State and county health officials say cases of pertussis, better known as whooping cough, are on the rise this year and another peak like the epidemic seen five years ago could be underway.
So far, California has reported 1,711 cases from January through May 12, more than triple the number for the same time last year.
And two infants have died -- one in Riverside and another in Placer County -- the first reported deaths from the respiratory disease since 2010.
In the central San Joaquin Valley, the increase in pertussis cases is hitting the hardest in Fresno County, which has reported 121 cases to the state compared to 41 cases in 2013. County health workers are investigating another 26 cases.
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Pertussis is a cyclical respiratory infection and Friday state health officials said it spikes every three to five years. It was epidemic in California in 2010: The state reported 9,159 cases and 10 deaths; Fresno County reported 550 cases and one death.
"As the last peak in California was in 2010, we are concerned that the recent increase in reported cases suggests that another cyclical peak is beginning," Dr. Ron Chapman, the state's health officer, said in a statement.
The illness is highly contagious and the bacteria is spread from person to person through droplets from coughs and sneezes. It typically starts with a cough and runny nose in children and worsens into a whooping sound.
In babies, a cough may not be present but the infant's face may turn red or purple. Adults may simply have a cough that lasts for several weeks but they can spread the disease to infants, who are vulnerable to complications.
Half of the 77 hospitalizations for pertussis in California this year have been babies, state health officials said.
In Fresno County, 33% of the cases this year have been in children younger than 5 and 40% have been children from ages 5 to 14.
At Children's Hospital Central California in Madera County, the emergency department has seen a dramatic increase in pertussis cases this spring, said Marsha Meyer, in charge of infection prevention and epidemiology.
Last week, Children's saw 29 children with pertussis, Meyer said. Six weeks ago, there were only two patients.
An increase in pertussis cases coincides with a deadly flu season. California has not issued a final report on influenza deaths, but a preliminary count shows 342 flu-related deaths in adults younger than 65. In the central San Joaquin Valley, 41 flu deaths have been reported with 22 of them in Fresno County.
While public concern mounted about the flu this past winter, Fresno County health officials were keeping a wary eye on pertussis.
"Pertussis was a concern of ours way back in January," said Joe Prado, manager of community health.
Vaccine is the best prevention, but Jared Rutledge, epidemiologist for the county's Department of Public Health, said: "If you have a cough for more than two weeks, please go see your physician."
The cough is unique, said Tom Booth, the county's supervising public health nurse. "It could be equated to machine gun fire. They cough until the magazine is empty and then they draw in some more air and cough some more."
Pertussis can be prevented by vaccination, and on Friday county health officials encouraged people to become immunized.
At a glance
Pertussis (whooping cough) vaccine information
should receive a vaccine booster during the third trimester of each pregnancy, even if they received it before.
Infants should be vaccinated as soon as possible. The first dose is recommended at 2 months of age. Young children need five doses of pertussis vaccine by kindergarten (ages 4-6).
Seventh-graders in California are required to get a vaccine booster shot.
Adults need vaccine booster shots, especially if they are in contact with infants or are health-care workers or have contact with pregnant women.
Source: California Department of Public Health