Recent epidemiologic studies have shown that the incidence and prevalence of Bordetella pertussis infection in adults are much greater than previously reported. In studies of adults with chronic cough, 20 to 25 percent were found to have serologic evidence of recent B. pertussis infection. However, pertussis is rarely considered in adults because the signs and symptoms are nonspecific. Yaari and colleagues investigated the clinical presentation and course of B. pertussis infection in persons who had been previously immunized.

Data regarding cases of serologically confirmed pertussis were obtained from the Israeli Ministry of Health. (Physicians in Israel are required to report all cases of pertussis.) The cases were reported between 1986 and 1991. During the five-year period, 552 cases of pertussis were reported. Of these, 283 cases occurred in persons from five to 30 years of age, which is the population the authors studied.

Criteria for serologic evidence of pertussis included a single antibody titer of 1:640 or greater, a fourfold rise in titers obtained two to three weeks apart or a positive pertussis IgM level by enzyme-linked immunosorbent assay. Physicians who reported the cases were contacted and asked to complete a questionnaire about the patients' signs and symptoms, laboratory results, the suspected initial diagnosis, treatment and complications.

Clinical information was available in 95 of the 283 cases in persons between five and 30 years of age. Females were affected more frequently than males, with 61 percent of the cases occurring in females. Seventy cases occurred in children who were between five and 10 years of age; 16 in children who were 11 to 14 years of age; and nine cases in persons who were 15 to 30 years of age.

A history of contact with other persons with pertussis was reported in 66 percent of the 95 cases. The mean duration from the onset of symptoms to diagnosis was 23 days (median: 14 days). The correct diagnosis was made initially in 46 percent of the cases. Mycoplasma pneumoniae infection was the most common incorrect diagnosis. It was the initial diagnosis in 17 percent of the cases, followed by sinusitis (7 percent), upper respiratory infection (4 percent), asthma (4 percent), laryngitis (3 percent) and suspected cystic fibrosis (1 percent).

All of the patients had a history of cough. The mean duration of cough was four weeks (median: three weeks). The cough was described as dry in 93 percent of the cases, but only 6 percent had the classic whoop. Paroxysmal nocturnal coughing was noted in 21 percent, and post-tussive vomiting in 13 percent. Fever was present in only 13 percent, and wheezing was found in 7 percent.

Five of the patients sought treatment in an emergency department because of severe cough and dyspnea, and two required hospital admission. These two cases occurred in a five-year-old girl with hypoxemia and a 10-year-old boy with secondary pneumonia and pleural effusion.

Before pertussis was diagnosed, 68 (71 percent) of the patients received antibiotics. Most of them (78 percent) received erythromycin. In the majority of cases, no clinical improvement was noted in response to antibiotic therapy.

The authors conclude that apart from the prolonged cough, there are no typical symptoms suggestive of pertussis in older persons who have been immunized. They believe pertussis should be considered in the differential diagnosis of persistent cough in previously immunized children and adults. Prompt administration of erythromycin may be a consideration in patients presenting with persistent cough. Prophylaxis of exposed persons before culture or serologic results are available would be another consideration. Early treatment should limit the spread of infection to persons whose immunity has waned or in unimmunized children. The development of an acellular vaccine may allow booster immunization, which in the future may be a method of preventing B. pertussis infection after immunity from the pertussis vaccination has waned.

editor's note: Clinical trials of an acellular pertussis vaccine have been underway for several years. Final approval may take several more years and is contingent on efficacy and safety data. Eventually, we may be giving a pertussis booster vaccine in the same way we now administer tetanus boosters.—j.t.k.