The Heat Is Online

West Nile Spreads to Younger Victims -- Heat Seen As Factor

West Nile threat recedes with drop in temperatures

The Boston Globe, Oct. 19, 2002

The arrival of chilly temperatures in Massachusetts this week signaled the approaching end of this year's West Nile virus season, the deadliest since the mosquito-borne disease arrived in the state two years ago.

In 2002, West Nile killed three people in Massachusetts and left 18 seriously ill, with state health authorities cautioning that a few more cases may still be confirmed.

Nationally, West Nile this year became the most lethal mosquito-borne illness in decades, with 3,104 cases resulting in 172 deaths.

And while that pales in comparison to a disease, such as the flu, that spreads person to person, the number of deaths blamed on West Nile has eclipsed other infectious diseases on the rise in the United States, including the hantavirus, spread by rodents, and food-borne illness related to E. coli and other bacteria.

''It has emerged as the most important mosquito-borne viral disease of our time in this country,'' said Thomas Chambers, a St. Louis University scientist who has been studying West Nile for two decades.

In Massachusetts, autumn's first frost means that mosquito colonies have begun their annual cold-weather slumber, especially in the western fringes of the state. Typically, when temperatures reach 40 or lower, mosquitoes begin dying off and breeding ceases as potential spawning pools cover over with ice. The mosquitoes that survive seek shelter in barns or beneath other buildings, waiting for springtime.

''We're beyond the point where you're going to see any cluster or surge in cases,'' said Ralph Timperi, assistant commissioner of the state Department of Public Health. ''There are definitely parts of the state where you can say there's no continuing risk for the rest of the year.''

As they review this year's mosquito season, specialists say the disease has continued to surprise them. The relentless march of West Nile this year from coast to coast - it reached California for the first time - showed the virus's adaptability, as well as the flaws in a public response system relying on town-by-town defenses to cope with an enemy that does not respect man-made borders.

''Mosquitoes don't care about our boundaries,'' said Dr. Bela T. Matyas, a state disease specialist. ''That's why a regional approach makes so much sense.''

Some also suggest that the appearance of animal-borne illnesses, such as West Nile and hantavirus, demonstrates broader ecological shifts.

''These have to be seen as symptoms of environmental change happening on a global scale,'' said Dr. Paul Epstein, associate director of the Center for Health and the Global Environment at Harvard Medical School. ''And in terms of addressing this, it's beyond the capacity of the Centers for Disease Control and Prevention, in the sense that they need collaboration with the wildlife service, with the geologic service, with the meteorologic service.''

Scientists know that mosquitoes carry the West Nile virus, transmitting it to birds. The birds then serve as reservoirs, providing blood meals to mosquitoes that go on to infect other birds and, potentially, horses and humans.

Beyond that, understanding of the disease is continuing to evolve. Early in this West Nile season, the virus hit hard in a muggy, marshy swath of the Southeast, especially Louisiana and Mississippi - prime grounds for mosquito breeding.

But as the season progressed, the virus wound up being most devastating in an arc of four Midwestern states stretching from Michigan to Illinois, which together accounted for more than half of all the cases.

The two most surprising - and, to many specialists, disturbing - developments involved West Nile's ability to penetrate the blood supply and to produce conditions in victims strikingly similar to polio. The CDC this week reported that it is now investigating 29 West Nile cases among patients who had received blood transfusions. So far, four cases have been definitively linked to tainted transfusions. The CDC also continues to review reports of West Nile patients developing symptoms that some neurologists have labeled as polio caused by the mosquito virus.

Looking to next year, public health officials foresee a battle that is equal parts science and politics, including the recurring question of whether to spray with insecticides - a summer ritual in some communities, a forbidden approach in others.

''Just recently, you get these educated people who are really against spraying,'' said Mike Rolli, chairman of Stoneham's health board. ''During the old days, you just did it and nobody said anything. But today, you have everybody looking at you.''

So, some cities and towns this year increasingly emphasized preventive strategies. In Malden, health inspectors scour the city for the favorite haunts of mosquitoes. Sometimes, that takes them into backyards, where they find pools that haven't been properly maintained to minimize bug breeding.

