Mosquitos are sorted at the Dallas County mosquito lab in Dallas. U.S. health officials say there's been an alarming increase in the number of West Nile cases. So far there have been more than 1,100 cases reported through the middle of August. That's three times as many as usually seen at this point in the year. (LM Otero/AP)

The West Nile virus epidemic in the United States this year is on track to be the deadliest ever since the mosquito-borne disease was first detected here 13 years ago, federal officials said Wednesday.

The number of severe infections, such as brain inflammation, is the highest year-to-date since 1999, when the virus was detected in New York City, according to figures released by the U.S. Centers for Disease Control and Prevention. Severe infections are considered the best indicator of the epidemic’s scope because those cases are most consistently reported to health authorities. Most people who get bitten by infected mosquitoes don’t develop symptoms, and their cases don’t get reported.

“This is the largest outbreak to date and certainly the most serious,” said Lyle Petersen, director of the CDC’s Division of Vector-Borne Infectious Diseases.

The Mid-Atlantic region has not been among the hardest hit. Maryland reported its second death Wednesday, bringing the region’s total to four fatalities. Virginia and the District have each reported one death. Texas, particularly the Dallas area, has been the most heavily affected region in the country.

Officials said they believe the worst of the outbreak is over.

But case numbers will rise through October because of the time lag between the onset of symptoms, doctor visit and reporting to authorities. People who were infected two to three weeks ago are just getting their cases reported now.

As of Tuesday, a total of 2,636 cases nationwide, including 118 deaths, had been reported to the CDC, about a 35 percent increase over the numbers reported the previous week.

Of the total cases, 1,405, or more than half, were the most severe type of infection, classified as neuroinvasive, which can include brain inflammation or polio-like paralysis of the arms or legs. About 8 to 10 percent of those people eventually die.

“If this year turns out to have the most neuroinvasive cases of any year, which is what we’re on track for, we expect this year to have the largest number of deaths of any year,” Petersen said.

The record number of U.S. cases for a full year was reported in 2003, with 9,862 cases and 264 deaths, but the most West Nile deaths were reported in 2002, with 284. The numbers in 2003 are somewhat of an aberration, officials said. The reported case total was extremely high primarily because one state tried to test for as many cases of West Nile fever, a milder illness that typically doesn’t get reported, Petersen said. Officials said later the state was Colorado.

This year’s epidemic has created a huge demand for West Nile test kits, leading to a temporary shortage of West Nile test kits earlier this week. State health departments told the CDC that several commercial laboratories were running out of kits on Monday and Tuesday. But back orders were packed and shipped Tuesday and Wednesday, and the shortage has been resolved, said Roger Nasci, a CDC expert on the West Nile virus.

The major manufacturer of test kits said orders for the kits have skyrocketed.

“We have eight times the demand, and we don’t have eight times the supply,” said Jay M. Lieberman, medical director for infectious disease at Quest Diagnostics. A Quest subsidiary is the major kit supplier, and four Quest labs have been among the commercial labs doing West Nile virus testing.

“No one anticipated that we were going to have a West Nile season of this magnitude,” he said.

Even with increased production, the company has not been able to provide labs that order a two-month supply of kits in advance. Instead, the company has been “allocating” kits as they become available, and will be able to meet the need for the remainder of the season, he said.

A person who gets infected with West Nile develops immunity for life. The disease is not contagious from person to person. It is only transmitted by mosquitoes who bite infected birds.

About 80 percent of people who get bitten show no symptoms at all. About 20 percent will develop West Nile fever, with symptoms that range from fever, headache and tiredness to more serious illness that requires hospitalization. About 1 percent of cases are the most severe type of infection, known as neuroinvasive, because they cause inflammation of the brain or tissues surrounding the brain and spinal cord.