A cluster of suspected whooping cough cases in Colorado was actually most likely a "pseudo-outbreak," according to an investigation by the U.S. Centers for Disease Control and Prevention (CDC).

The cases, reported in the summer of 2009, seem to have been mistakenly diagnosed when patients' test samples became contaminated at one medical clinic. However, the CDC says, a cluster of whooping cough cases from the winter before likely was a true outbreak.

The pseudo-outbreak does not diminish the importance of the "real" cases of whooping cough that continue to crop up each year, said lead researcher Sema Mandal of the CDC.

"We've had strong evidence of other outbreaks," Mandal told Reuters Health, citing a 2010 outbreak in California as an example.

Whooping cough, also known as pertussis, is a bacterial infection that causes uncontrollable, severe coughing. Worldwide, it infects between 30 million and 50 million people a year, and kills about 300,000 -- mostly children in the developing world.

In the U.S., most children are immunized against whooping cough with the DTaP vaccine, which is given as a series of shots starting at the age of 2 months.

But cases still occur. According to the CDC, 27,550 cases were reported nationwide in 2010, with many more probably going unreported. There were also 27 deaths, nearly all in infants younger than one year.

California saw an outbreak of more than 9,000 cases, including 10 infant deaths, that year.

But in the Colorado outbreak, researchers suspected something was amiss.

Between November 2008 and September 2009, 125 cases of whooping cough were reported in the southwestern part of the state, mostly in children.

But in many cases, patients did not have classic symptoms -- like the distinctive "whoop" sound that's heard when people try to inhale during a coughing attack.

Many of the suspected infections were also in people who'd been vaccinated.

When the CDC investigated, it found differences between the cluster of "winter" cases that were diagnosed through April, and the "summer" cluster diagnosed from May through August.

While nearly all people diagnosed in the winter had classic whooping cough symptoms, only 63 percent of the summer cluster did. And people diagnosed in the summer were more likely to have only small amounts of pertussis DNA in their test samples -- which suggested the samples may have been contaminated.

Indeed, the CDC found pertussis DNA on surfaces at the clinic where most of the cases were seen. Of the sites the team swabbed -- from nurses' laptops to sinks to glove containers -- 61 percent had detectable pertussis DNA.

The researchers suspect that pertussis DNA from vaccines contaminated clinic surfaces, and then contaminated samples taken from patients to be sent to out for testing.

On top of that, Mandal pointed out, after the true winter outbreak, doctors were on heightened alert for whooping cough, which means they may have been testing more people than necessary. And that alone raises the odds of mistaken diagnoses.

Pertussis pseudo-outbreaks have been reported a few times before, for instance, in New Hampshire, Massachusetts and Tennessee between 2004 and 2006.

In response, the CDC has published "best practice" guidelines to try to avert the problem. One recommendation is for clinics to separate the area where providers prepare and give the DTaP vaccine from the area where they collect throat secretions from patients for pertussis testing.

Other "simple steps," Mandal said, are for providers to always wear gloves for collecting samples and for preparing and giving vaccines, and then immediately dispose of the gloves.

But people should not get the impression that whooping cough outbreaks aren't real, the CDC researchers stress.

"Pertussis is a real problem, and we strongly recommend vaccination," Mandal said.

Immunity against whooping cough from the childhood DTaP vaccine can wane over time, so experts recommend that pre-teens get a booster shot with the "Tdap" vaccine. Pregnant women who haven't had the booster should get one dose of Tdap after the 20th week of pregnancy, according to the CDC.

Maternal antibodies against pertussis will be passed on to the baby, which offers protection against the infection early in life.