Three hundred eight cases of pertussis (6.3 per 100,000 population) were reported during 2001, compared with 575 cases in 2000. Laboratory confirmation was available for 204 (66%) cases, of which 105 were confirmed by culture and 99 by polymerase chain reaction. The remainder of cases either were epidemiologically linked to culture-confirmed cases (24%) or met a clinical case definition (10%). Two hundred seven (67%) cases occurred among residents of the seven-county Twin Cities metropolitan area. No deaths due to pertussis were reported in 2001.
Very young children, older individuals, and previously immunized persons may not exhibit the typical "whoop" associated with pertussis ("whooping cough"); paroxysmal coughing is the most commonly reported symptom. In 2001, nearly all (94%) reported cases had paroxysmal coughing, and 37% experienced whooping. Post-tussive vomiting was reported in over half (56%) of the cases; 36% reported apnea.
Due to waning immunity following natural infection or vaccination, pertussis can affect persons of any age and increasingly is recognized in older children and adults. Case-patients in 2001 ranged from 7 days to 80 years of age. Thirty five (11%) cases occurred in infants less than 6 months of age, and 74 (24%) occurred in children 6 months to 4 years of age. The most common age group among cases was children 5-12 years. Persons 13-17 years of age and persons 18 years of age or older accounted for 13% and 24% of cases, respectively.
The severity of pertussis increases with decreasing age; pertussis is most severe in infants and young children. Pneumonia was diagnosed in 18 (6%) cases, seven (39%) of whom were less than 18 months of age. Thirty (10%) cases were hospitalized, of which 17 (57%) were infants less than 6 months of age.
In Minnesota, pertussis infection in older children and adults may result in exposure of unprotected infants, who are most at risk for severe consequences of infection. During 2001, 43 cases of pertussis were reported in infants less than 1 year of age. A likely source of exposure was identified for 17 (40%) cases, of whom eight likely were infected by an adult and nine likely were infected by children. Of the remaining 26 (60%) cases with no identified source of infection, exposure likely occurred outside the household.
Although unvaccinated children are at highest risk for pertussis, fully immunized children also can develop disease. Vaccine efficacy for currently licensed vaccines is approximately 71-84% for preventing serious disease. Evaluation of the vaccination status of pertussis case-patients 2 months to 15 years of age indicated that 105 (56%) of 187 cases with a known vaccine history had received age-appropriate immunization for pertussis. (This includes infants 2-5 months of age, for whom a primary series is not yet indicated.) Among 169 case-patients 7 months to 15 years of age, 147 (87%) had received at least a primary series of three doses. Disease in previously immunized persons usually is mild. Of 88 cases among children 7 months to 7 years of age, 16 (18%) were considered preventable - i.e., the patient had received fewer than three doses of DTP vaccine before onset of illness.
Physicians should include pertussis in the differential diagnosis of coughing illnesses in persons of all ages, regardless of immunization status. Until an approved booster vaccination for pertussis is available to protect older children and adults, prompt diagnosis and treatment of cases and prophylaxis of contacts are the only options for limiting transmission.
Of 105 culture-confirmed cases reported in 2001, 93 (89%) had B. pertussis isolates subtyped by pulsed-field gel electrophoresis (PFGE) and tested for antibiotic susceptibility to erythromycin, ampicillin, and trimethoprim/sulfamethoxazole using ETEST. Twelve distinct PFGE patterns were identified; five of these patterns occurred in only a single case isolate. The two most common patterns accounted for 76 (82%) isolates.
The first case of erythromycin-resistant B. pertussis in Minnesota was identified in October 1999. Statewide, all 798 other isolates tested to date have had low minimum inhibitory concentrations, falling within the reference range for susceptibility to erythromycin and the other antibiotics evaluated. Only five other erythromycin-resistant B. pertussis cases have been identified in the U.S.