Indiana Epidemiology Newsletter
Karee L. Buffin, MS
ISDH Vaccine-Preventable Disease Epidemiologist
Pertussis, more commonly known as “whooping cough,” is caused by the Bordetella pertussis bacterium. It has gained attention in recent years due to the increase in the number of reported cases in the United States. In 2004, 25,827 cases were reported nationwide to the Centers for Disease Control (CDC). This was the largest number of reported cases of pertussis since 1959.1 The increase in pertussis disease activity was also significant in Indiana. In 2005, the Indiana State Department of Health (ISDH) reported 396 cases of pertussis to the CDC. This was the highest number of reported cases in Indiana since 1964. In 2006, the ISDH reported 280 cases of pertussis.
Figure 1: Pertussis Incidence in Indiana: Counties Reporting Five or More Cases
Because of the cyclic nature of pertussis (marked by a saw-toothed pattern every 3-5 years, see Figure 2)2 , a decrease in the number of cases from 2005 to 2006 was theoretically expected. However, the continued drop in reported cases from 2006 through 2007 was more dramatic than expected. In 2007, 67 cases of pertussis were reported in Indiana. In the previous 10-year period (1994-2003), the baseline for pertussis was 124 cases. Surprisingly, only two Indiana counties reported five or more cases in 2007.
Although the reporting of vaccine-preventable diseases has been a challenge, awareness of the increase in pertussis cases due to media attention likely would have caused an increase in reporting and would not have contributed to the decrease in reported cases documented in 2007. Therefore, reporting should be considered a constant, because there are no known factors that would have caused a decrease in reporting.
Because pertussis can be clinically confused with other cough illnesses, it is critically important to perform the most effective laboratory testing in order to differentiate pertussis from other illnesses.3
Performing currently recommended laboratory testing, rather than relying on less accurate serologic testing, may have led to more accurate reporting and may have eliminated the reporting of some false positive cases in 2007. Although nasopharyngeal culture is considered to be the “gold standard” for detecting the presence of pertussis, only three cases in 2007 were culture confirmed in Indiana. Polymerase chain reaction (PCR), although not currently available at the ISDH Laboratories, was used to detect 21 of the 67 reported cases of pertussis in Indiana in 2007. All of the cases met the CDC case definition for pertussis: a cough illness lasting at least two weeks with one of the following: paroxysms of coughing, inspiratory “whoop,” or post-tussive vomiting, and without other apparent cause (as reported by a health care professional).
Tdap vaccine was licensed in 2005. The Tdap booster has been labeled as the newest tool in fighting the rising number of pertussis cases. Since the introduction of the Tdap vaccine, the number of reported cases of pertussis in Indiana has fallen to levels not seen since 1992, when there were 64 cases reported. In 2006, the CDC stated that the decrease in pertussis incidence was more likely due to the cyclic nature of the illness and less likely to be related to the use of Tdap.4
In addition to introducing Tdap boosters to both adults and eligible children, local health departments aggressively administered Tdap as an outbreak response tool. Outbreak response clinics may also have aided in preventing an increase in cases. Infection control personnel have also promoted Tdap administration to health care workers.
To what degree the use of Tdap has influenced the decline in pertussis cases in Indiana remains an interesting topic for discussion. Arguably the numbers should continue to cycle below expected baseline values if the vaccine continues to be administered and has contributed to the reduction of cases.