Public health officials warn that whooping cough poses the greatest risk to young children as California deals with its worst outbreak of the respiratory disease since 2010, with nearly 3,500 reported victims so far this year.

"Prevention of pertussis (whooping cough) is particularly important in young infants because they are the ones at risk for severe disease and death," Dr Gil Chavez, state epidemiologist and deputy director of the California Department of Public Health, said on Monday.

Infants, in particular, are the most vulnerable to the disease, which causes severe, uncontrollable spasms of coughing that can make breathing difficult, and public health officials urged parents to make sure their youngsters are properly vaccinated.

Vaccines also are recommended for pregnant women and adults who have increased contact with young children.

California public health officials said on Friday that whooping cough has reached epidemic proportions in the state.

More than 800 cases have been confirmed statewide in the first two weeks of June alone, with Napa, Sonoma and Marin counties near San Francisco Bay reporting the highest rates of infection per capita.

The highly contagious bacterial infection strikes 30 million to 50 million people each year worldwide and kills about 300,000 annually, mostly children in the developing countries.

In the United States, where outbreaks tend to run in cycles, most children are immunized against pertussis with a vaccine given as a series of shots beginning as early as six weeks of age.

Pregnant women should get the vaccine in their third trimester because antibodies will be passed on to their newborns, Chavez said.

Pertussis in young children usually begins with a runny nose and occasional coughing for up to two weeks, escalating into bouts of intense coughs punctuated with a characteristic "whooping" sound.

Infants typically do not exhibit the classic symptoms but may gag or gasp, and their faces may turn red or purple during those spells, Chavez said.

As of June 10 of this year, 3,458 cases of pertussis have been reported throughout California, far surpassing the 2,530 people diagnosed with the disease statewide for all of 2013.

Of this year's tally, 119 patients have been hospitalized, most of them under four months of age, and one, a 5-week-old infant, has died.

This year's outbreak so far pales in comparison with a whooping cough epidemic that struck California in 2010, when 9,000 cases, including 10 infant deaths, were reported.