Even if inspectors cite a homeowner, Malden health inspector Chris Webb said, the city's objective is not to rake in fines.

This story ran on page B1 of the Boston Globe on 10/19/2002.

West Nile virus claims 2 Mass. residents

The Boston Globe, Sept. 14, 2002

Two Boston-area residents in their 80s have died from West Nile virus, public health authorities reported yesterday, the first deaths this year in the state from an illness that now spans the nation.

Traces of the mosquito-borne virus first became evident four years ago in New York City, and over the next three summers, West Nile made a steady march across the United States, finally reaching California this month. The latest CDC tallies show the virus is active in 42 states, with 1,295 human cases nationwide resulting in at least 54 deaths.

The victims are an 87-year-old South Boston woman who fell ill Sept. 4 and an 81-year-old Weymouth man whose illness had not previously been reported to disease trackers. The Weymouth man's infection was diagnosed after his Sept. 6 death through examination of blood samples.

Since its arrival on US shores in 1999, West Nile virus has been recognized as a particular threat to the health of the elderly, whose ability to fight disease is often compromised by age and chronic illness. State and city health authorities have targeted prevention efforts at the elderly, with Boston City Hall dispatching a team of workers to South Boston armed with informational pamphlets. The city also placed small bricks of mosquito-killing bacteria in water catch basins, while the town of

Weymouth on the marshy South Shore intensified insecticide spraying to eliminate the bugs.

''The elderly really need to pay attention to the precautions,'' said Dr. Alfred DeMaria, the state's director of communicable disease control. ''People should be thinking of their family members who are at risk, at least for the next few weeks. The mosquitoes are still out there and will be for the next six weeks or so.''

News of the deaths came one day after US Senator Patrick Leahy, speaking on a Burlington, Vt., radio talk show, raised the specter of terrorism as a possible source of the rapid spread of West Nile from coast to coast.

The Vermont Democrat's office afterward issued a statement expanding on the radio remarks.

''In the times in which we live, questions about our vulnerabilities are unavoidable, and finding all the answers we can is more important than ever,'' Leahy's statement said. ''I have no way of knowing what the answers are, but some legitimate questions have been asked, especially before September 11 last year, and no doubt they are being asked anew by the agencies that are working on this.''

But a spokeswoman for the US Centers for Disease Control and Prevention said yesterday that the agency had previously considered whether West Nile might be the work of terrorists and had dismissed the possibility. She said there are no indications that the CDC is planning to reopen that line of inquiry.

Traces of the mosquito-borne virus first became evident four years ago in New York City, and over the next three summers, West Nile made a steady march across the United States, finally reaching California this month. The latest CDC tallies show the virus is active in 42 states, with 1,295 human cases nationwide resulting in at least 54 deaths.

In many respects, the evolution of West Nile in the United States represents the natural, and predictable, spread of a viral agent that relies on a delicate interplay between mosquitoes, birds, horses, and humans, said a Harvard specialist.

''There are plenty of biological, ecological, and climatological reasons why this disease has spread so fast,'' said Dr. Paul Epstein, associate director of the Center for Health and the Global Environment at Harvard Medical School. ''I don't think this would exactly be the first agent that someone who wanted to attack the US would use. It's not very efficient.''

West Nile entered Massachusetts in 2000, with the first human cases last year, resulting in one death. This summer, the virus has made at least 11 people in the state ill, with the most serious cases causing encephalitis, brain swelling that can lead to death.

Authorities yesterday were still investigating the deaths of the South Boston woman and Weymouth man, who were not identified because of confidentiality rules. Disease investigators from the Boston Public Health Commission intend to speak with the family of the 87-year-old woman for clues as to what might have made her vulnerable to the virus, said John Auerbach, executive director of the city's public health agency. Because of the severity of her illness, epidemiologists were unable to interview her before she died, DeMaria said.

Her death did not alter Boston's strategy on how to prevent contracting the virus.

''Obviously, any tragedy like this is a reminder about the dangers of the West Nile virus,'' Auerbach said. ''But in terms of the assessment of appropriate steps to minimize exposure of the public, we believe that hasn't changed. Prevention is the most important step.''

