile outbreak was confirmed, if you’ll recall back then the public health departments and the CDC were focusing efforts strictly on crows, because those seemed to be the indicators that there was virus circulating in the urban setting but based on what we had seen in our zoo, we were very concerned that the virus effected not only birds, but also a number of mammals. And we had a clinically ill,we had a symptomatic and neurologic black rhinos, we had sick elephants, we had a sick snow leopard. And subsequent to that now here we are a number of years later, and we’ve done a great deal of work on zoo species and we know that, in fact West Nile is pretty good at making a lot of the you know megaverterbates, the real special animals, pretty sick, and it’s a much more serious problem then anyone at first realized.

– My real concern three years into this investigation is that, although a lot of people seem to have the impression that if you are exposed to West Nile and you don’t show any signs or if you survive, that you’re perfectly fine. That has not been my experience in looking at the long-term effects of this virus on captive zoo animals…

– What I’m seeing is – even in animals that survived – there is evidence of severe heart disease, from the inflammation that was there and they survived but it’s now replaced by non functional connective tissue, and I don’t know how much effect that will have on the longevity of some of these species, and even if they survive the viral infection are they going to go on and die from heart disease, at a much younger age than they would have had they not been exposed to West Nile virus. I’ve seen involvement in the intestines, severe involvement in the kidneys, severe involvement in the pancreas, and I don’t know, are the birds going to have difficulty digesting normal food? are they going to become diabetic, if it effected the islet tissue? we don’t have the answers to any of those questions at this point.

– The one that is perhaps the greatest concern from a captive, endangered species point of view is the fact that, based on the fact that I’m seeing so much virus in the ovarian tissues and testicular tissues of infected birds, I suspect that there may be what we call vertical transmission, that infected birds may be able to, pass the infection to their chicks in the egg, and if that’s the case I think it would be very difficult to predict what the long term effects of this virus may beyond both captive and free ranging populations, are we going to lose a whole generation of birds or not? Those kinds of studies really have not yet been done and they definitely need to be.

– The other thing that’s a great concern is uh based on the long term follow up we’ve been able to do on zoo animals, because unlike wild birds we can follow zoo animals over their entire lifetime, and when they die for whatever reason, we do a complete autopsy/necropsy on them, and I’ve been able to look at the brains of many birds and mammals that recovered from West Nile virus or were shown to have been exposed and developed, you know, by blood test we can prove that they actually were exposed to the virus, but may never even have shown any signs of being sick at all, the frightening thing is – both the neurologic or the completely asymptomatic birds and mammals are showing evidence of encephalitis, months to as much as a year after infection, raising questions as to whether or not this virus causes chronic persistent nerve degenerative disease, I think we are just looking at the tip of the iceberg in terms of what this virus may do, its not just when you’re sick it’s what’s going on after you’ve recovered, and we’re going to need many more long term studies to be able to evaluate that in the years to come.

– I suggested to the CDC that we look at captive pop of at risk, two years, excluded from network…

– The CDC agreed to help us establish a nationwide netork for the testing of zoo species for active West Nile surveillance, this made sense from a public heath point of view in that at least if a bird tested positive for West Nile virus, you knew where it was infected, this wasn’t a wild bird flying in from five miles away, there was no doubt as to where the exposure would have occured.

– And it was also helpful to them in that, the birds, unlike wild birds that often come in with no history, birds in zoos are watched closely on a daily basis, so they have terrific clinical histories, you know any kind of work up that needs to be done, and from a zoo point of view, zoos were desperate to get a diagnosis on their sick birds, and at that point, no public health labs were accepting flamingo samples ..

– … in offering free testing to zoos, quid pro qou was that zoos would then have to report that result to their local health departments, that was what the CDC wanted done …

– So it’s worked out very well – we were able to create a partnership with zoos, with public health on an emerging zoonotic disease threat and what’s unique about the zoo project is unlike human or domestic or animal, like food animal data, it’s pretty easy to share data across state borders when you’re dealing with zoo animals, we don’t have all the economic and trade issues that are attached to cows, and, we’re really, as far as I know, the only existing disease surveillance network wherein a positive diagnosis can be seen and shared with public health and real-time fashion and not only can we report it quickly, but I can sit down and look at the Cornell database and see what’s going on across the United States every 24 hours, that’s, given our current concerns about biologic agents, and other emerging disease threats, that’s a pretty powerful statement.

– The extraordinary thing about being a pathologist in a zoo setting is that it’s a frontier, and unlike other areas of medicine, where I think a great deal of things have been pretty well described and you don’t see a lot of new things, those of us who work in the zoo field are so few in number and the species we’re working on are so rare and unusual that that you can’t help but see new things… it’s the new and exciting age of discovery in zoos, that’s something we experience on a daily basis, its an extraordinary field and quite gratifying because as a pathologist I know that, unless you know what it is, unless you know how it makes animals sick, you can’t begin to prevent or treat a disease, comparative pathology in this case really was the foundation of really understanding West Nile Virus

– it could be, what if its something new, just part of the routine when you’re, it’s new and exciting , age of discover, quite gratify, as a pathologist, have to understand it before you can begin to prevent or treat a disease

– When this virus hit the United States, or hit our hemisphere, essentially it was such a novel virus that hit a naive population that it just swept through the ecosystem and it caused mass mortalities, We’re very concerned about a similar event happening when this virus reaches the tropical rainforests to the south of the United States…the predictions are not heartening…not major losses further south, not in a position to stop them, this could end up being a cataclysm event for the entire ecosystem

– The bottom line is that people may be the most important thing to public health, but there needs to be recognition that you can not possibly understand a disease transmission cycle unless you put as much weight in the animal data and the animal studies, as you do the human work,

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