Metro

Q&A: Bad season predicted for West Nile virus

The weather conditions this year have been a mosquito’s delight.
AFP/GETTY IMAGES
The weather conditions this year have been a mosquito’s delight.

It’s West Nile virus season in Massachusetts. Every year around this time, the state Department of Public Health sends out periodic alerts about infected mosquitoes and diagnosed cases. Here’s what you need to know about the mosquito-borne illness.

What can we expect this year?

It’s shaping up to be a bad year, said Dr. Catherine M. Brown, state epidemiologist and state public health veterinarian. As of Tuesday, 11 human cases had been confirmed. (The tally is updated online each day: http://www.mosquitoresults.com/) Most cases are diagnosed between August and October. Last year, only two cases had been diagnosed by the end of September.

In another bad sign, 8.6 percent of the mosquitoes trapped this year are infected with West Nile, while in a typical year the infection rate is around 1 percent to 3 percent. Last year, the rate was higher than normal, at 5.9 percent.

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Also, Brown said, “The risk is high throughout the state, not just in the urban areas where it is usually elevated.”

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The weather conditions this year have been a mosquito’s delight. The heat promotes their growth and speeds their ability to spread the virus. The humidity keeps them active for longer periods of the day. And the intermittent rains have left stagnant puddles where the critters breed.

Still, few people get seriously ill from West Nile virus. So why does the state track it so closely?

“We track it because we’re able to predict when it will show up,” said Dr. Brian D. W. Chow, an infectious disease physician at Tufts Medical Center.

You can’t say that about every infectious disease — and this knowledge alerts people to take precautions against mosquitoes: using insect repellent, covering up, avoiding outdoor activities at dawn and dusk, repairing screens, and removing standing water.

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“West Nile virus doesn’t make many people sick, but it does make a small proportion very sick,” Brown said. “There are ways we can estimate the risk and inform people, and help them prevent those cases.”

What are the symptoms of West Nile?

Most people who get infected experience no symptoms.

About 20 percent suffer fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or a rash on the torso.

In fewer than 1 percent of cases, the virus invades the central nervous system, leading to encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord). Symptoms of those rare, severe illnesses include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis.

People older than 60 and people with conditions that weaken their immune systems are at greater risk of severe illness.

How common is West Nile virus?

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It’s hard to know exactly because most people who become infected don’t have any symptoms and don’t seek care. The numbers the state collects reflect only those who became so ill their doctors decided to run a test. Between 2008 and 2017 in Massachusetts, 81 people were reported to have West Nile infection. Four of them died.

The numbers vary year to year, with six cases in 2017 and 16 in 2016. The worst year in recent memory was 2012, when 33 people in Massachusetts were reported ill.

How is West Nile diagnosed?

Sometimes a blood test will provide the diagnosis, but most people who seek medical care have neurological symptoms, Chow said. “In most cases, people will need to have a spinal tap,” a procedure that removes a small amount of fluid from the spinal column, he said. The fluid is tested for antibodies to West Nile.

How is it treated?

There is no treatment or vaccine for West Nile. Instead, doctors work to relieve symptoms. Those who are hospitalized receive intravenous fluids, pain medication, and nursing care to support them until their immune systems fight off the infection.

If you get West Nile once, are you immune?

It appears so. People who become infected are likely to remain immune to West Nile for the rest of their lives, Brown said. But it’s not known how many people have been exposed.

Why is West Nile more prevalent in urban areas?

“The species of mosquitoes that are most likely to spread West Nile virus tend to be urban dwellers,” Brown said. “They don’t like swamps or lakes. They like these small pockets of standing water — birdbaths, gutters, buckets. . . . That type of water source is more common in urban areas.”

Where did West Nile virus come from?

It was first identified in the West Nile region of Uganda in 1937. Outbreaks have occurred in Europe and the Middle East.

West Nile wasn’t seen in the Western Hemisphere until the summer of 1999, when a cluster of encephalitis cases in New York City drew the attention of public health officials. By the time health authorities identified the virus, 62 cases had been confirmed in New Yorkers.

But no one knows exactly how the virus made it across the ocean. It’s unlikely that an infected person spread it because there is no human-to-human transmission, and the amount of virus in human blood isn’t enough for mosquitoes to pick up. Perhaps an infected mosquito or bird in the Middle East hopped a New York-bound airplane.

Once in the Western Hemisphere, the virus quickly made itself at home and spread rapidly. By 2000, it had reached 10 counties in the Northeast. Mosquitoes in Massachusetts, as well as a horse, tested positive for West Nile that year.

In 2001, 66 human cases were found in 10 states, including one in Massachusetts. In 2002, the largest West Nile outbreak ever recorded anywhere occurred in North America, as the virus reached the Pacific Coast by year’s end.

Felice J. Freyer can be reached at felice.freyer@globe.com. Follow her on Twitter @felicejfreyer.