Bug off! First, it sickened birds. Then, it struck humans: The mysterious spread of the West Nile virus

Science World, April 26, 2004 by David George Gordon

Veterinarians are used to treating a wide variety of animal patients. But when a woman carrying a black crow walked into John Charos's waiting room in June 1999, the Queens, New York, doctor was surprised. "It looked so comfy," he says. Crows are wild birds with a reputation for loud behavior. "But this one wasn't cawing or flapping its wings." Turns out, the bird was uncharacteristically calm because it was ill.

What Charos didn't know at the time was that he was treating one of the earliest victims of West Nile virus (nonliving particle that invades and reproduces in a living cell) in the U.S. The mosquito-borne (transmitted by mosquitoes) virus had never before been reported outside of Africa, Asia, the Middle East, or Europe. But since its emergence in North America, West Nile has spread rapidly, killing thousands of wild birds and scores of reptiles, amphibians, and mammals--including humans. In 2003, the virus killed 223 people nationwide.

Summer's blazing heat, which draws swarms of mosquitoes, is fast approaching. And, along with the heat, scientists predict a renewed virus season. Researchers, still baffled by how West Nile reached the U.S., are racing against time in their efforts to curb its spread.

VIRUS FACTORY

The mosquito-home virus gets its name from the West Nile region of Africa, where it was first identified in the 1930s. And like most viruses, the West Nile virus is a parasite (organism or nonliving particle that feeds off another organism); it can't unleash its damage until it finds a host (organism in or on which a virus or another organism lives or feeds). But once the virus enters a healthy cell, it hijacks the cell's functions, ordering it to replicate (make copies of) the virus. When the infected cell is stuffed with new West Nile particles, it erupts and dies, releasing an army of viruses to attack more cells.

Most viruses target specific cells of certain animals or plants. For example, most cold viruses only invade cells in the nose and throat of humans. If your dog is exposed to the cold virus, "Rover" is unlikely to develop a runny nose. But West Nile virus is trickier; it can attack a wide range of animals--from sparrows to horses to humans.

To spread from host to host, West Nile depends on a carrier, or vector. When a mosquito feeds on the blood of an infected animal--especially birds--it delivers the virus to other creatures it bites. For humans, a nibble from a West Nile-infected insect could cause flu-like symptoms such as headache, high fever, and body pain. But for people whose immune (body's disease-fighting) systems are weakened by old age or disease, the symptoms could be severe. In these victims, West Nile has caused deadly diseases such as encephalitis (brain inflammation). But compared with wildlife, most humans are lucky. For unknown reasons, birds--especially crows--are the most easily killed by the virus.

MYSTERIOUS ORIGINS

In the New York City area, crows died by the hundreds during the summer of 1999. "At first, nobody paid attention," says ornithologist (bird scientist) Kevin McGowan of Cornell University. "That's because most people consider crows to be nuisance birds." However, in July of that same year, people took notice when a flamingo and several exotic birds at New York City's Bronx Zoo were found dead.

Soon, another strange illness hit the area--this time targeting humans. In September, the New York City government announced an outbreak of a rare disease called St. Louis encephalitis. Within a month, seven people were dead and 55 others had been hospitalized.

Was St. Louis encephalitis killing the city's birds too? Not according to Tracy McNamara, Bronx Zoo's veterinarian. St. Louis encephalitis isn't deadly to birds. "Birds may carry the virus, passing it to the mosquitoes that drink their blood," she says. "However, the virus is lethal only to human beings."

Turns out, West Nile was responsible for the human illnesses, not St. Louis encephalitis. It was an understandable diagnostic error. Both St. Louis and West Nile are viral diseases transmitted by mosquitoes, and both cause similar symptoms in people. Naturally, few people suspected West Nile to be the bird-and-human killer because the virus was once limited to foreign shores.

West Nile has since expanded its range beyond New York to 46 U.S. states and five Canadian provinces. Experts anticipate it could soon reach Mexico and Central America.

PEST CONTROL

What can be done to rein in West Nile's spread? One tactic: Knock out the swarms of virus-transmitting mosquitoes. During virus season, some parts of the country are routinely sprayed with pesticides. The down-side: Toxic bug-spray could accidentally poison other wildlife and people.

The most difficult to protect in West Nile's web of destruction is its frontline victim: wild birds, especially crows. While it's impossible to predict the bird's future, McGowan hopes some crows may be naturally gifted with West Nile-fighting genes. (Similar to how some people are genetically better able to fight colds.) If fit birds pass virus-resistant traits to offspring over several generations, more crows may be better adapted to fight West Nile. Then, mosquitoes may find fewer infected birds to bite, and the spread of West Nile would diminish.