Pertussis, 2018

In 2018, 397 pertussis cases (7 per 100,000 population) were reported. Laboratory confirmation was available for 280 (71%) cases, 19 (7%) of which were confirmed by culture and 262 (94%) by PCR. In addition, 60 (15%) cases met the clinical case definition and were epidemiologically linked to laboratory confirmed cases, and 56 (15%) met the clinical case definition only. One hundred ninety-two (48%) cases occurred in residents of the metropolitan area.

Paroxysmal coughing was the most commonly reported symptom, which 369 (93%) cases experienced. Approximately one third (118) 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 185 (47%) cases. Infants and young children are at the highest risk for severe disease and complications. Pneumonia was diagnosed in 8 (2%) cases, only 2 of which were in infants; 3 were 2 to 16 years old, 2 were 20 to 70 years old. Five (1%) cases were hospitalized; 2 (33%) hospitalized patients were <6 months of age. No deaths occurred.

Pertussis is increasingly recognized in older children and adults. During 2018, cases ranged in age from <1 month to 86 years. One hundred (25%) cases occurred in adolescents 13-17 years, 105 (26%) in children 5-12 years, 92 (23%) in adults ≥18 years, 74 (19%) in children 6 months through 4 years, and 14 (4%) in infants <6 months of age. The median age of cases was 13 years. Infection in older children and adults may result in exposure of unprotected infants. During 2018, 24 cases were in infants <1 year of age. A likely source of exposure was identified for 11 of those cases; 3 were infected by adults ≥18 years (one mother and two fathers), 1 by an adolescent 13-17 years, 6 by a child <13 years of age, and for 1 the age was unknown. Eleven infant cases had no identified source of infection. 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.

Although unvaccinated children are at highest risk for pertussis, fully immunized children may also develop 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 97 (24%) cases who were 7 months to 6 years of age, 40 (41%) were known to have received at least a primary series of 3 doses of DTP/DTaP vaccine prior to onset of illness; 54 (56%) received fewer than 3 doses and were considered preventable cases.

Isolates of Bordetella pertussis must be submitted to the PHL in order to track changes in circulating strains. Isolates for 17 (90%) culture-confirmed cases were received and sub-typed, with two distinct PFGE patterns identified. Nationally, isolates have had low minimum inhibitory concentrations (falling within the reference range for susceptibility) to erythromycin and azithromycin. Only 11 erythromycinresistant B. pertussis cases have been identified in the United States.

Laboratory tests should be performed on all suspected cases. 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 those with coughs >2 weeks.

• For up to date information see>> Pertussis
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2018