incidence of whooping cough Approximately 5 to 10 percent of clinical whooping cough is caused by B. parapertussis. The animal pathogen B. bronchiseptica is responsible for a very minor percentage of cases. On primary isolation, these organisms are indistinguishable from B. pertussis but can be differentiated by further bacteriologic or serologic procedures. Incidence and Epidemiology. In communities of susceptibles, the family attack rate is 80 to 90 percent, which is extremely high for a bacterial dis-ease approaching that seen in varicella or measles. Transmission is by droplet infection. Carriers of pertussis are found infrequently, and the reservoir is therefore unknown. The disease usually occurs in late winter in the northern climates and in late spring in southern zones, bat there is great variation.
The mortality rate from whooping cough fell markedly after the beginning of this century, before the advent of antimicrobial therapy and the general use of the pertussis vaccine. In 1920 the mortality rate in the United States was 12.5 per 100,000; in 1930 it was 4.8 per 100,000. For more than the past decade, the mortality rate has been below 1 per 100,000. Three-fourths of the deaths -occur in infants, persons under one year of age.
The incidence of whooping cough, however, did not change until after 1940, when immunization of young children became standard practice. incidence of whooping cough Large epidemics among the very young is no longer seen except in areas where prophylactic immunization is not carried out. In 1957 there were some 28,000 cases; in 1966 approximately 8000 cases of pertussis were reported in the United States.
incidence of whooping cough It is not generally appreciated that immunization against pertussis is not lifelong. Thus, approximately 10 to 20 percent of immunized young children exposed to whooping cough will acquire the disease. It is also significant that the incidence of pertussis in children over ten years of age has not changed. Indeed, in local epidemics, the family attack rate among older children who have been immunized early in life is greater than 50 percent.
Similarly, natural disease does not lead to protection for life. Second attacks or attacks occurring after primary immunization may not be diagnosed because they are often atypical and less severe than classic whooping cough in young children. Pathology Interpretation of pathologic material obtained at autopsy is difficult because of the common presence of complicating respiratory infections. Lesions caused by B. pertussis are found principally in the bronchi and bronchioles, but changes are also seen in the nasopharynx, larynx, and trachea.
intensity, and then with a deep inspiration the air is drawn into the lungs, making the “whoop.” A tenacious mucus plug is usually expelled, and vomiting frequently follows the spasmodic episode. Paroxysms may occur as often as every half hour and are accompanied by signs of increased venous pressure. The conjunctivae are deeply engorged: there is periorbital edema; and Petechial hemorrhages, particularly about t’ he forehead, as well as epistaxis are common. During the attack, the infant may be cyanotic until the crow-ing whoop occurs. In between paroxysms the child usually feels well though justifiably apprehensive.
Physical examination of the chest is usually unremarkable, though scattered rhonchi may be heard. The chest roentgenogram sometimes reveals hilar and mediastinal nodal enlargement. The presence of fever immediately suggests the development of a secondary infectious process. Convalescent Stage. Gradually the paroxysms become less frequent and less intense, vomit-ing ceases, and slow recovery ensues. Often for many months, even a mild, unrelated respiratory infection will be manifested by a return of paroxysmal cough and whoop.
It is important to recognize those patients with whooping cough in whom variation from the pat-tern frequently occurs. In young infants the paroxysms and the whoop are often absent; instead, choking spells and apneic periods may be the major manifestations. Second attacks of whooping cough as well as a disease occurring in older, previously immunized children are usually mild or abortive and difficult to recognize as clinical pertussis.