With that in mind, 20 city workers blanketed South Boston with prevention pamphlets and the Public Health Commission contacted groups that work with the elderly, offering to provide educators to teach West Nile prevention, such as wearing long-sleeved clothing and draining water from wading pools and unused tires, which can be mosquito breeding grounds.

The city also plopped bricks of a bacterial agent known as bacillus sphaericus into water catch basins. The bacteria eliminate mosquito larvae but do not harm the environment.

Boston's prevention methods do not include spraying insecticide, a practice that has engendered controversy in the past. In Weymouth, though, mosquito spraying is a summer fixture. In fact, spraying in the town has accelerated during the past four weeks, said Jane Hackett, chief of staff to Mayor David M. Madden.

Like the South Boston woman, little was known yesterday about how the Weymouth man might have been exposed to West Nile. The recent upsurge in cases, though, has had a palpable effect.

''There is a heightened awareness with people,'' Hackett said.

This story ran on page A1 of the Boston Globe on 9/14/2002.
Copyright 2002 Globe Newspaper Company.

West Nile Virus Claims 2 in Illinois

The Associated Press, Sept. 11, 2002

SPRINGFIELD, Ill. -- The West Nile virus has killed two more Illinois residents, pushing the state's death total to 13, the most in the nation, health officials said Wednesday.

Laboratory tests have confirmed that more than 1,200 people have been infected with the virus from coast to coast, including one case in California, according to the Centers for Disease Control and Prevention.

If the two latest Illinois fatalities are confirmed by the CDC, the national death toll would be at least 48.

The state's public health department said the new deaths were an 81-year-old woman from southern Cook County, in the Chicago area, who died Sept. 1 and a 76-year-old man from Madison County, near St. Louis, Mo., who died Sept. 5.

The department reported 22 new laboratory-positive human cases of the mosquito-borne disease, bringing Illinois' total this year to 314.

Louisiana is second with 222 confirmed illnesses and 10 deaths, the CDC says.

Health officials say most people who get infected with West Nile virus have either no symptoms or mild symptoms, but a few individuals -- especially the elderly -- can develop a more severe form of the disease.

West Nile virus spreading west, CDC says

The Boston Globe, Aug. 12, 2002

NEW ORLEANS - West Nile virus is an ''emerging, infectious disease epidemic'' that could be spread to the Pacific Coast by birds and mosquitoes, the director of the Centers for Disease Control said yesterday.

The Northeast and the South have been hardest hit by the virus, but Dr. Julie Gerberding said birds and mosquitoes infected with West Nile are now in most states east of the Mississippi River and some to the west of it.

West Nile is ''a problem that is having an unusually high human toll this year. So it is serious, and we have to continue our public health action to combat it,'' Gerberding said on CBS's ''Face the Nation.''

Seven people with West Nile virus have died in Louisiana this year. The virus has been detected in 35 states and Washington, D.C.

In Louisiana, state and local workers are spraying insecticide in residential areas where the Asian tiger mosquito and the Southern house mosquito typically lay eggs, under the assumption that those two species are the most likely carriers of West Nile.

''We have made an assumption about which species are involved in transmission of the disease here based on what has happened in other parts of the United States,'' said Dawn Wesson, a medical entomologist at the Tulane University School of Public Health and Tropical Medicine.

Gerberding said Louisiana's experience last year with the deaths of four people from St. Louis encephalitis, a mosquito-bourne virus similar to West Nile, has helped officials deal with this year's outbreak.

''I think the investments that we've made over the past several years in this kind of public health response have really paid off,'' she said.

This story ran on page A2 of the Boston Globe on 8/12/2002.

Hot summer may be behind West Nile outbreak in US, Aug. 9, 2002

ATLANTA - An unusually warm summer may be fueling the worst outbreak of West Nile virus in the United States since the mosquito-borne disease surfaced here three years ago, federal health officials said on Thursday. Five people have died of encephalitis, a severe brain inflammation sometimes associated with West Nile, and more than 100 others have been infected with the virus this summer, the U.S. Centers for Disease Control and Prevention said in an update on the outbreak.

