by Janet A. Ginsburg

There it was, by buried in the Metro section of the morning paper: one short paragraph announcing the first West Nile crow fatality in the Chicago area for 2006. It’s the season and reports are trickling in: Positive mosquito pools in Missouri. Dead birds in Ontario. A sick horse in Idaho. A 27-year-old man hospitalized in central California (no need to worry, though, at least according to the news report…).

A few years ago, West Nile was headline. It was news that hit home -- my home -- almost literally, when an exquisitely handsome crow fell from of the sky one day, landing face-down in some nearby grass. I happened to be working on a West Nile story for National Geographic television and scrambling for crow footage, so it was strangely fortuitous. My afternoon spent on “Golf Course Safari,” tooling around the suburban wilds in a golf-cart-cum-Land-Rover, had been a bust. The dozens of crows that in years past had taken up seasonal residence to dine on the grubby bounty of manicured greens had vanished. There wasn’t a caw to be heard.

I took some pictures and called the local health department to turn in the carcass for testing. No one was interested. They already knew West Nile was in the area and spraying plans were in the works. What was the point?

Well….as Tracey McNamara, the Bronx Zoo veterinary pathologist who played such a pivotal role identifying West Nile in 1999* noted when I asked her about a surveillance program involving tens of thousands of dead birds, only half the birds tested positive for  West Nile. But they all died. Of something. At the time, McNamara suggested that it might have been useful to test for other pathogens, such as avian influenza. Strains of bird flu (though not the infamous H5N1 strain) had been circulating in the U.S. for several years. A few outbreaks were even serious enough to call for massive culls on poultry farms.

Vittorio Guberti, director of Italy’s National Wildlife Institute, had also spoken to me about the limits of limited surveillance. We met last Spring in Bologna, just as millions of birds were migrating up the Italian boot -- including ducks from parts of Africa where H5N1 had been found.  12,000 dead birds had been tested but all were negative for the virus. Yet they died of something. And once again, the evidence was in the dumpster…

How much easier it would be to predict -- and possibly even prevent -- killers like H5N1 if we only had a better understanding of how regular business-as-usual flu strains travel and spread between wildlife, farm animals and people. That’s a story for another day…

Along the Mighty Mississippi

Science in the field often has a sweetly low-tech feel to it: a card-table in the woods with some camping chairs and a cooler to store samples. For that same National Geographic story, I headed with my cameraman to a forest near the Mississippi to rendezvous with a team from the USGS National Wildlife Health Center. Led by Bob McLean (now with the USDA Wildlife Research Center), they were chasing the virus by chasing birds migrating north up one of the continent’s busiest flyways.

There are four major North American flyways: The Atlantic, the Mississippi, the Central and the Pacific. Look at a map and you’ll see lots of smaller parallel flyways fanning out alongside the big four across the continent. In the fall when birds reverse course, the flyways funnel back together as the land narrows into Mexico, the Caribbean, and Panama.

The West Nile virus spread from sea to shining sea in a mere three years because birds from the Atlantic flyway, where the virus made its initial beachhead in New York City, mingled with Midwestern birds during migration. It wasn’t just a matter of birds infecting birds, but birds infecting mosquitoes en route. Mosquitoes, in turn, infected more birds and what started out as a local problem quickly exploded into an entrenched continental disaster. With each migration, more birds and bugs were infected until by 2002 every flyway was covered.

The virus has now been found in over-wintering mosquitoes, too. These little “syringes with wings” can go into a state called diapause, a sort of bug hybernation, and survive cold weather. When they warm up in the Spring, the virus within resumes replicating and West Nile is back in business. It is also possible, although very rare, for mosquitoes to transmit the virus transovarially to their young, giving rise to a generation of natural born killers.

Bob’s team strung what looked like a black badminton net on some trees to catch songbirds resting between flights northward. Every 20 to 30 minutes, we would trek into the forest wearing water-proof boots (near the mighty Mississippi in the Spring, the ground is as squishy as the air is fragrant...). We found two or three birds each time, trapped like, well, birdies in a badminton net. Each little bundle of feather and song was extracted with incredible delicacy and promptly deposited in a laundry wash-bag -- full of air-holes -- for transport back to the card-table lab. There, the birds were measured, aged, weighed and banded, blood drawn and feathers plucked before being released again into the wild. (Feathers can reveal all sorts of surprising “CSI”-type information, including where a bird has been feeding.)

