Pertussis, 2011

During 2011, 662 cases of pertussis (12 per 100,000 population) were reported. Pertussis annual incidence in Minnesota exceeded this number every year since 2007, when 393 cases (7.6 per 100,000 population) were reported. Laboratory confirmation was available for 428 (65%) cases, 8 (2%) of which were confirmed by culture and 420 (98%) of which were confirmed by PCR. In addition to the laboratory-confirmed cases, 135 (20%) cases were epidemiologically linked to laboratory-confirmed cases, and 97 (15%) met the clinical case definition only. Three hundred forty-nine (53%) of the reported cases occurred in residents of the metropolitan area.

Paroxysmal coughing was the most commonly reported symptom. Six hundred thirty-three (96%) cases experienced paroxysmal coughing. Almost one third (200, 30%) 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 283 (43%) of the cases. Infants and young children are at the highest risk for severe disease and complications. Pneumonia was diagnosed in 21 (3%) cases, 49 (7%) of whom were <1 year of age. Nineteen (3%) cases were hospitalized; 10 (52%) of the hospitalized patients were <6 months of age.

Due to waning 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 2011, cases ranged in age from <1 week to 77 years. Ninety-four (14%) cases occurred in adolescents 13 to 17 years of age, 148 (22%) in adults 18 years of age and older, 278 (42%) in children 5-12 years of age, 117 (18%) in children 6 months through 4 years of age, and 25 (4%) in infants <6 months of age. The median age of cases was 11 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 2011, 49 pertussis cases were reported in infants <1 year of age. A likely source of exposure was identified for 16 (33%) cases; 6 (31%) were infected by adults 18 years of age and older, 1 (6%) was infected by an adolescent 13 to 17 years of age, and 9 (50%) were infected by a child <13 years of age. For the 33 (67%) cases with no identified source of infection, the source was likely from outside the household. 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. 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 154 cases who were 7 months to 6 years of age, 107 (69%) were known to have received at least a primary series of 3 doses of DTP/DTaP vaccine prior to onset of illness; 43 (28%) received fewer than 3 doses and were considered preventable cases.

Vaccine history was unavailable for the remaining 4 (3%) cases.

MDH reporting rules require that clinical isolates of Bordetella pertussis be submitted to the PHL. Of the 8 culture-confirmed cases, 6 of the isolates were received and sub-typed by PFGE with 6 distinct PFGE patterns identified. In 2011 no case-isolates of pertussis were tested in Minnesota for susceptibility to erythromycin, ampicillin, or trimethoprim-sulfamethoxazole. However, 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 wanes approximately 5-10 years after completion of the primary series, leaving adolescents and adults susceptible.

• For up to date information see>> Pertussis
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2011