The agency noted that the bulk of the cases occurred in the deep Southern states of Louisiana, Texas and Mississippi, though isolated infections have been reported as far away as Illinois and Washington, D.C.

Dr. Lyle Petersen, a West Nile expert with the CDC, said it was possible that high temperatures this summer had created an ideal incubator for mosquitoes. The insects contract the virus from infected birds and then spread it to humans. "The kinds of mosquitoes that transmit West Nile virus like to breed in very small, murky pools of water, and during hot weather larger pools condense into smaller pools," said Petersen, who added that the CDC was working with local health officials to monitor and control mosquito populations.

Although previous outbreaks in Israel, Romania, Russia and New York City coincided with bouts of hot weather, Petersen cautioned that other environmental factors likely had also played a part in the current outbreak. West Nile is common in Africa and Asia but was unknown in the Americas until 1999 when an outbreak killed seven people in the New York borough of Queens. It has since spread quickly.

At least 23 people have died after contracting the virus in the United States. On Thursday, one day after confirming the first human case of the disease in Washington, D.C., officials in the nation's capital urged the public to be vigilant. "We need to do this together," said Theodore Gordon, the district's senior deputy director for public health assurance, at a news conference. "People have to go in their backyards and look for those situations and conditions that may lend themselves to mosquito breeding."


Most people who contract West Nile suffer nothing more than headaches and flu-like symptoms, but the elderly, chronically ill and those with weak immune systems can develop fatal encephalitis and meningitis when infected.

The average age of 98 people infected this year is 55, according to the CDC. About 60 percent were men.

At least 36 states, stretching from Massachusetts to Texas, and the District of Columbia have reported some West Nile activity in 2002. The virus, which is spread largely through bird migrations, has also been detected in parts of Canada.

Mosquitoes contract West Nile from infected birds and then pass it on to human hosts. The virus can not be spread from person to person or from birds to humans. Incidence of the disease tends to drop off when temperatures fall below 55 degrees (13 Celsius) and mosquitoes become dormant. But West Nile can spread again when the insects become active in spring.

Although more common, the risk of contracting the virus is still extremely low and can be reduced further if people use mosquito repellent, wear long sleeves and pants at night, and eliminate pools of standing water where mosquitoes breed.


Specialists see younger West Nile virus victims

La. is hit hardest, with 71 infections, 5 deaths this year

The Boston Globe, Aug. 9, 2002

ATLANTA - For reasons health officials cannot explain, this year's victims of the mosquito-borne West Nile virus are younger than usual.

The median age for this year's infections is 55, the Centers for Disease Control and Prevention said in a report released yesterday. In previous years, the median age was in the mid-60s.

''The reason patients seem to be younger this year is unknown and certainly something we're looking into,'' said Dr. Lyle Petersen, a CDC expert. However, CDC officials said the declining median age is not a reason to worry.

The CDC has reported 112 human cases in 2002, more than half in the past week.

Most of the human cases were in Louisiana, which has reported 71 infections and five deaths in the nation's biggest outbreak since the disease was first detected in New York in 1999. The CDC said Louisiana would get $3.4 million to help control mosquitoes.

The CDC also reported human cases in Mississippi, Texas, and Illinois. Cases were also announced this week in Washington, D.C., and Alabama, but came too late to be included in the CDC report.

In addition, Illinois reported its second human case yesterday, a 57-year-old man who became critically ill with encephalitis.

And Kansas officials said yesterday the virus has spread to their state, where it was found in a dead horse.

All of the West Nile human deaths so far this year have been in Louisiana.

Most people bitten by an infected mosquito will suffer no more than flulike symptoms, but the weak and the elderly can get encephalitis, a potentially fatal brain inflammation.

Petersen said advanced age remains the biggest risk factor for getting sick from the bite of an infected mosquito. Health officials do not compile statistics on health problems of the younger West Nile victims, but suspect that many had conditions that weakened their immune systems.

Men made up 60 percent of this year's West Nile cases, consistent with years past. Petersen said that is probably because men spend more time outdoors.

