Pertussis, 2012

During 2012, 4,144 cases of pertussis (75 per 100,000 population) were reported. This is the highest reported incidence of pertussis since the 1930s. Laboratory confirmation was available for 3,207 (77%) cases, 95 (3%) of which were confirmed by culture and 3,189 (99%) of which were confirmed by PCR. In addition to the laboratory confirmed cases, 1,353 (33%) cases were epidemiologically linked to laboratory-confirmed cases, and 278 (7%) met the clinical case definition only. Two thousand five hundred three (60%) of the reported cases occurred in residents of the metropolitan area.

Paroxysmal coughing was the most commonly reported symptom. Four thousand eight (97%) cases experienced paroxysmal coughing. Nearly one fourth (917, 22%) 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 1,561 (38%) of the cases. Infants and young children are at the highest risk for severe disease and complications. Pneumonia was diagnosed in 60 (1%) cases, 6 (10%) of whom were <1 year of age. Fifty-eight (1%) cases were hospitalized; 25 (43%) of the hospitalized patients were <6 months of age.

Pertussis can affect persons of any age. The disease is increasingly recognized in older children and adults. During 2012, cases ranged in age from <1 week to 99 years. Nine hundred forty-two (23%) cases occurred in adolescents 13-17 years of age, 850 (21%) in adults 18 years of age and older, 1,765 (43%) in children 5-12 years of age, 463 (11%) in children 6 months through 4 years of age, and 110 (3%) in infants <6 months of age. Age was missing for 14 (≤1%) cases. The median age of cases was 12 years.

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 2012, 176 (4%) pertussis cases were reported in infants <1 year of age. A likely source of exposure was identified for 55 (31%) of those cases; 12 (22%) were infected by adults 18 years of age and older, 9 (16%) were infected by an adolescent 13-17 years of age, 24 (44%) were infected by a child <13 years of age, and 10 (18%) were of unknown age. For the 121 (69%) infant cases with no identified source of infection, the source was likely from outside the household. ACIP recently recommended vaccination of women at ≥20 weeks gestation during each pregnancy in an effort to protect young infants. Ensuring up-to-date vaccination of children, adolescents, and adults, especially those in contact with young children is also important. Vaccinating adolescents and adults with Tdap will decrease the incidence of pertussis in the community and thereby minimize infant exposures.

Although unvaccinated children are at highest risk for pertussis, fully immunized children may also develop the disease, particularly as the years since vaccination increase. 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 674 (16%) cases who were 7 months to 6 years of age, 555 (82%) were known to have received at least a primary series of 3 doses of DTP/DTaP vaccine prior to onset of illness; 98 (15%) received fewer than 3 doses and were considered preventable cases. Vaccine history was unavailable for the remaining 21 (3%) cases.

MDH reporting rules require that clinical isolates of Bordetella pertussis be submitted to the PHL. Isolates for all 95 culture-confirmed cases were received and sub-typed by PFGE, with 16 distinct PFGE patterns identified. In 2012, no case-isolates of pertussis were tested in Minnesota for susceptibility to erythromycin, ampicillin, or trimethoprim-sulfamethoxazole. Nationally, isolates have had low minimum inhibitory concentrations, falling within the reference range for susceptibility to the antibiotics evaluated. Only 11 erythromycinresistant B. pertussis cases have been identified in the United States to date.

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. Serological tests are not standardized and are not acceptable for laboratory confirmation at this time.

Pertussis remains endemic in Minnesota despite an effective vaccine and high coverage rates with the primary series. Reported incidence of pertussis has consistently increased over the past 10 years, particularly in adolescents and adults. One of the main reasons for the ongoing circulation of pertussis is that vaccine-induced immunity to pertussis begins to wane 3 years after completion of the primary series.

• For up to date information see>> Pertussis
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2012