What is the pathophysiology of Haemophilus influenzae infections?

Updated: Jun 11, 2021
  • Author: Joseph Adrian L Buensalido, MD; Chief Editor: Pranatharthi Haran Chandrasekar, MBBS, MD  more...
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The nomenclature (Haemophilus is Greek for "blood loving") acknowledges the fact that H influenzae requires 2 erythrocyte factors for growth: X (hemin) and V (nicotinamide-adenine-dinucleotide). These factors are released following lysis of red blood cells, thereby allowing growth of this fastidious organism on chocolate agar. H influenzae consists of 8 biotypes; biotype 3 (Haemophilus aegyptius) is associated with Brazilian purpuric fever, and biotype 4 is a neonatal, maternal, and genital pathogen. Humans are the only natural hosts. NTHi strains are a common resident of the nasopharyngeal mucosa and, in some instances, of the conjunctivae and genital tract.

Transmission is by direct contact or by inhalation of respiratory tract droplets. Nasopharyngeal colonization of encapsulated H influenzae is uncommon, occurring in 2-5% of children in the prevaccine era and even less after widespread vaccination. The incubation period is not known. A larger bacterial load or the presence of a concomitant viral infection can potentiate the infection. The colonizing bacteria invade the mucosa and enter the bloodstream. The presence of antibodies, complements, and phagocytes determines the clearance of the bacteremia. The antiphagocytic nature of the Hib capsule and the absence of the anticapsular antibody lead to increasing bacterial proliferation. When the bacterial concentration exceeds a critical level, it can disseminate to various sites, including meninges, subcutaneous tissue, joints, pleura, pericardia, and lungs.

Host defenses include the activation of the alternative and classical complement pathways and antibodies to the PRP capsule. The antibody to the Hib capsule plays the primary role in conferring immunity. Newborns have a low risk of infection, likely because of acquired maternal antibodies. When these transplacental antibodies to the PRP antigen wane, infants are at high risk of developing invasive H influenzae disease, and their immune responses are low even after the disease. Therefore, they are at high risk of repeat infections since prior episodes of H influenzae do not confer immunity. By age 5 years, most children have naturally acquired antibodies. The Hib conjugate vaccine induces protection by inducing antibodies against the PRP capsule. The Hib conjugate vaccine does not provide protection against NTHi strains. Since the widespread use of the Hib conjugate vaccine, NTHi has become a more common pathogen.

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