Chapter 139. Haemophilus Infections (Kỳ 2)

PathogenesisHib strains cause systemic disease by invasion and hematogenous spread from the respiratory tract to distant sites such as the meninges, bones, and joints. The type b polysaccharide capsule is an important virulence factor affecting the bacteriums ability to avoid opsonization and cause systemic disease.Nontypable strains cause disease by local invasion of mucosal surfaces. Otitis media results when bacteria reach the middle ear by way of the eustachian tube. Adults with chronic bronchitis experience recurrent lower respiratory tract infection due to nontypable strains.In addition, persistent nontypable H. influenzae colonization of the lower airways of adults with chronic obstructive pulmonary...
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Chapter 139. Haemophilus Infections (Kỳ 2) Chapter 139. Haemophilus Infections (Kỳ 2) Pathogenesis Hib strains cause systemic disease by invasion and hematogenous spreadfrom the respiratory tract to distant sites such as the meninges, bones, and joints.The type b polysaccharide capsule is an important virulence factor affecting thebacteriums ability to avoid opsonization and cause systemic disease. Nontypable strains cause disease by local invasion of mucosal surfaces.Otitis media results when bacteria reach the middle ear by way of the eustachiantube. Adults with chronic bronchitis experience recurrent lower respiratory tractinfection due to nontypable strains. In addition, persistent nontypable H. influenzae colonization of the lowerairways of adults with chronic obstructive pulmonary disease (COPD) contributesto the airway inflammation that is a hallmark of the disease. The incidence ofinvasive disease caused by nontypable strains is low. Immune Response Antibody to the capsule is important in protection from infection by Hibstrains. The level of (maternally acquired) serum antibody to the capsularpolysaccharide, which is a polymer of polyribitol ribose phosphate (PRP), declinesfrom birth to 6 months of age and, in the absence of vaccination, remains low until~2 or 3 years of age. The age at the antibody nadir correlates with that of the peak incidence oftype b disease. Antibody to PRP then appears partly as a result of exposure to Hibor cross-reacting antigens. Systemic Hib disease is unusual after the age of 6 yearsbecause of the presence of protective antibody. Vaccines in which PRP is conjugated to protein carrier molecules havebeen developed and are now used widely. These vaccines generate an antibodyresponse to PRP in infants and effectively prevent invasive infections in infantsand children. Since nontypable strains lack a capsule, the immune response to infection isdirected at noncapsular antigens. These antigens have generated considerableinterest as immune targets and potential vaccine components. The human immuneresponse to nontypable strains appears to be strain-specific, accounting in part forthe propensity of these strains to cause recurrent otitis media and recurrentexacerbations of chronic bronchitis in immunocompetent hosts. Clinical Manifestations Hib The most serious manifestation of infection with Hib is meningitis (Chap.376). The age of peak incidence varies somewhat among populations, dependingin part on the use of vaccine, but this infection primarily affects infants about one-fourth have a significant handicap of some type. If more subtlehandicaps are sought, up to half of survivors are found to have some neurologicsequelae, such as partial hearing loss and delayed language development. Epiglottitis (Chap. 31) is a life-threatening Hib infection involving cellulitisof the epiglottis and supraglottic tissues. It can lead to acute upper airwayobstruction. Its unique epidemiologic features are its occurrence in an older agegroup (2–7 years old) than other Hib infections and its absence among NavajoIndians and Alaskan Eskimos. Sore throat and fever rapidly progress to dysphagia,drooling, and airway obstruction. Epiglottitis also occurs in adults. Cellulitis (Chap. 119) due to Hib occurs in young children. The mostcommon location is on the head or neck, and the involved area sometimes takes ona characteristic bluish-red color. Most patients have bacteremia, and 10% have anadditional focus of infection. Hib causes pneumonia in infants. The infection is clinicallyindistinguishable from other types of bacterial pneumonia (e.g., pneumococcalpneumonia) except that Hib is more likely to involve the pleura. Several less common invasive conditions can be important clinicalmanifestations of Hib infection in children. These include osteomyelitis, septicarthritis, pericarditis, orbital cellulitis, endophthalmitis, urinary tract infection,abscesses, and bacteremia without an identifiable focus. As has been mentioned,Hib infections are unusual among patients >6 years old.