counting crows

By Bruce Goldfarb
Baltimore magazine, June 2000

West Nile virus killed seven people in New York City last summer. Does an infected bird found downtown last fall mean Baltimore is next?

On the afternoon of October14, bustling traffic on the 300 block of East Lombard Street, in the heart of the city's business district, slowed as it made its way around officers from the Baltimore City Health Department who were gingerly removing the carcass of an American crow. Although the foot-long specimen resembled hundreds of others that had been picked up in years past, this crow was different. This crow had the government watching it.

A public health alert had arisen 200 miles to the northeast: Earlier in the summer, New York City had experienced an outbreak of lethal West Nile encephalitis virus – a disease never before seen in the Western Hemisphere. More than 60 people had become ill with brain infections, and seven people had died. At the direction of the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, state and local health departments along the East Coast were asked to step up surveillance of birds and other wildlife. The virus had been found in animals in New York, Connecticut and New Jersey. And since it was carried by mosquitoes, it was critical to know how far the disease might have spread.

So, like dozens of other dead animals found on Baltimore’s streets and alleys that summer, the Lombard Street crow was warpped in two plastic bags, packed with ice, and shipped by UPS to the National Wildlife Health Center in Madison, Wisconsin.

Within days, the laboratory tests came back positive. West Nile encephalitis had come to Baltimore.

The rapid movement of a previously unheard-of virus from Europe to New York to the streets of downtown Baltimore offers a rare and alarming opportunity for infectious disease experts at Johns Hopkins and University of Maryland -- who typically travel to remote and inhospitable areas of the world -- to study an emerging disease locally.

It is “very uncommon” for a potentially major disease to appear in our own backyard, says geographic medicine specialist Robert Edelman, MD, director of the Traveler’s Health Clinic at the University of Maryland. “Everyone realizes the potential for this,” says Edelman. “The chances that [West Nile] has spread down here are very, very good.”

That’s the bad news. The West Nile virus causes encephalitis, an inflammation of the brain that can, in extreme cases, result in permanent brain damage and death. Common symptoms include fever, stiff neck, mental confusion, and, in the case of severe infection, paralysis.

The good news is that most people who are bitten by an infected mosquito will not become sick. Although the elderly tend to be more susceptible to the disease, West Nile “causes illness only in a small proportion of individuals,” says Edelman. “Even if they do get sick,” he explains, “most people will never notice.”

Right now, there’s no way to know how serious an issue West Nile encephalitis is for Baltimoreans. But there’s enough reason for concern that we should know about the disease – and what to do about it.

Culex pipiens, the most common urban mosquito in North America and a vector for the West Nile virus.

According to the CDC, more than 30 new infectious diseases have been identified since 1973, including legionnaires disease, toxic-shock syndrome, the human immunodeficiency virus (HIV), and now, West Nile encephalitis.

An emerging disease, explains Edelman, is one that was not previously recognized, one that enlarges in a known territory, or moves into a new territory. The viruses and bacteria that cause emerging diseases aren’t literally new, but had been festering in some remote spot until an animal or human gave them an opportunity to travel.

While the HIV virus that causes acquired immune deficiency syndrome (AIDS) wasn’t identified until the 1980s – it was a kind of virus, a retrovirus, which until then wasn’t thought to infect humans – further research has shown that the virus has been around in humans and primates for decades.

The last major disease to emerge in the United States was hantavirus, a genus of at least 14 viruses that erupted in 1993 among impoverished Navajo in the “Four Corners” region of Arizona, Utah, Colorado, and New Mexico. In May of 1993, several healthy Navajo youth died of a mysterious pulmonary illness. The agent responsible for the disease, hantavirus, is transmitted by breathing air contaminated with rodent feces. A mild winter and wet spring produced an abundant food supply that allowed the population of mice and rats to multiply dramatically in the Southwest.

There have been 238 cases of hantavirus pulmonary syndrome and nearly 100 deaths in the U.S. through February 2000, according to the CDC. Although hantavirus has been found in all 48 contiguous states, the disease has not grown into a major public-health problem because the conditions under which it flourishes are not common. Fortunately, Baltimore, whose humid climate is very different from the arid Southwest’s, doesn’t have many rats that are infected with the disease.

What we do have is mosquitoes.

Diseases emerge in areas of the world where different populations of humans, animals and viruses mingle, says Donald Burke, MD, professor of the Vaccine Research Center at Johns Hopkins. Usually, viruses are choosy about the hosts they infect. There are animal viruses and there are human viruses, but rarely do they pass from one species to another. However, in certain places around the world – the tropics, African rainforests, Hong Kong – humans and wildlife co-exist in close proximity, with animals coming and going through human settlements, and humans hunting wild meat.

In those cases, viruses that are adapted to humans have a chance to mix and swap genes with viruses that are adapted to other animals, leading to new hybrids that can cross from species to species. “An animal-adapted virus recombines with a human-adapted virus,” explains Burke. Under the right conditions, there can be an “exchange of genetic material between human and animal microbes that allows accelerated evolution of the organism.”

This scenario – the adaptation of a simian virus to humans – is likely how HIV emerged from Africa years ago. The human T-lymphocyte, an essential component of the immune system, was not at the time known to be vulnerable to an infectious agent like HIV. But it is. In fact, viruses are very receptor-specific, and for every receptor in the body – and there are hundreds of thousands of different kinds – there exists the potential for a virus. The white blood cell was an untapped market for viruses, an opportunity waiting to happen.

