For reasons health officials cannot explain, the victims of the mosquito-borne West Nile virus are younger than usual this year.

The median age for this year's infections is 55, the Centers for Disease Control and Prevention said in a report released Thursday. In previous years, the median was in the mid-60s.

"The reason patients seem to be younger this year is unknown and certainly something we're looking into," said Dr. Lyle Petersen, a CDC expert.

However, CDC officials said the declining median age is not a cause for immediate concern.

Most people bitten by an infected mosquito will suffer no more than flu-like symptoms, but the weak and the elderly can get encephalitis, a potentially fatal brain inflammation.

Petersen said advanced age remains the biggest risk factor for getting sick from the bite of an infected mosquito. Health officials do not compile statistics on any health problems the younger West Nile victims had, but suspect many had conditions that weakened their immune systems.

Men made up 60 percent of this year's West Nile cases, consistent with years past. Petersen said that is probably because men spend more time outdoors.

Gary Simon, an infectious-disease expert from George Washington University, said doctors may be more aware of the disease and likely to look for it in patients with less serious symptoms, explaining the lower median age.

"It may be because we're testing people more," he said. "It's the only possible explanation from the information I've seen so far."

Since its first appearance in the United States, the virus has been detected in 35 states and Washington, D.C.

State and local officials have boosted mosquito-spraying efforts and urged people to protect themselves by using bug repellent and wearing long sleeves.

Encephalitis is usually seen in August and September, but Louisiana's first patients became ill in June. The West Nile virus is showing up earlier in the summer as it spreads to warmer climates, according to the CDC.