Health officials say pertussis, commonly known as whooping cough, reached record numbers in Wisconsin in 2004.

Around 20 to 25 percent of the nation’s whooping cough cases are from Wisconsin, and officials are attempting to discover why.

“Our provisional data indicates that 2004 had an excess of 5,100 confirmed or probable cases of pertussis in Wisconsin, with numbers still being counted,” said Wisconsin Director of Immunization Dan Hopfensperger.

The Madison Public Health Department estimates the city saw 408 cases in 2004, compared to only 40 in 2003.

Whooping cough is an infectious disease caused by the colonization of the bacteria Bordatella pertussis in the respiratory tract. The disease is characterized by severe coughing, which leaves the individual gasping for breath and making “whooping” sounds.

Whooping cough is typically seen in children when common cold-like symptoms progress into coughing spells. Symptoms can last for months if untreated.

Some health officials in Wisconsin say the numbers are heightened due to new testing procedures that have led to increased sensitivity in detecting the disease.

Two testing procedures commonly used in the Wisconsin State Laboratories include a traditional culture method, which isolates the bacteria causing whooping cough, and a molecular method known as the Polymerase Chain Reaction.

“PCR increases our sensitivity and detection a little bit over two-fold,” Dave Warshauer, Assistant Director of Communicable Diseases of the State Laboratory, said. “We’ve been using the test since the beginning of 2002, so a portion of the increase [in whooping cough cases last year] is due to increased sensitivity, but this does not account for the large increase that is being seen.

Many health officials agree while the improved tests play a role in the large incidence of whooping cough, it is not the primary reason for the heightened numbers seen in 2004.

“This seems to be a true increase in disease, rather than an increased testing sensitivity,” Amanda Kita-Yarbra, a Madison disease epidemiologist, said. “Certainly [the increased sensitivity of testing] is a part of it, but this is a significant increase from last year’s [incidence].”

A decreased immunity to the bacterial agent may be yet another cause for the rise in whooping cough in 2004.

A pertussis vaccine approved only for children younger than seven years old is available; however, the immunity to the disease wanes with age, according to Warshauer.

Some health officials believe Wisconsin’s high incidence rate of the disease is due to the strength of the public health-response, compared to that of other states less equipped to deal with outbreaks.

“Part of [the increase] may be due to our active and aggressive public-health response to outbreaks, and we are probably seeing more than other states for that reason,” Warshauer said.

While health officials cannot predict what 2005 will bring, most are confident the numbers will be lower than last year.

The CDC says whooping cough seems to follow a cyclical pattern, peaking every three to five years.

While whooping cough is commonly thought of as a disease of the early 1900s, its incidence has risen in the past few decades. The World Health Organization estimates 20 to 40 million cases of whooping cough worldwide, with an estimated 200,000 to 400,000 fatalities annually.

“Someone who has a cough or respiratory infection should be considered [to have a possible case of whooping cough],” Warshauer said. “It is important to identify these people and have them treated so they do not transmit the disease to others.”