Gary Simon, an infectious-disease specialist at George Washington University, said doctors may be more aware of the disease and more likely to look for it in patients with less serious symptoms, explaining the lower median age.

''It may be because we're testing people more,'' he said. ''It's the only possible explanation from the information I've seen so far.''

Since its first appearance in the United States, the virus has been detected in 35 states and Washington, D.C.

State and local officials have boosted mosquito-spraying efforts and have urged people to protect themselves by using bug repellent and wearing long sleeves.

Encephalitis is usually seen in August and September, but Louisiana's first patients became ill in June. The West Nile virus is showing up earlier in the summer as it spreads to warmer climates, according to the CDC.

Dr. Lyle Petersen, a West Nile specialist with the CDC, said it was possible that high temperatures this summer had fostered ideal conditions for mosquitoes, according to a Reuters report.

''The kinds of mosquitoes that transmit West Nile virus like to breed in very small, murky pools of water, and during hot weather larger pools condense into smaller pools,'' said Petersen.

He added that the CDC was working with local health officials to monitor and control mosquito populations.

Although previous outbreaks in Israel, Romania, Russia, and New York City coincided with bouts of hot weather, Petersen cautioned that other environmental factors probably had also played a part in the current outbreak.

© Copyright 2002 Globe Newspaper Company.

Fifth Louisana West Nile Death Confirmed
The Associated Press,
August 6, 2002

NEW ORLEANS -- The West Nile virus has killed a fifth Louisiana resident, and about a dozen other new cases were confirmed, making the outbreak the worst ever for a single state, health officials said Tuesday.

The death was among about 14 new cases confirmed by the federal Centers for Disease Control and Prevention, state health department spokesman Bob Johannessen said Tuesday.

Details on the new cases were to be discussed at an afternoon news conference.

Fifty-eight cases had already been confirmed, bringing the new total to about 72. Before now, the nation's largest outbreak had been the first, when 62 people became ill and five of them died in 1999 in New York.

Mississippi has 22 cases confirmed by the CDC this year. Texas has 10 suspected cases and Arkansas one suspected case.

Since 1999, the virus has been detected in birds and people in 34 states and Washington, and health officials expect it will continue spreading west.

The Louisiana outbreak is also the nation's earliest since the one in New York. Encephalitis is usually seen in August and September, but Louisiana's first patients became ill in June.

Dr. Jim Hughes, director of the CDC's National Center for Infectious Disease, said West Nile is showing up earlier in the summer as it moves to warmer climates.

"The disease will not necessarily behave the same in all geographic areas," he said.

Copyright © 2002, The Associated Press

Four Are Killed in Big Outbreak of West Nile Virus on Gulf Coast

The New York Times, Aug. 3 2002

Federal and state officials said that 58 Louisianans had become ill from the West Nile Virus in recent days and that four have died. The outbreak of the virus is the largest since it was first detected in the United States, in New York City, in 1999.

In addition to the cases in Louisiana, 13 have been reported in the adjacent states of Texas and Mississippi.

The Louisiana outbreak prompted Gov. Mike Foster to declare an emergency yesterday, partly in an effort to obtain millions of dollars in federal aid for local governments that are rapidly using up their mosquito spraying budgets.

The federal Centers for Disease Control and Prevention has sent a team of 10 scientists to investigate how widely the virus has spread in Louisiana and elsewhere in the gulf region, and to trap insects and the animals they may have infected. One aim is to determine whether species of mosquitoes not previously identified as transmitters of the virus are in fact spreading it. Such identification would be crucial to controlling the outbreak.

Most people who become ill with West Nile virus suffer only mild flulike symptoms. But a small percentage — mostly older people and those whose immune systems have become impaired by cancer therapy, by H.I.V. or for other reasons — can suffer more serious illness, including meningitis or encephalitis. These inflammations of the brain and its covering can bring serious, long-lasting health problems or even death.

Health officials said they could not explain precisely why the West Nile virus had struck so fiercely in Louisiana and neighboring states. They noted, however, that encephalitis caused by other viruses had long been a significant health problem in the gulf region.

