Mosquitoes trapped near Cox Creek in northern Anne Arundel County have tested positive for the West Nile virus, the first appearance in the state this year of a disease that killed seven Maryland residents in 2002.
State officials battling a rain-enhanced mosquito boom this summer say it's the earliest they have seen West Nile-infected mosquitoes since the viral invasion began in the United States in 1999.
The demand from Maryland communities for spray attacks on adult mosquitoes is so high that hard-pressed mosquito control officials have closed 13 counties to new requests.
"I don't think we've ever had a year like this," said Cyrus Lesser, chief of the state Department of Agriculture's mosquito control division, who has been in the business since 1970. "We're just looking at a lot more work than we can handle."
No human West Nile illness has been reported in Maryland so far this year. Last year, 36 people were stricken, and seven died. Thirty Maryland horses fell ill, and 13 died.
The virus is now active in all but four of the continental United States. Three human cases have been reported nationwide this year: one in South Carolina and two in Texas. Last year, more than 4,100 cases were reported, of which 284 were fatal.
This year, Maryland is not collecting or testing birds, which carry the disease, because the virus is known to be entrenched statewide. But mosquito surveillance continues for both the West Nile virus and eastern equine encephalitis.
Equine encephalitis is a rarer but more deadly mosquito-borne illness, killing half the people who get it. It has been reported this season in horses, birds and mosquitoes in Florida, Georgia, Mississippi, the Carolinas and Virginia. A Georgia man died from the infection June 21.
Maryland's first West Nile-infected mosquitoes for this year turned up among 40 Culex salinarius mosquitoes trapped July 1 at a Maryland Port Authority dredge spoil site near Cox Creek, just south of the Baltimore City line.
"Unfortunately, it's the earliest we have ever recovered a virus-carrying mosquito ... in the 10 years we've been doing surveillance for mosquito-borne viruses," Lesser said. Usually, they don't show up until late July.
It's not clear what their early appearance means. "Only a very small percentage of mosquitoes are actually carrying the virus at any time, so to collect one is significant in itself," he said.
"We do anticipate we're going to see more positive mosquito pools this year, simply because there are a lot more mosquitoes out there this year, and over a wider geographical area," Lesser said.
The Cox Creek site had never yielded West Nile-infected mosquitoes before. If more turn up there in the coming weeks, "that would probably indicate that a lot more virus is being circulated now than we would hope to see," Lesser said.
Residents of the area, especially those older than age 50, should take precautions to avoid being bitten, Lesser said. But so should people elsewhere. "Mosquitoes carrying the virus are in a large part of the state right now," he said.
Tracy S. DuVernoy, acting state public health veterinarian, urged everyone to be "proactive" by minimizing standing water where mosquitoes breed, applying repellent when outdoors and making sure window and door screens are intact.
People have heard all this before, she said, but "it's now paramount, absolutely."
The state responded to the discovery at Cox Creek by conducting aerial spraying Friday evening, sooner than it originally had planned.
The Agriculture Department said yesterday that the demand for mosquito spraying is now 20 percent above normal statewide, with 2,200 communities participating. Stretched thin, the agency has closed more than half the state to new requests for service, including Anne Arundel, Baltimore, Carroll, Charles, Frederick, Howard, Montgomery, Prince George's, St. Mary's, Talbot, Washington, Wicomico and Worcester counties.
A short break from the rains on the Eastern Shore recently reduced mosquito counts in some locations there, Lesser said. But personnel sent to offer their arms as bait "can still get 20- to 40-a-minute landing rates, as well as collecting hundreds, to a couple thousand per night in light traps.
"If it keeps on at this rate, it's going to be the worst year we've had," he said.
There is some good news: Populations of the Asian tiger mosquito, a daytime biter and a particularly bothersome new urban and suburban pest, have been low so far this season. But that, too, could change if the rains let up, water levels in backyard containers drop and female tiger mosquitoes get a chance to lay their eggs just above the waterline.
The West Nile virus is carried by birds and transmitted by mosquitoes to other birds, horses and people. Most infected people experience no illness, or only mild flu-like symptoms.
Fewer than 1 percent suffer West Nile encephalitis or meningitis, which are potentially debilitating or fatal inflammations of the brain or spinal cord. Older people and those with weakened immune systems are particularly vulnerable.
A West Nile vaccine for humans is under development. A horse vaccine is available.