The number of West Nile virus cases reported in the United States through early September is the highest year-to-date total since the mosquito-borne disease was first detected in this country in 1999, federal officials said Wednesday. The number of fatalities had jumped by nearly a third from the previous week, they said.

Texas continues to be the state hit hardest, accounting for about half of all reported U.S. cases this year. Aerial spraying of insecticide in some areas has reduced the population of mosquitoes that carry the virus, officials there said. But the number of human cases is expected to rise through October because of the lag time between infection and reporting of the illness.

As of Tuesday, a total of 1,993 cases nationwide, including 87 deaths, had been reported to the U.S. Centers for Disease Control and Prevention, a 25 percent increase in the number of cases and a 32 percent increase in deaths from the previous week.

Asked about recent disease outbreaks, including the hantavirus outbreak traced to Yosemite National Park, Lyle Petersen, director of the CDC division of vector-borne infectious diseases, said pathogens are spreading faster because people and goods are moving around the planet at record rates. “The world is a smaller place right now,” he said.

U.S. health officials have notified 39 countries that their citizens might be at risk from the rodent-borne hantavirus after traveling recently to Yosemite. Six hantavirus cases, two of which were fatal, have been linked to the park. The CDC said that as many as 10,000 people were at risk after staying in Yosemite’s “signature tent cabins” between June 10 and Aug. 24.

For Texas, 2012 is “the worst year ever for West Nile virus,” the state health commissioner, David Lakey, told reporters during a conference call. The state had 1,013 confirmed cases and 40 deaths.

More than 70 percent of the cases this year have been reported in six states: Texas, South Dakota, Mississippi, Oklahoma, Louisiana and Michigan.

Most people who become infected have no symptoms. In Texas, 89 people found out they had been infected during routine screening for blood donation, Lakey said. A spokeswoman for the American Red Cross said volunteer blood donors are routinely screened for West Nile virus. As of Wednesday, more than 200 would-be donors in 28 states had tested positive this year for West Nile, and the numbers are expected to rise, Red Cross spokeswoman Karen Stecher said. Blood infected with the virus is destroyed or used for research, she said.

Public health experts said it is hard to know for sure what is behind this year’s large outbreak, or why Texas — particularly the Dallas area — has been hit so hard. But this summer’s heat waves and record temperatures are likely factors, the CDC’s Petersen said.

“We know that West Nile virus tends to occur when temperatures are above normal,” he 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.

Mosquito-borne outbreaks have always been difficult to predict, experts said. A complex set of environmental factors is involved in West Nile transmission, said Katherine Feldman, Maryland’s public health veterinarian. The virus lives in the blood of birds, and mosquitoes spread it to people and horses.

There is no vaccine for humans.

The extent of an outbreak is influenced by the number of mosquitoes and how infectious they are, the population of susceptible bird species, rainfall and temperature. Bringing “all these things together in the right combination at the right time facilitates that the virus go faster and to a greater degree in certain areas of the country,” said Roger Nasci, chief of arboviral diseases at the CDC.

“The hotter it is, the mosquitoes tend to be more infectious, and it also affects how long a mosquito may live,” Petersen said.

Floods wash out mosquito breeding sites. But the right amount of rain can produce ideal breeding conditions.

Robert Haley, director of the epidemiology division at the University of Texas Southwestern Medical Center at Dallas, said he suspects that particular local weather conditions made Dallas ground zero for West Nile virus this year.

A mild winter allowed more female mosquitoes to survive. West Nile infection among birds was relatively mild last year, meaning more birds would be susceptible this year. Haley, who lives in North Dallas, said he suspected that something was amiss when he saw two dead bluejays in his yard in July. Bluejays and crows are among those species that tend to die from the virus, he said.

Dallas also had a hot, dry summer with rain every three to four weeks that replenished the stagnant pools in which mosquitoes breed, he said.

West Nile disease can vary in severity. The onset of symptoms can take from a few days to two weeks. People 50 or older have the highest risk of severe illness.

About 80 percent of people who are infected will not develop any illness. About 20 percent will develop West Nile fever. Symptoms include fever, headache, tiredness and body aches. Occasionally, there will be a skin rash and swollen lymph glands.

The most severe type of infection causes inflammation of the brain or of the tissues surrounding the brain and spinal cord. In those cases, symptoms include headache, fever, stiff neck, muscle weakness and paralysis.

Of the West Nile disease cases reported to the CDC this year, 1,069, or 54 percent, were considered severe.

Health officials say residents should use insect repellent when outdoors, especially at dusk and dawn, when mosquitoes are most active. Residents should also eliminate mosquito-breeding areas by emptying birdbaths, flowerpots, buckets and barrels where rainwater collects.