Washington ? 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 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.