State health officials are urging Marylanders to be wary of mosquito-borne diseases like West Nile virus and, now, the dengue-like virus chikungunya — raising concerns after it was reported in a Florida man Thursday.

The chikungunya case is believed to be the first that was contracted in the U.S.; other cases had been reported in people who had recently traveled to areas where the virus is prevalent.

That is raising concern over the possible spread of the virus, which is not usually fatal but can cause fever and debilitating joint pain and cannot be treated. Though experts considered chikungunya's spread to the United States inevitable, they are unsure if it will bring any outbreaks or how widespread they might be.

"It's certainly expected — a number of us had predicted it several years ago," said Barry Beaty, a professor of virology at Colorado State University and a member of the Baltimore-based Global Virus Network's chikungunya task force. "Mother Nature is conducting an experiment on us, unfortunately."

State health officials said this week they have begun annual monitoring for West Nile and also for chikungunya, urging residents to avoid mosquito bites by using repellent, wearing long sleeves and pants when concerned about mosquito exposure, and clearing yards and other outside areas of potential mosquito breeding grounds.

An invasive species known as the Asian tiger mosquito, which can carry both West Nile and chikungunya, prefers to breed in small, shady, man-made containers like empty cans or bottle caps, plant holders, wheelbarrows, garbage can lids and birdbaths. The Asian mosquitoes are known for aggressively pursuing humans, feeding during the day and following people into their homes and cars.

Neither disease is spread directly from person to person but is transmitted by bites from infected mosquitoes. Native mosquitoes also can carry the viruses.

Locally contracted cases of chikungunya have now been reported in 24 countries, including the United States. The virus spread quickly through the Caribbean, starting on the French side of the island of Saint Martin, and into South and Central America late last year. Outbreaks have previously been reported in Africa, Asia and Europe.

"It has just explosively spread," Beaty said. "The question will be: 'Will it explode in a big epidemic in the U.S.?' I think maybe not, but we need to see."

Experts with the federal Centers for Disease Control and Prevention said they believe chikungunya will behave similarly to dengue fever in the United States, with cases imported from other parts of the world causing sporadic local transmission but no major outbreaks. There have been more than 200 cases of chikungunya reported in the U.S. since 2006 among people who have traveled to other parts of the world, according to the CDC.

Most people who contract West Nile virus, meanwhile, do not develop symptoms, but those who do typically develop a combination of fever, headache, body aches, skin rash and swollen lymph glands.

"We expect to see West Nile virus cases each year because it is established in Maryland, but we are also looking for imported mosquito-borne diseases in Maryland residents," Dr. Katherine Feldman, Maryland public health veterinarian, said in a statement. "Maryland residents returning from a visit in the Caribbean who experience fever should seek medical care and make their provider aware of their recent travel."

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