Pertussis, 2014

During 2014, 950 cases of pertussis (18 per 100,000 population) were reported. Laboratory confirmation was available for 717 (75%) cases, 26 (4%) of which were confirmed by culture and 691 (96%) of which were confirmed by PCR. In addition to the laboratory-confirmed cases, 164 (17%) cases met the clinical case definition and were epidemiologically linked to laboratory-confirmed cases, and 69 (7%) met the clinical case definition only. Four hundred sixty-three (49%) of the reported cases occurred in residents of the metropolitan area.

Paroxysmal coughing was the most commonly reported symptom which 863 (91%) cases experienced. Approximately one fourth (274, 29%) reported whooping. Although commonly referred to as “whooping cough,” very young children, older individuals, and persons previously immunized may not have the typical “whoop”. Post-tussive vomiting was reported in 415 (44%) of the cases. Infants and young children are at the highest risk for severe disease and complications. Pneumonia was diagnosed in 22 (2%) cases, only 1 of which was in an infant; 15 (68%) were between the ages of 2 and 16 years, 6 (27%) were between the ages of 20 and 81 years. Nineteen (2%) cases were hospitalized; 9 (47%) of the hospitalized patients were <6 months of age. No deaths occurred (note, the lab findings in an UNEX case [p. 23] did not count as a confirmed or probable pertussis case).

Pertussis can affect persons of any age. The disease is increasingly recognized in older children and adults. During 2014, cases ranged in age from <1 month to 92 years. Two hundred fifty-four (27%) cases occurred in adolescents 13-17 years of age, 168 (18%) in adults 18 years of age and older, 346 (37%) in children 5-12 years of age, 148 (16%) in children 6 months through 4 years of age, and 30 (3%) in infants <6 months of age. Age was missing for 4 (<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 2014, 52 (5%) pertussis cases were reported in infants <1 year of age. A likely source of exposure was identified for 17 (33%) of those cases; 2 (12%) were infected by adults 18 years of age and older, 3 (18%) were infected by an adolescent 13-17 years of age, 10 (59%) were infected by a child <13 years of age, and 2 (12%) had an unknown age. For the 35 (67%) infant cases with no identified source of infection, the source was likely from outside 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 number of years since vaccination increase. Disease in those previously immunized is usually mild. Efficacy for currently licensed DTaP vaccines is estimated to be 71 -84% in preventing typical disease within the first 3 years of completing the series. Waning immunity sharply increases at 7 years of age, and most are susceptible by 11-12 years of age when TDaP booster is recommended. Recent studies suggest that immunity wanes sharply 2 years from receipt of TDaP. Of the 202 (21%) cases who were 7 months to 6 years of age, 135 (67%) were known to have received at least a primary series of 3 doses of DTP/ DTaP vaccine prior to onset of illness; 67 (33%) received fewer than 3 doses and were considered preventable cases.

Reporting rules require clinical isolates of Bordetella pertussis be submitted to the PHL in order to track changes in circulating strains. Isolates for all 26 culture-confirmed cases were received and sub-typed, with 5 distinct PFGE patterns identified. In 2014, no case-isolates of pertussis were tested in Minnesota for antimicrobial susceptibility. Nationally, isolates have had low minimum inhibitory concentrations, falling within the reference range for susceptibility to erythromycin and azithromycin. Only 11 erythromycin-resistant 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 a specimen of nasopharyngeal mucus 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. Whenever possible, culture should be done in conjunction with PCR testing. Serological tests may be useful for coughs >2 weeks.

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 middle school-aged children, adolescents, and adults.

• For up to date information see>> Pertussis
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2014