Two elderly men from the San Fernando Valley are the first Los Angeles County residents to die this season of complications from West Nile virus, public health officials announced Friday.

Both men were hospitalized in August and died of West Nile virus-related encephalitis, according to an alert.

Their deaths cast a somber reminder that the mosquito-borne virus can be deadly, especially among older adults or those with compromised immune systems, said Dr. Jeffrey Gunzenhauser, interim health officer for Los Angeles County’s Department of Public Health.

“This serves as a warning that West Nile virus is a serious disease that may lead to hospitalization and can even result in death,” Gunzenhauser said in a statement. “Older adults and people with weak immune systems are at highest risk of severe illness.”

So far this season, 155 people have been sickened by West Nile virus in California, which is already a higher number than the same time last year. There have been a total of four deaths statewide.

In July, state health officials warned the public that this may be a severe West Nile virus season. Local vector control agents noticed that while the mosquito population has appeared to decrease this season, more mosquitoes caught in traps are infected with the virus.

Residents from the San Fernando and San Gabriel valleys and East Los Angeles are at heightened risk, vector control agents noted. But all Los Angeles County residents are encouraged to take protective action to prevent mosquito bites. That includes wearing insect repellent and dumping and draining stagnant water in the front and backyards of homes and properties.

“We expect to see West Nile virus activity in mosquito populations through the fall,” said Truc Dever, general manager for the Greater Los Angeles County Vector Control District, in a statement.

Last year, 53 Californians died after contracting West Nile virus. That’s the most since the state’s department of public health began recording cases in 2003 and the most in any U.S. state. Of those deaths in California, 24 were from Los Angeles County.

Known as a disease of wild birds, West Nile virus is spread to humans by the bite of an infected mosquito. Infection occurs between mosquitoes and birds. In rare cases, a person can be become ill through a blood transfusion.

Most people infected with the virus do not become seriously ill, but some experience flu-like symptoms and about 1 percent of them can develop serious neurologic illness. People who are 50 and older or who have pre-existing medical conditions such as diabetes or high blood pressure have the greatest risk of developing complications such as meningitis and encephalitis, health officials say.

There is no vaccine or specific treatment for the disease.