Mosquitoes -- flying hypodermic needles -- are the only species that...

Mosquitoes -- flying hypodermic needles -- are the only species that can transmit West Nile, and on Long Island it is spread by the species Culex pipiens, the Suffolk health department said. Credit: USGS

Twenty years ago this month, the first evidence of a mystifying illness emerged in Queens but quickly spread to Long Island — a puzzling infection that within weeks would be identified as West Nile virus.

To this day, no one knows precisely how New York became the gateway to an infectious disease that never had been detected in the United States.

“We had no idea what it was when it came to town,” said Dr. Scott Campbell, chief of the Arthropod-borne Disease Laboratory in Yaphank, a division of the Suffolk County Department of Health. The 20th anniversary of a virus that was new to the Western Hemisphere seems to have come about quickly, he said.

At the time, crows sickened by the virus were dropping dead throughout the metropolitan region, a sign that birds were a critical link to the virus. Among people, a flu-like illness that morphed into more serious symptoms for some was being diagnosed with increasing frequency, especially among the elderly. It took several weeks for medical experts to connect the dots between dying crows and ailing people.

Experts in infectious diseases, veterinary medicine, entomology and public health converged on New York in the summer of 1999, including a battalion of virus hunters from the Centers for Disease Control and Prevention.

“Suffolk County went into the city and the CDC was there when we all figured it out,” said Campbell, noting that in a scant three years the virus — carried by migrating birds — made its way from New York to California.

“We’ve seen it every year in Suffolk County since then, so it’s not something that is going away,” Campbell said.

By fall of 1999, nearly 2,000 people in the greater metropolitan area had been infected with a virus that is incubated in birds but is spread throughout the human population by mosquitoes.

Dr. Luis Marcos, a specialist in infectious diseases at Stony Brook University Hospital, said the number of people diagnosed on Long Island has been diminishing over the years. He attributes the decline to vast swaths of the population having been exposed to the virus, a factor that has blunted the possibility of an outbreak on the scale of the one that occurred 20 years ago.

Since 2000, the New York State Department of Health has estimated about 500 cases of the disease and 37 deaths. Most infections produce mild flu-like symptoms. But in the population at large, many people have antibodies to the virus, Marcos said, a sign they had been infected at some point in the past.

“Once you get the infection, you are immune to West Nile virus,” Marcos said.

The disease is a summertime affliction, the time of year when mosquitoes are in flight.

“I see about one to two cases a year in the county,” said Marcos, who defined the infectious agent as a flavivirus, a type of pathogen that is generally transmitted by mosquitoes. Other types of flaviviruses include the pathogens that cause dengue, Zika, yellow fever and Japanese encephalitis, among others. Ticks are also carriers of flaviviruses.

Although he rarely sees severe cases now, Marcos said elsewhere in the country, particularly in the Southeast, West Nile remains a menace capable of causing paralysis and brain swelling.

Campbell, meanwhile, said there is no turning back the clock to a time when West Nile did not exist in this hemisphere.

Before its arrival in Queens in the summer of 1999, West Nile virus was a mosquito-borne infection that occurred in Africa, the Middle East and western Asia. It was episodically detected in Europe. The viral strain that emerged in New York 20 years ago had the genetic fingerprints of a viral type mostly seen in Israel.

Mosquitoes — flying hypodermic needles — are the only species that can transmit West Nile, and on Long Island it is spread by the species Culex pipiens, Campbell said.

Birds harbor West Nile virus, and mosquitoes ingest it when they feed on birds. Only female mosquitoes are blood-hungry and pass along the virus in their saliva when they bite people. Female mosquitoes need blood to produce and lay eggs.

“Most people in this business were surprised how quickly it spread in three years, from New York to California,” Campbell said of the West Nile pathogen. “It was an incredible expansion of the virus from coast to coast, and what drove it was the birds.”

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