Five hundred seventy-five cases of pertussis (11.7 per 100,000 population) were reported in 2000. This is the largest number of cases reported since 1955 when 594 cases were reported. Laboratory confirmation was available for 376 (65%) reported cases; 194 were confirmed by culture and 182 by polymerase chain reaction (PCR). The remaining cases were either epidemiologically linked to culture-confirmed cases (140, 24%) or met a clinical case definition (53, 9%). Four hundred sixty (80%) cases occurred in residents of the Twin Cities metropolitan area. No deaths due to pertussis were reported in 2000.
Although pertussis often is referred to as “whooping cough,” very young children, older individuals, and previously immunized persons may not exhibit the typical “whoop.” Paroxysmal coughing is the most commonly reported symptom. In 2000, nearly all (94%) reported pertussis cases experienced paroxysmal coughing, and 32% experienced whooping. Post-tussive vomiting was reported in 53% of the cases. Nearly a third (31%) of cases reported apnea.
Due to waning immunity following either natural infection or vaccination, pertussis can affect persons of any age and increasingly is recognized in older children and adults. During 2000, ages of reported cases ranged from 10 days to 92 years. Eighty-one (14%) case-patients were infants less than 6 months of age, and 97 (17%) were children between 6 months and 4 years of age. The largest age group was children between 5 and 12 years of age (32%). Persons 13 to 17 years of age and persons 18 years of age or older accounted for 16% and 21% of cases, respectively. The severity of pertussis increases significantly with decreasing age; pertussis is most severe in infants and young children. Pneumonia was diagnosed in 26 (5%) cases, thirteen (50%) of whom were less than 18 months of age. Fifty-four (9%) case-patients were hospitalized; thirty-eight (70%) of the hospitalized patients were younger than 6 months of age.
In Minnesota, pertussis 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 2000, 96 cases of pertussis were reported in infants less than 1 year of age. A likely source of exposure was identified for 51 (52%) of these cases. Twenty-six (51%) cases likely were infected by an adult (most often a parent, grandparent, or another adult relative); six (12%) cases likely were infected by an adolescent (usually a sibling or other relative), and 18 (35%) cases likely were infected by children (usually a sibling). One infant’s likely source of infection was of unknown age. Forty-five (46%) cases had no identified source of exposure. These cases highlight the need for a high index of suspicion for pertussis in adolescents and adults with cough illness and the importance of antibiotic treatment.
Although unvaccinated children are at highest risk for pertussis, fully immunized children also may develop disease. The efficacy of current pertussis vaccines is approximately 71 to 84%. Among 373 pertussis cases reported in 2000 who were 2 months to 15 years of age and had a known vaccination history, 64% had received age-appropriate immunization for pertussis. (This includes infants 2 to 5 months of age for whom a primary series is not yet indicated.) Two hundred eight (63%) of the 331 cases 7 months through 15 years of age had received at least a primary series of three doses. Of 113 cases among persons 7 months to 7 years of age, 35% were considered preventable – i.e., pertussis occurring in a patient who is between 7 months and 7 years of age and has received fewer than three doses of DTP vaccine before onset of illness.
Clinical isolates of B. pertussis must be submitted to the MDH Public Health Laboratory. Of the 194 culture-confirmed cases, 189 (97%) B. pertussis isolates were subtyped by pulsed-field gel electrophoresis (PFGE) and tested for antibiotic susceptibility to erythromycin, ampicillin, and trimethoprim-sulfamethoxazole using E-test. Nine distinct PFGE patterns were identified; three (33%) occurred in only a single case isolate. The two most common patterns identified accounted for 80% of the total isolates and occurred throughout the year.
The first case of erythromycin-resistant B. pertussis in Minnesota was identified in October 1999. Statewide, all 705 other isolates tested to date have had low minimum inhibitory concentrations, falling within the reference range for susceptibility to the antibiotics evaluated. Only five other erythromycin-resistant B. pertussis cases have been identified in the United States.