Pertussis in Childhood

The incidence in the United States of pertussis, a potentially fatal disease, has increased during the past decade and new recommendations for vaccination have been made in recent years.After reading this article, readers should be able to:Pertussis, commonly known as “whooping cough,” is a respiratory illness caused by the bacterium Bordetella pertussis. The classic clinical syndrome causes morbidity by affecting the upper respiratory tract in patients of all ages. The disease can be modified greatly and prevented by primary vaccination. An ongoing resurgence of clinical pertussis has been seen in the United States over the past decade, with increasing numbers of young infants affected despite the availability of effective vaccines. It is important to understand the biological properties of the bacterium, the clinical presentation, and the factors contributing to the continuing burden of this disease.B pertussis is a small Gram-negative coccobacillus that infects only humans. It is aerobic and grows best at 35°C to 37°C. Bordetella species, including B pertussis and B parapertussis, are fastidious and difficult to grow on media usually used in the laboratory to grow respiratory pathogens; B pertussis requires supplemental growth factors including charcoal, blood, and starch. Media such as Bordet-Gengou, which contains potato starch, and charcoal-based Regan-Lowe media typically are used in microbiology laboratories for culturing the organism.B pertussis causes irritation and inflammation by infecting the ciliated respiratory tract epithelium. The ensuing tissue necrosis and epithelial cell damage recruits macrophages, and reactive lymphoid hyperplasia of peribronchial and tracheobronchial lymph nodes occurs.The bacterium has several virulence factors and toxins that are important in the pathogenesis of the disease and also play a role in inducing protective immune responses. Filamentous hemagglutinin and fimbriae are adhesins required for tracheal colonization. These substances are highly immunogenic and are major components of acellular vaccines. Other virulence factors such as pertactin and pertussis toxin (PT) can act as adhesins as well. PT can inactivate or suppress signaling pathways of the immune system in the lung, which delays recruitment of neutrophils. The role of pertussis toxin in the pathogenesis of pertussis is not fully understood. The toxin has been shown to cause leukocytosis with lymphocytosis and possibly the rare encephalopathy seen in the clinical disease. Other direct systemic effects of PT include sensitization of the beta-islet cells of the pancreas. This effect can lead to hyperinsulinemia with a resistant hypoglycemia, and sometimes occurs in young infants who have poor feeding, which exacerbates the symptoms. Adenylate cyclase toxin inhibits migration and activation of phagocytes and T cells.Worldwide, an estimated 50 million cases and 300,000 deaths due to pertussis occur annually. (1) In the United States, pertussis is an endemic disease, with periodic epidemics every 3 to 5 years and frequent outbreaks. The last peak in the incidence of pertussis occurred in 2005, when ∼25,000 cases were reported nationally. Increasing incidence has been noted in the United States and other countries despite widespread immunization. In 2009, nearly 17,000 cases of pertussis were reported in the United States, with many more going unreported. (2)In the past year, 9,477 cases of pertussis (including 10 infant deaths) were reported in California, the highest incidence in the state since the cyclical peak in 2005. (3) According to the Centers for Disease Control and Prevention (CDC), 50% of infants under age 1 year who are infected with pertussis will require hospitalization. Of these, 50% will develop pneumonia and 1% will die of complications from their infection. Pertussis morbidity and mortality is most significant in infants younger than age 3 months. Infants in this age group have the highest incidence of hospitalization, admission to intensive care units, and death from pertussis.In a review of a national pediatric inpatient database from 2000 to 2003, 86% of all hospitalizations for pertussis were in infants age <3 months. (4) In the United States and other industrialized countries, the resurgence of pertussis is being seen in very young infants who are not fully immunized, and in children and adolescents aged 10 years and older. Waning vaccine-induced immunity and lack of natural booster events may account for many of those cases in this older age group. Other factors that might be contributing to the increase in reported cases of pertussis are heightened awareness and reporting among health-care providers, increased use of polymerase chain reaction (PCR) testing for diagnosis, and decreased pertussis vaccination rates in some areas (Fig 1).Pertussis in developing countries is still the source of the highest disease burdens, primarily in Asia, Africa, and South America. Pertussis worldwide remains one of the top 10 causes of mortality in infants younger than age 1 year. Varying case definitions make comparisons of incidence between countries difficult. Hospitalization rates sometimes are used to follow incidence and the severity of outbreaks.A clinical case is defined commonly as a cough illness lasting at least 2 weeks with one of the following: paroxysms of coughing, inspiratory