United States health officials say four people recently infected with West Nile virus also recently received blood transfusions. The government is trying to determine whether the donated blood carried the virus. So far this year, more that 1,200 Americans have been infected with the mosquito-borne disease, and at least 46 have died.

The Centers for Disease Control and Prevention says it is looking for several blood donors in four states - North Dakota, Georgia, Louisiana and Mississippi - as it looks for a link between four cases of West Nile virus and blood transfusions. Dr. Jesse Goodman of the U.S. Food and Drug Administration says it is still possible the four infections were caused by mosquito bites, but officials want to make sure.

"It is an evolving situation, we are not sure there is a risk, it is biologically plausible, we are concerned about these reports, so we do want to prepare as though this is a problem," Dr. Goodman said.

Officials say the blood recipients live in parts of the country that have seen a high number of West Nile cases this year.

West Nile virus can cause encephalitis, a severe brain inflammation. Most people who get the virus suffer no serious effects, though it can be harmful to the elderly or those already ill. There have been fears recently that the virus could be transmitted through blood since four organ recipients were infected from a single donor in Georgia.

The spread of West Nile Virus this year was initially centered in the Southern states of Louisiana, Mississippi and Texas, but a growing number of cases are turning up in the Upper Midwest. Illinois now leads the nation in West Nile cases and deaths.

In Chicago spokeswoman for the Cook County Public Health Department, Kitty Loewy, said Chicago and nearby suburbs that have reported West Nile cases have been spraying to kill mosquitoes and their larvae. "What they did was apply some sprays last week. They will be doing more spraying next Tuesday and Wednesday, weather permitting," Ms. Loewy said.

This month, Chicago sprayed against mosquitoes for the first time in more than 25 years.

At the Centers for Disease Control and Prevention, Dr. Lyle Petersen said there is hope that the number of new West Nile cases will taper off soon. "Our data suggest that the epidemic has peaked in the Southern United States, although transmission is still ongoing and people need to take precaution," Dr. Petersen said.

He says the epidemic should peak in the Northern United States in another week or two. New cases should disappear completely in the North next month when the first freezing temperatures of the year kill any remaining mosquitoes.

West Nile Virus first appeared in the United States in 1999. It has spread to just about every state east of the Rocky Mountains. California also recently reported its first case of the virus.