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BACKGROUND: Elevated serum IgG antibody to pertussis toxin (PT) after 2 weeks of cough is suggestive of recent B pertussis infection1but historically IgG positive cases in Colorado were not investigated because of concerns among public health staff and providers about the limited utility of this test. Involvement in the Emerging Infections Program (EIP) Enhanced Pertussis Surveillance (EPS) led to a statewide change whereby all serology-positive pertussis patients were investigated. During 2014, Colorado analyzed the outcomes of cases reported through IgG serology amid an ongoing outbreak of pertussis.
METHODS: Pertussis cases are reported to state or local health departments by clinical, reference, or public health laboratories following a positive pertussis laboratory result (e.g. PCR, culture, serologic assay) and maintained in a disease reporting database. Following notification of a positive laboratory result, relevant clinical information is collected through an interview with the patient and the patient’s physician. The patient is assigned a case status based on the Council of State and Territorial Epidemiologists (CSTE) case definition or removed from the database if found to not meet the CSTE case definition.
RESULTS: A total of 226 cases of pertussis were reported following a positive pertussis IgG antibody test during January 1- October 31, 2014. Ninety-nine (99) cases (44%) met CSTE’s probable case definition compared to 605/698 (87%) of cases with a PCR positive test. Pertussis cases with a positive IgG were older and had longer cough duration than those who tested PCR positive (median 50 years vs. 11 years; 32 days vs. 21 days).
CONCLUSIONS: Investigating cases of pertussis identified by a positive IgG antibody test added to the burden of pertussis disease documented in Colorado. In contrast to previous assumptions, physicians of patients who tested IgG antibody positive were often testing to diagnose acute disease and not for immunity. While there are difference in the epidemiology of PCR positive and serology positive cases, identifying these cases has allowed public health to implement disease control measures surrounding cases that were not previously investigated. Further examination of serology positive pertussis cases reported throughout 2014 is ongoing. 1American Academy of Pediatrics. Pertussis (Whooping Cough). In: Pickering LK, Baker CJ, Kimberlin DW, Long SS, eds. Red Book: 2012 Report of the Committee on Infectious Diseases. Elk Grove Village, IL: American Academy of Pediatrics; 2012: 554.