Health officials are investigating whether a Mississippi woman contracted the West Nile virus through a blood transfusion, the second suspected case of West Nile transmission through blood.

With no blood screen test for West Nile available, the investigations are prompting concern that the virus could travel through the blood supply undetected. Still, health experts said, any risk is minimal and far outweighed by the medical need for blood.

Nationally, the number of confirmed West Nile cases this year has topped 850, with 43 dead, the Centers for Disease Control and Prevention said Thursday. Many others are infected but never show symptoms, so there could be 110,000 to 150,000 people who have been infected in the United States, most of whom will never suffer its effects or know they have the virus, said Dr. Anthony A. Marfin of the CDC.

That number is likely to grow in the coming weeks as West Nile peaks, but cases should drop off as the weather gets colder and disease-carrying mosquitoes disappear, officials said.

The Mississippi woman received 18 units of blood during an obstetrical procedure in late July, Marfin said. Soon after the surgery, she was diagnosed with West Nile encephalitis. The woman is now recuperating at home.

Health officials cautioned that the woman may have been infected by a mosquito, as other West Nile victims have, noting that she lives in an area with many cases of the disease. She reported having been bitten by mosquitoes many times, investigators said.

Even as they investigate whether the disease can be transmitted through the blood, officials are certain that West Nile can be spread by organ transplants. Four patients were infected with the disease after receiving organs from an infected patient.

Three of the four organ recipients had been diagnosed earlier in the week; the fourth, a 71-year-old Florida woman, was confirmed with the disease Thursday.

Also Thursday, a House committee approved legislation offering new money to control mosquitoes, which spread the disease.

Authorities now have two similar investigations under way aimed at determining whether blood transfusions were the source of infection for the Georgia organ donor and for the woman in Mississippi as they try to determine whether the virus it can be transmitted through blood.

The organ donor had received blood from 63 people as doctors tried to save her after a severe car accident. The Mississippi woman received blood from 18 people during her surgery.

CDC investigators were collecting samples from all of these blood donors to check for traces of the virus. It is also possible that either or both of the women contracted West Nile from a mosquito bite.

In particular, the Mississippi woman lived in an area where there are many infected mosquitoes. "It's very hard to sort that out," said Marfin, an epidemiologist with the CDC's Division of Vector-Borne Infectious Diseases.

Her case was discovered by state epidemiologists who were reviewing a sample of infected patients and reviewing their histories to see what they may have in common. They were also looking for anyone who might have received a significant amount of blood before becoming infected.

CDC officials cautioned that many people are now infected with West Nile, so some of them are likely to have received blood as well. That alone does not prove that the virus is carried by blood, said Dr. Lyle Petersen, deputy director of the CDC's vector-borne disease division.

Transmission by blood, he said, remains a "theoretical possibility at this point."

Health officials are now virtually certain that the virus can be spread through organ transplants, and the CDC was advising transplant doctors to be alert for West Nile in their patients. But that information does not warrant changes to national blood or transplant policies, they said.

Federal officials have urged blood banks to pay particular attention to would-be donors, screening out anyone who is sick and may have West Nile. But many people infected do not appear to be sick, and there is no blood screening test available now. So even if the disease does prove blood-borne, there is little more than can be done.

"We're obviously very concerned," Eve Slater, assistant secretary for health at the Department of Health and Human Services, said Thursday. "Screening methods can't be developed overnight."

For each person who becomes severely ill, experts believe there are an additional 30 who get mildly sick and 120 to 150 others who do not show any symptoms. It is possible those healthy but infected people could carry the virus to others.

In the most severe cases, West Nile causes a potentially fatal brain inflammation. Others get a flu-like illness, with fever, headache and muscle pains, that lasts two or three days.

West Nile, which emerged in the United States just three years ago, has struck other countries for decades, from the tip of Africa up to Europe and throughout Asia. It had not appeared on this continent until 1999. It is spread by mosquitos, who are infected by birds and then infect people when they bite.

As of Thursday, the CDC had confirmed 854 cases in 28 states and the District of Columbia this year.

Based on similar diseases and West Nile on other continents, Petersen said that in the future he expects the number of cases to be quite low in many years, with small outbreaks in others and occasional epidemics like the nation is seeing now.

On Thursday, the House Energy and Commerce Committee approved legislation authorizing $100 million in grants to communities for mosquito-control programs. The measure must compete with other programs for actual funds during the budget process.

Too many communities have not have conducted assessments of the problem or prepared plans to control mosquitos, said the committee chairman, Republican Rep. Billy Tauzin of Louisiana, the state hardest hit by West Nile. He said just 18 of 64 Louisiana parishes have programs going.