The introduction of West Nile into the area only adds to the burden on the health system there.

The virus spread widely elsewhere last year, moving from the Northeast as far south as the Florida Keys and 750 miles to the west of them. In all, it covered an area of about half a million square miles. Sixty-six cases of illness were reported then, from 10 states.

Though the 71 cases reported so far this year — all from Louisiana, Mississippi and Texas — are little more than the 2001 total, health officials said they were deeply concerned, for two reasons: first, the onset of illness this year occurred relatively early in the summer; second, the illness seems to be afflicting a younger population than the people sickened in the three previous years since the virus's detection.

From 1999 through 2001, when a total of 149 cases were reported in the United States, most were identified from mid-August through September; the earliest onset of illness in those years occurred on July 14.

This year, by contrast, cases were first identified in middle to late June, and have been occurring with increasing frequency ever since.

As for the age groups of the people made ill, the average age of patients in the first two years that West Nile virus caused infections in the United States was about 66, and last year it was 70, said Dr. Stephen M. Ostroff, a senior official of the disease control centers.

This year, on the other hand, the average age in the three affected states has been in the upper 50's. Health officials in Louisiana said that of the 55 patients there whose ages were known, 24 were 60 or older. But 12 were 45 to 59, and 19 were younger still.

There is no way to know, however, whether the age disparity from the previous years' experience will continue. And it is too early to know whether the disparity recorded so far signals any change in the virus.

Of the four people killed in the current outbreak, two were from Baton Rouge, one from Folsom, near the Mississippi line, and one from the town of Iowa, not far from Texas.

The Gulf Coast's natural features will make efforts to control the virus there more difficult than they were in New York and elsewhere.

"Mosquitoes are breeding prolifically at this time" in the gulf region, Dr. Ostroff said in an interview.

"We're just going into the season for encephalitis," he added. "More cases will occur."

Dr. Ostroff said that "even with aggressive measures, the outbreak is not going to come to a screeching halt as it did in New York," and that "control will require a continuing effort for months."

Dr. Ostroff was a leader in the epidemiological investigation of the outbreak in New York, which infected 62 people and killed seven. He said that while he would not advise people to refrain from visiting the Gulf Coast, it was important to keep in mind that groups like the elderly and those with impaired immune systems were at greater risk than others for developing severe illness from West Nile virus.

There is no specific treatment for West Nile other than supportive therapy like intravenous fluids, assistance in breathing with a mechanical ventilator and nursing care to prevent secondary infections like pneumonia.

Many of those infected during the current outbreak have been hospitalized, and some have required treatment in intensive care units, Dr. Ostroff said. A "significant" number of those hospitalized have recovered and gone home, he said.

Federal and Louisiana health officials have advised people in the region to try to avoid mosquito bites by reducing the amount of time they spend outdoors, particularly in early morning and early evening, when the mosquitoes that carry the virus are most likely to bite.

The officials also recommended wearing long pants and loose long-sleeved shirts and applying mosquito repellent to exposed skin. As a further step, they urged residents to help eliminate mosquito breeding sites by draining stagnant water and spraying.

In just three years, West Nile virus has infected 31 species of mosquitoes and 115 species of birds in the United States.

Last year, 27 states and the District of Columbia reported finding West Nile virus in humans or insects and animals. This year, seven states have been added to the list. In the North, the virus has spread as far west as the eastern Dakotas.


Louisiana confirms four West Nile deaths, Aug. 2, 2002

BATON ROUGE, Louisana (CNN) -- The West Nile virus has caused three more deaths in Louisiana, bringing the summer's death toll in the state to four, all in this week, health officials said Friday.

Since Monday, the outbreak has spread nearly throughout the state, the Department of Health and Hospitals said in a news release.

The three new deaths and 26 new cases of illness caused by the virus occurred in Orleans, Calcasieu and Ouachita parishes, the health department said.

In addition to an 83-year-old woman from Baton Rouge who died earlier in the week, the dead include a 53-year-old man from Folsom, a 75-year-old man from Baton Rouge and a 72-year-old man from Iowa in Calcasieu Parish.

