Bordetella

Transmission route: Inhalation

Definition

Most Bordetella species are obligate respiratory pathogens of animals and humans. B. pertussis causes a severe and potentially life-threatening disease (whooping cough, or pertussis) of infants and young children, characterized by repeated and violent coughing spells and the characteristic whooping sound that comes from breathing difficulties.

Natural Habitat and Features

The Bordetella species (except for B. petrii) are obligate respiratory pathogens of animals and humans. B. pertussis and B. parapertussis cause disease only in humans. The rest, except B. petrii, are found naturally in diseased animals, including birds. All have been found, in rare cases, to cause disease in immunocompromised humans.

All Bordetella species are gram-negative coccobacilli that are nonfermentative and are strict aerobes. B. pertussis requires enriched media containing charcoal or blood (or both) to grow in the laboratory because of their sensitivity to unsaturated fatty acids and sulfur compounds in regular agar media. On Bordet-Gengou agar, B. pertussis forms small (less than 1 millimeter) smooth, transparent, shiny colonies with a circular edge in about five to seven days of incubation at 98.6° Fahrenheit (37° Celsius). B. parapertussis forms similar but larger, duller brownish colonies after two days, and B. bronchiseptica forms larger, rougher, and pitted colonies in one to two days on this medium. Other species can be grown successfully on less stringent media.

Bordetella species can be differentiated by growth, biochemical, and antigenic characteristics. Molecular methods, including fluorescent antibody and other immunological (serological), polymerase chain reaction (PCR), and 16S rRNA (ribosomal ribonucleic acid) gene sequencing, have been employed to identify and study properties of various species.

It has been shown through 16S rRNA (ribosomal ribonucleic acid) gene sequencing that B. holmesii is closely related to B. pertussis, B. parapertussis, and B. bronchiseptica, whereas B. avium, B. hinzii, B. petrii, and B. trematum have diverged through time.

Pathogenicity and Clinical Significance

All Bordetella species are pathogens. B. pertussis and B. parapertussis affect only humans and cause whooping cough. Whooping cough is most severe in infants less than one year of age, with significant morbidity and mortality rates. Roughly 85 to 90 percent of those exposed get the disease, with the majority being hospitalized. Patients with B. parapertussis normally have a less severe form of the disease, indistinguishable from a mild upper-respiratory-tract infection.

Older children, adolescents, and adults can also contract whooping cough. Cases are normally milder because of increased immunity; however, immunocompromised persons can experience severe disease. Research suggests that adolescents and adults can infect susceptible infants, and vice versa. Therefore, health authorities recommend giving adolescent siblings, parents, and health care workers an additional pertussis booster immunization.

B. pertussis has been studied most extensively, so its pathogenesis and the disease-causing roles of its many virulence factors are well understood. The incubation period lasts five to twenty-one days after exposure. During this time, the organism employs adhesins, including filamentous hemagglutinin, pertussis toxin, pertactin, and fimbriae proteins.

Recognizable symptoms occur during the catarrhal stage, when the pathogen multiplies rapidly. Because these symptoms resemble a common cold, the organism can be transmitted before patients realize they have a serious disease. This stage normally lasts one to two weeks, with persons exhibiting rhinorrhea, mild fever, coryza, and mild cough (although even at this stage, infants can exhibit apnea and respiratory distress).

The paroxysmal stage occurs when numerous toxins, including pertussis toxin, adenylate cyclase, dermonecrotic toxin, and tracheal cytotoxin, cause biochemical abnormalities and tissue destruction that advance the disease process and battle the host’s immune defenses. During this stage, which lasts two to six weeks, characteristic multiple spasms of dry cough occur, often with projectile vomiting and exhaustion. In infants, the characteristic whoop occurs when he or she struggles to breathe.

During the convalescent stage (which last two to four weeks), patients have decreasing bouts of coughing and vomiting; however, secondary complications can occur, normally by other pathogens that can now colonize the host because of the biochemical and physical damage that occurred during B. pertussis pathogenesis. These complications include pneumonia, seizures, encephalopathy, and death. During this stage, recovery occurs when the host’s defenses revive and when tissue, especially the ciliated epithelium, regenerates.

Most Bordetella species can infect animals (including birds) and immunocompromised humans. B. bronchiseptica can establish asymptomatic infections or serious respiratory infections in various mammals: kennel cough in dogs, atrophic rhinitis in pigs, snuffles in rabbits, and guinea pig bronchopneumonia. B. avium causes a potentially fatal respiratory disease of birds, including chickens and turkeys, which can result in significant economic loss. B. hinzii is found naturally as a commensal organism in the respiratory tracts of poultry. The least understood species, B. trematum, has been found associated with wounds and ear infections. B. avium, B. hinzii, and B. petrii have all been found in the lungs of persons with cystic fibrosis. Any disease-causing role is unclear.

Drug Susceptibility

Treatment for whooping cough is primarily supportive; however, early antibiotic therapy can influence the severity and duration of the disease. It is critical to interfere with transmission to susceptible persons.

Traditionally, erythromycin has been used, but some infants experienced infantile hypertrophic pyloric stenosis. Another macrolide antibiotic, clarithromycin, has not been shown to be safe for infants. Azithromycin is effective and is preferred for infants younger than one month of age.

Azithromycin or clarithromycin are better for persons older than one month because they cause fewer side effects and less gastrointestinal upset. Persons older than two months of age who cannot tolerate a macrolide antibiotic can take trimethoprim-sulfamethoxazole for fourteen days. For persons exposed to clinically diagnosed pertussis cases, prophylaxis for five days with azithromycin or clarithromycin is recommended.

Although the efficacy of the pertussis portion of the diphtheria tetanus acellular pertussis (DTaP) vaccine is not 100 percent, immunization of infants, children, adolescents, and adults is the most effective way to combat the spread of pertussis.

Bibliography

EBSCO Publishing. DynaMed: Pertussis. Available through http://www.ebscohost.com/dynamed.

Levitzky, Michael G. Pulmonary Physiology. 7th ed. New York: McGraw-Hill Medical, 2007.

Long, S. S. “Pertussis.” In Nelson Textbook of Pediatrics, edited by Richard E. Behrman, Robert M. Kliegman, and Hal B. Jenson. 18th ed. Philadelphia: Saunders/Elsevier, 2007.

Mason, Robert J., et al., eds. Murray and Nadel’s Textbook of Respiratory Medicine. 5th ed. Philadelphia: Saunders/Elsevier, 2010.

Mattoo, Seema, and James D. Cherry. “Molecular Pathogenesis, Epidemiology, and Clinical Manifestations of Respiratory Infections Due to Bordetella pertussis and Other Bordetella Subspecies.” Clinical Microbiology Reviews 18 (2005): 326–382.

Sandora, Thomas J., Courtney A. Gidengil, and Grace M. Lee. “Pertussis Vaccination for Health Care Workers.” Clinical Microbiology Reviews 21 (2008): 426–434.

Weiss, Alison. “The Genus Bordetella.” In The Prokaryotes: A Handbook on the Biology of Bacteria, edited by Martin Dworkin et al. Vol. 5. New York: Springer, 2006.