Pertussis, 2007

During 2007, 393 cases of pertussis (7.6 per 100,000 population) were reported in Minnesota, compared to 320 in 2006 and a peak of 1,571 cases reported in 2005. Laboratory confirmation was available for 248 (63%) cases, 32 (13%) of which were confirmed by culture and 216 (87%) of which were confirmed by PCR. In addition to the laboratory-confirmed cases, 58 (15%) cases were epidemiologically linked to laboratory-confirmed cases, and 87 (22%) met the clinical case definition. Two hundred ninety-seven (76%) of the reported cases occurred in residents of the metropolitan area.

Paroxysmal coughing was the most commonly reported symptom. Three hundred seventy-one (94%) of the case-patients experienced paroxysmal coughing. Nearly one third (108, 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 158 (40%) of the cases. Infants and young children are at the highest risk for severe disease and complications. Pneumonia was diagnosed in four (1%) case-patients, two (50%) of whom were less than 18 months of age. Seventeen (4%) case-patients were hospitalized; 11 (65%) 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 2007, case-patients ranged in age from 1 week to 97 years. One hundred thirty-five (34%) cases occurred in adolescents 13 to 17 years of age; 110 (28%) cases occurred in adults 18 years of age and older; 93 (24%) occurred in children 5-12 years of age; 30 (8%) occurred in children 6 months through 4 years of age, and 25 (6%) occurred in infants less than 6 months of 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 2007, 30 pertussis cases were reported in infants less than 1 year of age. A likely source of exposure was identified for 14 (47%) cases; nine (30%) were infected by adults 18 years of age and older, two (7%) were infected by a child 13 years of age or older, and three (10%) were infected by a child less than 13 years of age. For the 16 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 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 34 case-patients who were 7 months to 6 years of age, 22 (65%) were known to have received at least a primary series of three doses of DTP/DTaP vaccine prior to onset of illness, 12 (35%) received fewer than three doses and were considered preventable cases.

MDH reporting rules require that clinical isolates of Bordetella pertussis be submitted to the PHL. Of the 32 culture-confirmed cases, 27 (84%) 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 15 (56%) 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 1,194 other isolates tested to date have had low minimum inhibitory concentrations, falling within the reference range for susceptibility to the antibiotics evaluated. Only eight 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, 2007