Gov. Mike Foster is declaring a state of emergency, making the state eligible for federal funds to fight a rash of infections of West Nile, said Steven Johnston, his deputy press secretary.

"We hope that the federal government is able to respond quickly and effectively," said the health department Secretary David Hood.

"It is becoming quite evident that the broad nature of this outbreak is going to quickly deplete both state and local funds dedicated to the epidemiological efforts, laboratory testing, surveillance and mosquito-control efforts."

The number of cases is expected to increase and to spread into other areas of Louisiana, according to department spokesman Bob Johannessen.

At least 10 other hospitalized patients suspected to have the disease are being tested for the virus, he said. Lab tests take about a day to complete.


W. Nile virus found in Texas; spread perplexes specialists

The Boston Globe, Aug. 2, 2002

Federal officials yesterday confirmed the first West Nile virus human outbreak in three years, a sudden cluster of 44 cases in Louisiana, Mississippi, and Texas that reflects a mysterious spurt beyond its established East Coast range.

An elderly Baton Rouge leukemia patient died of the virus Monday. Louisiana health officials investigated a second possible fatality yesterday that would bring the three-year US death toll to 20.

Yet even as Southern states scrambled to repel the unwelcome visitor, Atlantic Coast states, where the virus first appeared three years ago, have been aided by a drought that has lowered the population of the water-loving mosquitoes that spread the virus.

West Nile Virus spreads most rapidly in steamy, humid August weather, and Massachusetts public health officials planned aggressive testing of crows and C. pipiens mosquitoes, the virus's prime carriers, over the next three weeks.

The virus has turned up as far south as Florida before, but never as far west as Texas, where 10 people contracted it this summer. Federal disease detectives flew to Louisiana earlier this week in search of clues.

''What we've seen is a great leap forward, and it appears that that would be a leap to the West,'' said Dr. Anthony Marfin, a West Nile virus specialist for the US Centers for Disease Control and Prevention. ''When we don't completely understand reasons for the movement. ... It's very hard to speculate when it will show up in states further to the west.''

At this time last year, the virus had turned up in nine states. This year, it has appeared in 34 plus the District of Columbia. Clusters of virus-laden bird corpses were found in Ohio and Illinois. Two birds were recovered in Minnesota. Texas and South Dakota mark the virus's westernmost appearance to date, though specialists predicted a California sighting later this summer or next year.

Infectious disease specialists consider West Nile virus endemic on the East Coast, meaning that it is expected to appear summer after summer. Infected mosquitoes preserve the virus through winter hibernation. They awaken in summer and spread it to birds, which fly up and down the coast, spreading it to more mosquitoes.

That pattern created a narrow New England-to-Virginia plume considered the virus's habitat since 1999, when it first appeared in the United States. But its migration westward this summer was unexpected. Federal officials speculated yesterday that responsibility could lie with birds that simply veered off course. But they remain uncertain, with intensive tests underway in the heart of the current outbreak in Lousiana.

''They're trying to see if there's a different breed of mosquito involved,'' said Kyle Viator, spokesman for the Louisiana Department of Health and Hospitals.

Louisiana state officials identified 122 virus-killed birds in that state. More disturbing are the 32 human cases, including the death of an 83-year-old woman with leukemia whose illness was fatally exacerbated by the virus, according to her doctors.

The first outbreak, in 1999, occurred in Queens, N.Y., with 62 cases causing seven deaths. Health officials remain baffled over how it jumped the Atlantic Ocean from its longtime habitats in Africa and the Middle East.

Less than 1 percent of those infected develop severe illness; of these, less than 1 percent die. Most cases cause slight flulike symptoms, but the risk increases for the elderly. Two-thirds of the 161 cases and 18 fatalities since 1999 involved seniors.

In Massachusetts, state health officials remained cautious.

''If there's significant rainfall,'' said Assistant Public Health Commissioner Ralph Timperi, ''then there's still the potential risk for West Nile virus transmission to humans at a level that would be significant. ''

This story ran on page A11 of the Boston Globe on 8/2/2002.

© Copyright 2002 Globe Newspaper Company.