“There are lots of animal pathogens, many of which do not have a corresponding established human infection,” says Burke. “The AIDS virus is a good example of something that was not recognized in humans until the early 1980s. This kind of thing is probably occurring with lots of different organisms.”

more than 30 new infectious diseases
have been identified since 1973

Several alarming infectious diseases have emerged from the tropical rain forests in recent years – the painful and deadly Ebola virus, which nearly escaped from a Reston, Virginia, animal facility in 1989; dengue fever, which has rapidly spread through the Caribbean and Central America; and now West Nile encephalitis.

Originally identified in the 1930s, the West Nile virus has been responsible for sporadic cases of disease in Africa, Asia and Europe for decades. Outbreaks of West Nile occurred in the western Mediterranean and southern Russia in the early 1960s, Belarus and Ukraine in the 70s and 80s, Romania in 1996-97, the Czech Lands in 1997, and Italy in 1998. The last major outbreak of West Nile, in Bucharest, Romania in 1996-97, caused more than 500 bases of encephalitis.

But U.S. residents didn’t pay attention to the disease until the summer of 1999, when West Nile erupted in a residential neighborhood in Queens, New York. Initially identified as the more common St. Louis encephalitis, lab testing later proved the virus to be West Nile.

West Nile is carried by mosquitoes that thrive in densely populated urban areas and is able to cause illness in several different kinds of animals, apparently crossing species with ease. West Nile has been found in about 20 species of bird – particularly the common American crow – as well as horses, dogs and cats.

“When you see that kind of behavior, moving from one species to another,” says Burke, “that has all the earmarks of an emerging infection.”

The crow found in Baltimore marks the most southern point in the United States to which West Nile is known to have traveled. Public health officials and infectious disease experts from around the world will be watching the mid-Atlantic closely, Baltimore in particular, to assess how far West Nile may have spread since last summer.

On scenario is that the Baltimore crow is a bellwether that West Nile is creeping further south, heading toward the warmer climate similar to its African origins. “Our great fear is that it will move south,” says Edelman. “That’s a big mystery. We don’t know what’s going to happen.”

Last summer, federal, state and local health agencies launched a massive response to West Nile, monitoring mosquito traps and dead animals to identify traces of the disease. Other than the Baltimore crow and some birds found in New Jersey, no other infected bird has been found outside of the New York City area, according to Linda Glaser, wildlife disease specialist at the U.S.G.S. National Wildlife Health Center in Madison, Wisconsin. The last infected bird was a red-tailed hawk in Westchester County, New York, that died in February 2000.

However, monitoring of trapped mosquitoes revealed that West Nile had “overwintered,” surviving the cold weather in New York. If West Nile lived through the winter in New York, says Glaser, the virus could also have survived in Maryland. Nobody knows whether West Nile will be a problem this year until the weather warms up and mosquitoes become active again.

“There is a threat,” says Edelman. “Whenever you have mosquito transmission [of disease], you have potential for large-scale problems. But that doesn’t mean there will be large-scale illness.”

The absence of West Nile in mosquitoes caught in Maryland to date is not particularly reassuring, says Jeff Roche, MD, acting state epidemiologist in the disease control division of Maryland’s health department. “We’re not discounting the fact that there could be endemic disease in Maryland from West Nile virus,” he says.

In Maryland, three state agencies are involved in efforts to control the threat of West Nile. The Department of Natural Resources is monitoring signs of disease in birds and other wildlife. Mosquito-control efforts are under the direction of the state Department of Agriculture. Responding to the potential human health risks of West Nile is the jurisdiction of the Department of Health and Mental Hygiene. The State of Maryland established a West Nile hotline – 888-584-3110 – that will be staffed 24 hours a day, seven days a week.

Though scientists are concerned about a threat to public health, they can’t help butt be intrigued by this rare chance to study an emerging disease close to home. “This may be one of those fascinating times when we may be seeing the emergence of an important new disease,” says Roche.

We’ll be hearing more about West Nile as the virus settles in and finds its way around the new neighborhood. “We’re going to have to get used to it,” says Edelman. “The problem with these things is that once [emerging diseases] are ensconced, the general rule is that they stay around for a while. We’re going to have our hands full with West Nile.”

Reducing Risks of West Nile Virus

•  Stay indoors at dawn and dusk, when mosquitoes are most active.

•  Wear long-sleeved shirts and long pants when outdoors.

•  Apply insect repellent sparingly to exposed skin. An effective repellent has 20 to 30 percent DEET. High concentratins of DEET may cause side effects, particularly in children.

•  Spray clothing with repellent containing permethrin or DEET, as mosquitoes may bite through clothing.

•  Vitamin B and "ultrasonic" devices are not effective in preventing mosquito bites.

Source: Centers for Disease Control and Prevention

UPDATE: West Nile was reported in four states in late 1999. Within three years the virus spread from coast to coast to all but four of the lower 48 states. There were 3,852 confirmed cases of West Nile infection and 241 deaths during 2002. In Maryland there were 31 cases of West Nile -- including the nation's first in utero transmission from mother to baby -- and at least two deaths in 2002.


West Nile Home Page
Centers for Disease Control and Prevention

West Nile Virus
From Medline, sponsored by National Library of Medicine and National Institutes of Health

Research on West Nile Virus
Fact sheet from the National Institute of Allergy and Infectious Disease

West Nile -- Publications and Links
From National Institute of Allergy and Infectious Disease


© Copyright 2000, 2003 Bruce Goldfarb. All rights reserved.