During 2003, 207 (4.2/100,000 population) cases of pertussis were reported, compared to 429 cases reported in 2002, and 308 in 2001. Laboratory confirmation was available for 131 (63%) cases; 59 (45%) were confirmed by culture and 72 (55%) by PCR. Among the remaining cases, 52 (25%) were epidemiologically-linked to culture-confirmed cases, and 24 (12%) met the clinical case definition. Sixtysix percent of the reported cases occurred in residents of the Twin Cities metropolitan area.
Two deaths due to pertussis-related complications were reported in 2003. One case was a 13-year-old with neuroblastoma. The other case was an 82-year-old with chronic renal insufficiency and multiple myeloma. Both had pertussis-related pneumonia. Disease in the 82-year-old woman was confirmed by blood culture. There has been only one prior report of B. pertussis isolated from blood (CDC. Fatal case of unsuspected pertussis diagnosed from a blood culture – Minnesota, 2003. MMWR 2003;53:131- 2).
Paroxysmal coughing is the most commonly reported symptom. In 2003, 192 (93%) of the case-patients experienced paroxysmal coughing. Over one third (73, 35%) 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 100 (48%) of the cases. Sixty-nine (33%) of the case-patients reported apnea. Infants and young children are at the highest risk for severe disease and complications. Pneumonia was diagnosed in 8 (4%) case-patients, 3 of whom were less than 18 months of age. Nineteen (9%) case-patients were hospitalized; 13 of the hospitalized patients were younger than 6 months of age.
Due to waning immunity, of either natural infection or vaccine, pertussis can affect persons of any age. The disease is increasingly recognized in older children and adults; however, it is not clear whether it is a true increase or due to changes in surveillance and reporting. During 2003, case-patients ranged in age from 1 day to 82 years. Fifty-eight (28%) cases occurred in persons ages 13 to 17 years old. Fiftyfive (27%) cases occurred in persons 18 years of age and older. Persons 5- 12 years of age accounted for 21% (44) of all cases. Twenty-one (10%) of the total cases occurred in infants less than 6 months of age, and 29 (14%) occurred in children ages 6 months through 4 years.
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 2003, 23 cases of pertussis were reported in infants less than 1 year of age. A likely source of exposure was identified for 11 (48%) cases: 5 (46%) were infected by adults age 18 and over, 3 (27%) were infected by an adolescent, and 3 (27%) were infected by a child less than 13. Twelve (52%) cases had no identified source of infection. For these cases, the source of infection was likely outside the household.
Although unvaccinated children are at highest risk for pertussis, fully immunized children may also develop disease. Disease in those previously immunized is usually mild. Efficacy for currently licensed vaccines is estimated to be 71 to 84% in preventing serious pertussis disease, but waning immunity begins around 3 years after the last dose of DTaP. Of the 118 case patients who were 7 months to 15 years of age, 82 (70%) are known to have received at least a primary series of three doses. Of the 31 cases in persons 7 months to 7 years of age, 12 (39%) received fewer than three doses of DTP/DTaP vaccine before onset of illness, and were considered preventable cases.
MDH reporting rules require that clinical isolates of B. pertussis be submitted to the Public Health Laboratory. Of the 59 culture-confirmed cases, 58 (98%) isolates were received and subtyped by PFGE and tested for antibiotic susceptibility to erythromycin, ampicillin, and trimethoprim/sulfamethoxazole. Ten distinct PFGE patterns were identified; three of these patterns occurred in only a single case isolate. The two most common patterns identified accounted for 36 (62%) of the total isolates and occurred throughout the year.
No cases of erythromycin-resistant B. pertussis have been identified in Minnesota since the first case in October 1999. Statewide, all 968 other isolates tested to date have had low minimum inhibitory concentrations, falling within the reference range for susceptibility to the antibiotics evaluated. Only eight other erythromycinresistant B. pertussis cases have been identified to date in the United States.
The prompt diagnosis and treatment of cases, and prophylaxis of contacts, are the only options for limiting transmission, until an approved booster vaccination for pertussis is available to protect older children and adults. Pertussis should be included in the differential diagnosis of cough illness in persons of all ages regardless of immunization status.
Laboratory tests should be performed on all suspected cases of pertussis. Culture of B. pertussis requires inoculation of nasopharyngeal (NP) mucous on special media and incubation for seven to ten 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 nonviable 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. Direct fluorescent antibody (DFA), 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 solely for laboratory confirmation. Serological tests are not standardized and are not acceptable for laboratory confirmation.
Note: For up to date information on B. pertussis see Pertussis (Whooping Cough)
Go to full issue: DCN, August 2004: Volume 32, Number 4