During 2002, 429 cases of pertussis (8.7 per 100,000 population) were reported, compared to 308 cases in 2001 and 575 in 2000. Laboratory confirmation was available for 277 (65%) cases reported in 2002; 126 (45%) were culture-confirmed and 151 (55%) were confirmed by PCR. Among the remaining cases, 105 (24%) were epidemiologically linked to culture confirmed cases, and 47 (11%) met the clinical case definition. Sixty-seven percent of the reported cases occurred in the seven-county Twin Cities metropolitan area. No fatal cases were reported in 2002.
Pertussis commonly is called "whooping cough." However, very young children, older individuals, and previously immunized persons may not have the typical "whoop." Paroxysmal coughing is the most commonly reported symptom. In 2002, 396 (92%) case-patients experienced paroxysmal coughing, and nearly one-third experienced "whooping." Posttussive vomiting was reported in 255 (59%) cases, and 144 (34%) case patients reported apnea.
Due to waning immunity from natural infection or vaccination, pertussis can affect persons of any age. The disease increasingly is recognized in older children and adults; however, it is not known whether this represents a true increase or changes in surveillance and reporting procedures. case patients reported in 2002 ranged in age from 1 day to 86 years. Forty seven (11%) cases occurred in infants less than 6 months of age, and 67 (16%) occurred in children 6 months to 4 years of age. The largest number of cases (120, [28%]) occurred in children 5 to 12 years of age, followed by 113 (26%) cases among persons 18 years of age or older. Persons 13 to 17 years of age accounted for 82 (19%) cases. Infants and young children are at highest risk for severe disease and complications. Pneumonia was diagnosed in 19 (4%) case-patients, nine (47%) of whom were less than 18 months of age. Twenty-six (6%) case patients were hospitalized; 19 (73%) 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 2002, 54 cases of pertussis were reported in infants less than 1 year of age. A likely source of exposure was identified for nine (17%) cases. Of these nine cases, six (67%) were infected by adults 18 years of age or older, one (11%) was infected by an adolescent 13 to 17 years of age, and two (22%) were infected by a child less than 13 years of age. Forty-five (83%) pertussis cases in infants had no identified source of infection, which likely was outside the household.
Although unvaccinated children are at highest risk for pertussis, fully immunized children also can develop disease. The efficacy of currently licensed vaccines in preventing serious pertussis disease is estimated to be 71 to 84%. Among 208 pertussis cases 2 months to 15 years of age with a known vaccination history, 174 (84%) had received age-appropriate immunization for pertussis. Of the 243 cases who were 7 months to 15 years of age, 186 (77%) had received at least a primary series of three doses; this is not surprising, since waning immunity begins approximately 3 years after the last dose of vaccine. Disease in previously immunized persons usually is mild. Of 97 cases in persons 7 months to 7 years of age, 15 (15%) had received fewer than three doses of DTP/DTaP vaccine before onset of illness and therefore were considered preventable cases.
Physicians should include pertussis in the differential diagnosis of cough illness 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.
Laboratory tests should be performed on all suspected cases of pertussis. Culture of Bordetella pertussis requires inoculation of nasopharyngeal mucous on special media such as Regan-Lowe or Bordet-Gengou, and incubation for 7 to 10 days. However, B. pertussis rarely is 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; therefore, a positive PCR result does not necessarily indicate 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. Thus, whenever possible, culture should be done in conjunction with PCR testing. Direct fluorescent antibody (DFA) testing provides a rapid presumptive diagnosis of pertussis; however, because both false-positive and false-negative results can occur, DFA tests should not be relied upon as laboratory confirmation. Serologic tests for pertussis are not standardized and therefore do not indicate laboratory confirmation.
Minnesota Rules Governing Communicable Diseases require submission of all clinical B. pertussis isolates to MDH. Among 126 culture-confirmed cases, 112 (89%) had B. pertussis isolates submitted to MDH. These isolates were subtyped by PFGE and tested for antibiotic susceptibility to erythromycin, ampicillin, and trimethoprim/sulfamethoxazole. Twelve distinct PFGE patterns were identified; five of these patterns were represented by only a single case isolate. The two most common patterns accounted for 61% of the total isolates and occurred throughout the year.
The first case of erythromycin-resistant B. pertussis in Minnesota was identified in October 1999. All 910 other isolates tested to date have been susceptible to the antibiotics evaluated. Only eight other erythromycinresistant B. pertussis cases have been identified in the U.S.