Pertussis, 2013

During 2013, 865 cases of pertussis (16 per 100,000 population) were reported. Laboratory confirmation was available for 647 (75%) cases, 22 (3%) of which were confirmed by culture and 644 (>99%) of which were confirmed by PCR. In addition to the laboratory-confirmed cases, 154 (18%) cases were epidemiologically linked to laboratory-confirmed cases, and 64 (7%) met the clinical case definition only. Four hundred ninety-six (57%) cases occurred in residents of the metropolitan area.

Paroxysmal coughing was the most commonly reported symptom. Eight hundred seventeen (94%) cases experienced paroxysmal coughing. One fourth (221, 26%) 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 325 (38%) of the cases. Infants and young children are at the highest risk for severe disease and complications. Pneumonia was diagnosed in 20 (2%) cases, 4 (20%) of whom were <1 year of age. Twenty-five (3%) cases were hospitalized; 12 (48%) of the hospitalized patients were <6 months of age.

Pertussis is increasingly recognized in older children and adults. During 2013, cases ranged in age from <1 month to 91 years. One hundred ninety-two (22%) cases occurred in adolescents 13-17 years of age, 192 (22%) in adults 18 years of age and older, 324 (37%) in children 5-12 years of age, 113 (13%) in children 6 months through 4 years of age, and 41 (5%) in infants <6 months of age. Age was missing for 3 (<1%) cases. 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 2013, 55 (6%) cases were reported in infants <1 year of age. A likely source of exposure was identified for 30 (55%) of those cases; 12 (40%) were infected by adults 18 years of age and older, 1 (3%) was infected by an adolescent 13-17 years of age, 15 (50%) were infected by a child <13 years of age, and 2 (7%) were of unknown age. For the 25 (45%) infant cases with no identified source of infection, the source was likely from outside the household. One death occurred in a 1-month old child who had no underlying conditions. The likely source was another child in the household. ACIP recommends 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 157 (18%) cases who were 7 months to 6 years of age, 119 (76%) were known to have received at least a primary series of 3 doses of DTP/DTaP vaccine prior to onset of illness; 38 (24%) received fewer than 3 doses and were considered preventable cases.

MDH reporting rules require clinical isolates of Bordetella pertussis be submitted to the PHL. Isolates for 18 of the 20 culture-confirmed cases were received and sub-typed by PFGE, with 9 distinct PFGE patterns identified. In 2013, 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 isolates 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 may be useful for diagnosis in later phases of the disease; however, serological tests are not yet standardized and should be interpreted with caution.

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, 2013