LOS ANGELES, CA -- More mosquitoes collected in the San Fernando Valley have tested positive for the West Nile virus, according to health officials. Two samples from Sherman Oaks and Porter Ranch tested positive last week, officials with the Greater Los Angeles County Vector Control District announced this weekend.

Earlier this month, a sample from Panorama City tested positive for the virus, which can cause severe infections among adults age 50 and older. District health officials said West Nile activity was off to a slower start this year than during previous summers.

"Last year, by the end of week 29, we already had 43 positive samples reported," said Susanne Kluh, the District's director of scientific technical services. But Kluh and other officials said the prospect of additional hot days before the end summer could result in increased reproduction of mosquitoes, which can spread the virus.

Health officials reminded residents and business owners to remove standing water on their properties and to clear gutters, ditches, storm drain channels, basins, non-functioning pools and ponds, and to monitor other potential mosquito breeding sites. West Nile Virus is transmitted to people and animals through the bite of an infected mosquito, although most people who contract the virus do not experience any symptoms.

One in five persons infected with the virus will exhibit symptoms such as fever, headache, body aches, nausea, or a skin rash, which can last for several days to months.

One in 150 people infected with the virus may require hospitalization for severe symptoms including high fever, muscle weakness, neck stiffness, coma or paralysis.