Pertussis, 2005

During 2005, 1,571 (30.6/100,000 population) cases of pertussis were reported in Minnesota representing a continuation of the increase first seen in 2004 when 1,368 cases reported in Minnesota. This increase occurred nationally as well, and may be attributable to several factors including increased awareness of pertussis among health care providers and the public, increased availability of more sensitive diagnostic testing using PCR, as well as a true increase in incidence. Laboratory confirmation was available for 1,096 (70%) cases; 115 (10%) were confirmed by culture, and 981 (90%) were confirmed by PCR. In addition to the laboratory-confirmed cases, 215 (14%) cases were epidemiologically linked to culture-confirmed cases, and 260 (17%) met the clinical case definition. Seven hundred five (45%) of the reported cases occurred in residents of the Twin Cities metropolitan area.

Paroxysmal coughing is the most commonly reported symptom. In 2005, 1,519 (97%) of the case-patients experienced paroxysmal coughing. Over one third (475, 30%) 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 749 (48%) of the cases. Four hundred eighty-four (31%) case-patients reported apnea. Infants and young children are at the highest risk for severe disease and complications. Pneumonia was diagnosed in 44 (3%) case-patients, 14 (32%) of whom were less than 18 months of age. Thirty-two (2%) case-patients were hospitalized; 23 (72%) of the hospitalized patients were younger than 6 months of age.

Due to waning immunity, either of 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 2005, case-patients ranged in age from 1 day to 83 years. Four hundred twenty-three (27%) cases occurred in persons 13 to 17 years of age. Five hundred five (32%) cases occurred in persons 18 years of age and older. Persons 5-12 years of age accounted for 24% (372) of all cases. Ninety-two (6%) of the total cases occurred in infants less than 6 months of age, and 178 (11%) occurred in children 6 months through 4 years of age. Age was unknown for one case.

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 2005, 102 cases of pertussis were reported in infants less than 1 year of age. A likely source of exposure was identified for 49 (48%) cases; 33 (67%) were infected by adults 18 years of age and older, six (12%) were infected by an adolescent, and 10 (20%) were infected by a child less than 13 years of age. Fifty-three (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 disease, but waning immunity begins approximately 3 years after the last dose of DTaP. Of the 863 case-patients who were 7 months to 15 years of age, 615 (71%) are known to have received at least a primary series of three doses. Of the 203 cases in persons 7 months through 6 years of age, 34 (17%) received fewer than three doses of DTP/DTaP vaccine before onset of illness, and were considered preventable cases, 29 (14%) of cases in this age group had unknown vaccine history.

MDH reporting rules require that clinical isolates of Bordetella pertussis be submitted to the Public Health Laboratory. Of the 115 culture-confirmed cases, 100% of the isolates were received and subtyped by PFGE and tested for antibiotic susceptibility to erythromycin, ampicillin, and trimethoprim-sulfamethoxazole. Twenty-two distinct PFGE patterns were identified; 11 of these patterns occurred in only a single case isolate. The two most common patterns identified accounted for 53 (46%) of the total isolates and they occurred throughout the year.

No cases of erythromycin-resistant B. pertussis have been identified in Minnesota since the first case was identified in October 1999. Statewide, all 1,155 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 erythromycin-resistant B. pertussis cases have been identified to date in the United States.

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. 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.

Pertussis booster vaccines licensed in 2005 for persons 10 years of age and older will help to decrease the incidence and transmission of pertussis in the community. The ACIP recommends the routine use of Tdap vaccines in adolescents aged 11-18 years in place of tetanus and diphtheria toxoids (Td) vaccines. See page 46 of the May/August 2005 DCN for more information.

• For up to date information see>> Pertussis
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2005