Pertussis, 2008

During 2008, 1,034 cases of pertussis (19.9 per 100,000 population) were reported, compared to 393 in 2007 and a peak of 1,571 cases reported in 2005. Laboratory confirmation was available for 644 (62%) cases, 100 (16%) of which were confirmed by culture and 544 (85%) of which were confirmed by PCR. In addition to the laboratory-confirmed cases, 202 (20%) cases were epidemiologically linked to laboratory-confirmed cases, and 188 (18%) met the clinical case definition only. Seven hundred fourteen (69%) of the reported cases occurred in residents of the metropolitan area.

Paroxysmal coughing was the most commonly reported symptom. Nine hundred fifty-four (92%) of the case-patients experienced paroxysmal coughing. Nearly one-third (280, 27%) reported whooping. Although commonly referred to as “whooping cough,” very young children, older individuals, and persons previously immunized may not have the typical “whoop” associated with pertussis. Post-tussive vomiting was reported in 420 (41%) of the cases. Infants and young children are at the highest risk for severe disease and complications. Pneumonia was diagnosed in 29 (3%) case-patients, two (7%) of whom were less than 18 months of age. Twenty-four (2%) case-patients were hospitalized; 14 (58%) of the hospitalized patients were younger than 6 months of age.

Due to waning of immunity from either natural infection or vaccine, pertussis can affect persons of any age. The disease is increasingly recognized in older children and adults. During 2008, case-patients ranged in age from 16 days to 87 years. Two hundred thirty-five (23%) cases occurred in adolescents 13 to 17 years of age; 238 (23%) cases occurred in adults 18 years of age and older; 434 (42%) occurred in children 5-12 years of age; 85 (8%) occurred in children 6 months through 4 years of age; 40 (4%) occurred in infants less than 6 months of age, and 2 (<1%) occurred in persons of unknown age.

Infection in older children and adults may result in exposure of unprotected infants who are at risk for the most severe consequences of infection. During 2008, 53 pertussis cases were reported in infants less than 1 year of age. A likely source of exposure was identified for 20 (38%) cases; nine (17%) were infected by adults 18 years of age and older, two (4%) were infected by an adolescent 13 to 17 years of age, and 6 (11%) were infected by a child less than 13 years of age. For the 33 cases with no identified source of infection, the source was likely from outside the household.

Although unvaccinated children are at highest risk for pertussis, fully immunized children may also develop the disease. Disease in those previously immunized is usually mild. Efficacy for currently licensed vaccines is estimated to be 71 - 84% in preventing serious disease. Of the 102 case-patients who were 7 months to 6 years of age, 74 (73%) were known to have received at least a primary series of three doses of DTP/DTaP vaccine prior to onset of illness; 13 (13%) received fewer than three doses and were considered preventable cases. Vaccine history was unavailable for the remaining 15 case-patients.

MDH reporting rules require that clinical isolates of Bordetella pertussis be submitted to the PHL. Of the 100 culture-confirmed cases, 94 (94%) of the isolates were received and sub-typed by PFGE and tested for antibiotic susceptibility to erythromycin, ampicillin, and trimethoprim-sulfamethoxazole. Nine distinct PFGE patterns were identified; five of these patterns occurred in only a single case isolate. The most common pattern identified accounted for 36 (38%) of the total isolates and they occurred throughout the year.

No cases of erythromycin-resistant B. pertussis have been identified in Minnesota since the first case was identified in 1999. Statewide, all 1,288 other isolates tested to date have had low minimum inhibitory concentrations, falling within the reference range for susceptibility to the antibiotics evaluated. Only 8 other erythromycin-resistant B. pertussis cases have been identified to date in the United States.

Laboratory tests should be performed on all suspected cases of pertussis. Culture of B. pertussis requires inoculation of nasopharyngeal mucous on special media and incubation for 7 to 10 days. However, B. pertussis is rarely identified late in the illness; therefore, a negative culture does not rule out disease. A positive PCR result is considered confirmatory in patients with a 2-week history of cough illness. PCR can detect non-viable organisms. Consequently, a positive PCR result does not necessarily indicate current infectiousness. Patients with a 3-week or longer history of cough illness, regardless of PCR result, may not benefit from antibiotic therapy. Cultures are necessary for molecular and epidemiologic studies and for drug susceptibility testing. Whenever possible, culture should be done in conjunction with PCR testing. Direct fluorescent antibody (DFA) provides a rapid presumptive diagnosis of pertussis; however, because both false-positive and false-negative results can occur, DFA tests should not be relied upon solely for laboratory confirmation. Serological tests are not standardized and are not acceptable for laboratory confirmation at this time.

• For up to date information see>> Pertussis
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2008