During 2010, 1,143 cases of pertussis (22 per 100,000 population) were reported. This was the third consecutive peak incidence year, following 1,134 cases in 2009 and 1,034 cases in 2008. Laboratory confirmation was available for 729 (64%) cases, 33 (5%) of which were confirmed by culture and 696 (95%) of which were confirmed by PCR. In addition to the laboratory-confirmed cases, 276 (24%) cases were epidemiologically linked to laboratory-confirmed cases, and 134 (12%) met the clinical case definition only. Five hundred ninety-five (52%) of the reported cases occurred in residents of the metropolitan area.
Paroxysmal coughing was the most commonly reported symptom; 1,065 (93%) cases experienced paroxysmal coughing. Approximately one fourth (314, 27%) 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 493 (43%) of the cases. Infants and young children are at the highest risk for severe disease and complications. Pneumonia was diagnosed in 23 (2%) cases, 5 (22%) of whom were between 18 months and 4 years of age. Twenty-nine (3%) cases were hospitalized; 21 (72%) of the hospitalized patients were younger than 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 2010, cases ranged in age from 14 days to 87 years. One hundred sixteen (10%) cases occurred in adolescents 13 to 17 years of age, 299 (26%) cases occurred in adults 18 years of age and older, 480 (42%) occurred in children 5-12 years of age, 190 (17%) occurred in children 6 months through 4 years of age, 54 (5%) occurred in infants less than 6 months of age, and 2 (<1%) occurred in persons of unknown age. The median age of cases during 2010 was 11 years, compared to a median age of 13 years in 2005, the most recent previous peak incidence year.
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 2010, 75 pertussis cases were reported in infants < 1 year of age. A likely source of exposure was identified for 36 (48%) cases; 12 (33%) were infected by adults 18 years of age and older, 2 (6%) were infected by an adolescent 13 to 17 years of age, and 20 (56%) were infected by a child less than 13 years of age. For the 39 (52%) 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 252 cases who were 7 months to 6 years of age, 178 (71%) were known to have received at least a primary series of 3 doses of DTP/DTaP vaccine prior to onset of illness; 73 (29%) received fewer than 3 doses and were considered preventable cases. Vaccine history was unavailable for 1 case.
MDH reporting rules require that clinical isolates of Bordetella pertussis be submitted to the PHL. Of the 33 culture-confirmed cases, 19 (58%) of the isolates were received and sub-typed by PFGE. Nine distinct PFGE patterns were identified. Five of these patterns occurred in only a single case isolate. The most common pattern identified accounted for 7 (37%) of the total isolates.
In 2010 no case-isolates of pertussis were tested in Minnesota for susceptibility to erythromycin, ampicillin, and trimethoprim-sulfamethoxazole. However, nationally isolates have had low minimum inhibitory concentrations, falling within the reference range for susceptibility to the antibiotics evaluated. 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 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.
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