Maryland Newsline

• More Information on the virus from the Centers for Disease Control and Prevention

WASHINGTON - Reported cases of West Nile virus plummeted in Maryland this year in both humans and horses, a change that officials said was likely brought about by prevention efforts and weather that was unfavorable to the virus.

The Department of Health and Mental Hygiene reported only 11 human cases this year and no deaths, down from 73 people who were infected with the virus and eight deaths in 2003.

And while more than 200 horses got West Nile last year, and 75 died from it, the first case of an infected Maryland horse was just reported Friday in Cecil County.

The season is winding down, with formal surveillance stopping at the end of October.

"We would love to think it's because of all the public education messages we've been putting out," said Kim Mitchell, West Nile virus coordinator at the health department.

But, she said, "It's hard to pinpoint a single reason or factor responsible."

People infected with West Nile virus usually experience minimal to mild flu-like symptoms, but in individuals more than 50 years old or with other risk factors, the virus can lead to potentially fatal inflammation of the brain and spinal cord.

The disease is spread by mosquitoes that bite infected birds then bite humans, horses or other animals.

Cyrus Lesser, mosquito control chief at the Maryland Department of Agriculture, said less rain and cooler weather prevented mosquitoes from flourishing as they did last year, when the population was one of the largest since 1958.

With less standing water, there were fewer places for mosquitoes to breed. And the cooler weather made it harder for the virus to reproduce while it was carried in mosquitoes, he said.

By comparison, last year's mosquito population was so high that the largest collection by a single mosquito trap was more than 10,000 female mosquitoes. This year, the highest trap collection was about 1,400 females, Lesser said.

He said his department has been especially aggressive in controlling mosquito numbers this year by spraying larvicides from trucks and stocking storm-water ponds with thousands of "mosquito fish," a 2-inch fish that eats the bug's larvae. Those measures will likely continue through October, he said.

Mitchell said the decline of horses infected with West Nile could be due to horse owners taking more precautions such as vaccinating their animals, using fans to disperse mosquitoes and getting rid of standing water.

Another possibility, she said, is that the birds that carry the disease could have gone elsewhere. She said it is unlikely, but possible that people are starting to develop an immunity to the virus.

Despite the drop in cases this year, Mitchell said West Nile virus is still considered endemic in Maryland -- meaning that it is here to stay.

She said people should continue to take measures to protect themselves, including getting rid of standing water, wearing long sleeves at dawn and dusk and using insect repellent. It's impossible to predict what next year will hold, she said.

"It might hit us hard."