by Joanne Faryon | KPBS

The whooping cough our parents — or even grandparents — were exposed to is probably not the same whooping cough now causing the worst epidemic in California in 50 years. The Centers for Disease Control has confirmed six of the seven cases it’s studied so far have been caused by a strain which produces more toxins.

The number of whooping cough, or pertussis, cases has been steadily increasing in the U.S. since the 1990s, despite high vaccination rates.

At around the same time, scientists discovered a more virulent strain of the illness was also emerging.

In a paper published in the CDC’s Emerging Infectious Diseases, researchers in the Netherlands found the new strain produced almost twice as much toxin as its predecessor.

Dr. Tom Clark, with the Centers for Disease Control, said he wasn’t surprised to find that particular strain in the majority of cases because it is now the most prevalent strain worldwide.

The CDC can’t make any conclusions about whether that strain is influencing the current outbreak, he said.

“It doesn’t tell you if that makes it more likely to be transmitted or whether it’s more likely to cause more severe disease. We’re not seeing more deaths now than we would expect.”

In California, more than 3,000 people have been diagnosed with whooping cough and eight babies have died since January.

Scientists in the Netherlands did find a correlation between the increasing incidence of the more virulent strain and increased hospitalizations and deaths.

Joanne Faryon is a freelance reporter and former inewsource and KPBS reporter.