No matter how many birds Bob’s team sampled over the six weeks they were on the on the road, it would only be the tiniest fraction of the tens of millions migrating to their summer homes. The best they could hope for was to bring a very fuzzy picture into slightly less fuzzy focus. To give you an idea of just how daunting wildlife surveillance can be, Bob estimated that for every bird found dead of West Nile, there could be as many as 100 that would never be found, lost to decay and whatever ate them for dinner.

Bob’s team could, however, begin to get a handle on whether birds were infected as they migrated, or had antibodies from an earlier infection. This is important because it is the birds that survive to raise the next brood that help keep the virus entrenched. Birds are “amplifying hosts,” meaning that virus reproduces in them at levels high enough to infect mosquitoes, which then spread the virus to the next host. There is so much virus in a crow, it has been described it in terms of how much of the crow turns into virus. Still, when a crow dies, that’s that. Robins, sparrows and other birds don’t produce nearly as much virus, but still enough to infect mosquitoes. More important, these birds survive long enough and in large enough numbers to provide the next generation of amplifying hosts.

Like many pathogens, West Nile is a generalist capable of infecting a variety of species, from birds, alligators and squirrels, to horses and people. As we trekked through the beautiful Spring-humid forest, Bob began telling me about some of West Nile’s arbovirus cousins: St. Louis Encephalitis (SLE) and Eastern and Western Equine Encephalitis. All are amplified in birds and spread by mosquitoes. WEE and EEE, as their names suggest, can be particularly deadly to horses.

Round Up

The horses came thundering down through the mountain pass, lost in a cloud of blinding dust, desperate to escape the pursuit of a relentless predator/helicopter overhead. Exhausted and betrayed by instinct, they followed a “Judas horse,” a plant let loose at just the right moment to lead them into the corral.

Months after the West Nile story, I was working on another short piece for National Geographic television on the Pryor Mountain Mustangs, descendants of horses brought to the New World by Spanish conquistadors four centuries ago. The horses had found their way to a gorgeous bit of nowhere, tucked between three mountain ranges on the Montana / Wyoming border. Over the generations, they had begun to revert to primitive form, becoming smaller, and sporting leg and dorsal stripes. They looked like they could have galloped right off a French cave painting.

Every couple of years the Bureau of Land Management (BLM), which owns the land, holds a round up, culling the herd to keep the numbers down to somewhere between 150 and 200. The BLM is also experimenting with giving mares birth control shots in an effort to stabilize the population growth rate near zero. Some worry that if the herd gets too small, genetic variability could reach a point of no return, dooming the Pryor Mountain horses to extinction.

It isn’t as if the horses don’t already have plenty of natural predators: bears, mountain lions, lightning strikes, drought, wildfires, and winter all take their natural selection toll.

I watched a small band kick at a patch of snow, the only water in an otherwise dry landscape, preserved in the shadow of a rock from the previous winter. I thought about how tenuous their fierce existence really was.

What if a new disease came through? In the wild, illness can tip the balance. A woozy, wobbly, disoriented horse is dinner delivered to a hungry bear. A dead pregnant mare doesn’t give birth. An ailing pregnant mare could easily abort, or simply not be able to take of the young she already has. Sick stallions almost surely lose their bands, turning the social status quo topsy-turvy as survivors fight for the spoils.

About a third of horses diagnosed with West Nile die. A veterinary vaccine was

quickly developed to protect domestic horses, but wild horses are on their own. I wondered would happen if West Nile hit the Pryor herd. Then I began wondering if something similar could have happened long ago, something that could explain an ancient mystery…

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For more on the Pryor Mountain / Arrorwhead Mountain mustangs: Filmmaker

Ginger Kathrens has been following the herd for years, producing an extraordinary series of documentaries for the PBS show “Nature,” along with a series of companion books. Ginger's

Cloud Foundation is dedicated to preserving the herd, and to helping other wild horses on public lands. Check the “Links” section of her site for additional sources.

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* The CDC’s initial diagnosis was St. Louis Encephalitis, a related virus. But SLE doesn’t kill crows and McNamara had a freezer-full…