San Francisco,  California has confirmed its first human West Nile Virus (WNV) death in 2021 in San Luis Obispo County, health authorities said.

Tomas J. Aragon, director of the California Department of Public Health (CDPH), said on Friday that more cases of the fatal virus, which could be transmitted to humans and animals by the bite of an infected mosquito, had been reported recently, reports Xinhua news agency.

"West Nile virus activity in the state is increasing, so I urge Californians to take every possible precaution to protect against mosquito bites," he said.

As of July 9, WNV had been detected in 45 dead birds from six counties and 177 mosquito samples from 13 counties, the CDPH noted, adding hot temperatures this month are contributing to increasing numbers of mosquitoes and the increased risk of virus transmission to humans.

Nationwide, new reports from Centers for Disease Control and Prevention showed that Arizona, Arkansas, Illinois, Iowa, Massachusetts, New York and Colorado had detected mosquitoes that carry the WNVs, and humans had contracted the disease in four of these places.

The CDPH said WMV is influenced by many factors, including climate, the number and types of birds and mosquitoes in an area, and the level of WNV immunity in birds.

While the risk of serious illness to most people is low, some individuals, less than 1 per cent, can develop serious neurologic illnesses such as encephalitis or meningitis.