The presence of West Nile virus in the San Fernando Valley and Long Beach has picked up pace as more mosquitoes tested positive with the potentially deadly illness, vector control officials announced last week.

At least 15 mosquito samples were confirmed to be infected this past week, including those in Encino, Van Nuys, Sherman Oaks, Reseda, North Hills and Northridge, according to the Greater Los Angeles County Vector Control District.

In addition, West Nile antibodies were found in five of sentinel chickens used for virus surveillance in east Long Beach. Chickens do not become sick from the virus, vector officials said, but the presence of antibodies in their blood shows active transmissions between mosquitoes and birds.

“This confirmation is just further indication that the virus is very active,” said Levy Sun, a spokesman for the county’s vector control district. “As temperatures continue to stay hot, we can expect more virus activity.”

The acceleration of infected mosquitoes and birds can be seen statewide as well.

Data provided by the California Department of Public Health shows that so far, 19 people have been sickened with West Nile virus, compared with 14 last year. The number of dead birds infected this year compared with the same time last year has nearly doubled, from 454 to 1,014.

West Nile virus is transmitted to humans and animals by the bite of an infected mosquito. While the risk of illness is low, at least 1 percent of those infected can develop encephalitis or meningitis. Those 50 years of age and older, or those with diabetes and/or hypertension, are at greatest risk for serious illness, according to state health officials.

The increased presence of West Nile during a drought may be surprising, but actually mosquito season tends to pick up in dry times, said Kelly Middleton, spokeswoman for vector control.

“We actually see more West Nile virus activity in drought years,” she said. “This year in California, there are parts that are seeing more activity than usual.”

One reason is because mosquitoes and birds share limited water supplies, which then increases transmission between the two.

Meanwhile, health experts say the increase in activity is a good time to remind residents to remove stagnant water around homes, wear insect repellent and proper clothing and make sure to fix all window and door screens that have holes.