The West Nile virus epidemic of 2012, the worst in a decade, may be notorious for yet another reason: The virus, in some cases, is attacking the brain more aggressively than in the past, raising the specter that it may have mutated into a nastier form, say two neurologists who have extensive experience dealing with the illness.

One doctor, Art Leis in Jackson, Miss., has seen the virus damaging the speech, language and thinking centers of the brain — something he has never observed before. The other, Elizabeth Angus in Detroit, has noticed brain damage in young, previously healthy patients, not just in older, sicker ones — another change from past years.

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Mosquitos are sorted at a lab. Some doctors worry that the virus is mutating and is attacking the brain more aggressively. File photo/The Associated Press

Angus, who has treated West Nile patients for a decade, and Leis, who has more experience treating severe West Nile illness than perhaps any doctor in the country, both suspect the virus has changed — a view bolstered by a Texas virologist whose laboratory has found signs of genetic changes in virus collected from the Houston area.

“I’ve been struck this year that I’m seeing more patients where the brain dysfunction has been very much worse,” said Angus, of Detroit’s Henry Ford Hospital. “It makes you wonder if something’s different, if something’s changed.”

And while the virus in the past has typically invaded the brain and spinal cord only of people who have weakened immune systems, such as the elderly and transplant or cancer patients, Angus this summer treated a severely affected woman in her 20s and a man in his 40s.

Leis said he is seeing much more severe encephalitis — inflammation of the brain — than he has in the past. “It is clearly much more neuroinvasive, neurovirulent,” he said.

Four patients who Leis treated this summer had lost their ability to talk or write. Another was paralyzed on one side, as often seen in strokes, not West Nile infections. Others experienced recurring seizures.

In all, 11 of the first 12 patients Leis saw this year at the Methodist Rehabilitation Center in Jackson had more severe brain damage than he had seen previously. The outlook for such patients varies, but most will face years or a lifetime of disability.

“For the first time, we have radiographic evidence, clinical evidence of the virus attacking the higher cortical areas,” said Leis, who has published 15 scientific papers describing previous West Nile patients.

Marc Fischer, an epidemiologist at the Centers for Disease Control and Prevention who tracks the West Nile virus, said the federal agency has not noticed the changes described by Leis and Angus. “There’s just a lot more cases this year than anybody has seen in at least 10 years,” he said. “You’re just going to see more severe cases and probably a broader variety of manifestations.”

Last month, Leis asked a Food and Drug Administration scientist who studies the genetics of the virus whether a new, more virulent strain was circulating.

“You are absolutely right . . . that new genetic variants of WNV might have appeared this year,” the scientist replied in an Oct. 23 e-mail obtained by The Washington Post. The scientist continued that “it is not easy to correlate” the new mutations with any specific type of brain damage.

Thirty minutes after Leis received the message, another e-mail from the same scientist arrived. It said the previous message had been “recalled.”

When contacted by phone, the FDA scientist, who works at the agency’s Center for Biologics Evaluation and Research, declined to discuss the messages.

In an e-mail, FDA spokeswoman Heidi Rebello said that the agency is studying the genetics of West Nile viruses collected from 270 blood donors this year, but that “it is premature for us to draw any conclusions about new genetic variants . . . or of any possible association of new genetic variants with increased virulence.”

West Nile virus, made of error-prone RNA instead of the hardier DNA found in human cells, can evolve rapidly. In 2002, a new strain appeared that quickly churns out copies of itself inside mosquitoes. This fast-replicating version swiftly replaced the earlier dominant variety.

In 2003, another genetic variant, now dubbed the Southwestern strain, appeared in New Mexico and Arizona.

The West Nile virus, first described in Uganda in 1937, arrived in New York City in 1999, killing eight in the city. Infected birds transmit the virus to mosquitoes, which then infect people, who cannot infect one another. By 2003, the virus had crossed the country.

So far this year, health authorities have reported more than 5,000 cases of West Nile illness and 228 deaths in 48 states, with Texas, California, Illinois and Michigan having the most cases. Maine has had one confirmed case, a 34-year-old man from Cumberland County who was hospitalized in October. The man, who suffered from encephalitis and meningitis as a result of the virus, was released from the hospital and was recovering well as of the end of October, officials said.

Neurologists in Dallas have witnessed devastating encephalitis this year, but in different areas of the brain than Leis described. Steven Vernino, a neurologist at the University of Texas Southwestern Medical Center in Dallas, said he saw damage to the lower brain stem in several patients, but not to the higher language and thinking centers.

Virologist Alan Barrett will look at the genetics of viruses from North Texas as soon as he gets samples, which he expects any day. “Everybody wants to know what’s going on in Dallas,” he said.

Leis said it’s crucial to know whether the virus is mutating. “Otherwise,” he said, “we might be unprepared to deal with it in the future.”

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