Evaluation of the Pertussis Surveillance System in Illinois

BACKGROUND:  Pertussis, also known as “whooping cough”, is a highly contagious bacterial respiratory illness caused by Bordetella pertussis. In 2012, the incidence of pertussis in Illinois (13.5/100,000) was higher than the national incidence. High quality pertussis surveillance systems are useful for monitoring trends, identifying outbreaks, and targeting and evaluating interventions.

METHODS: The evaluation was based on the Centers for Disease Control and Prevention’s “Updated Guidelines for Evaluating Public Health Surveillance Systems”. Illinois pertussis reporting rules and laboratory standard operating procedures were reviewed, and in-person interviews were conducted with key stakeholders. Data stored in the Illinois National Electronic Disease Surveillance System (I-NEDSS) were examined.

RESULTS: Local health departments and hospital infection preventionists report that the pertussis surveillance system in Illinois is easy to use. The flexibility of the system is fair; a suspect case classification was introduced in 2012 that was easily integrated into the system. 475 suspect cases exist in I-NEDSS from years prior to 2012, when the suspect case classification did not exist. The data quality varies; missing data elements range from 18 to 95% for eight outbreak-related fields and 5 to 7% for three clinical fields. The acceptance of the system by stakeholders is excellent, as measured by the timeliness of disease reporting. The median number of days from date of earliest pertussis report to the date received by the local health department is zero. The positive predictive value of the system is good, despite potential false positive results for B. pertussis in I-NEDSS from the use of PCR assays that cannot rule out Bordetella holmesii. The representativeness of the system is fair. Differences in pertussis rates (e.g. 8/100,000 in Saline County compared to 14/100,000 in Cook County) may reflect relative lack of access to diagnostic testing in some areas of the state. Timeliness is excellent; the median length of time between the “open” date and “complete” date of pertussis investigations is three days.

CONCLUSIONS:  Strengths of Illinois’ pertussis surveillance system are ease of use, acceptability and timeliness. Weaknesses are flexibility, data quality, and representativeness. The findings from this surveillance evaluation will guide the Illinois Department of Public Health’s efforts to improve the system. Such efforts can include labeling suspect cases prior to 2012 as not meeting the new suspect case classification, and requiring outbreak-related fields to be completed for cases associated with an outbreak.