Haemophilus influenzae infection

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Haemophilus influenzae
H. influenzae on a blood agar plate.
H. influenzae on a blood agar plate.
Scientific classification
Kingdom: Bacteria
Phylum: Proteobacteria
Class: Gamma Proteobacteria
Order: Pasteurellales
Family: Pasteurellaceae
Genus: Haemophilus
Species: H. influenzae
Binomial name
Haemophilus influenzae
(Lehmann & Neumann 1896)
Winslow et al. 1917

Haemophilus influenzae infection Main page

Patient Information




Otitis media


This page is about clinical aspects of the disease.  For microbiologic aspects of the causative organism(s), see Haemophilus influenzae.

For patient information about Haemophilus influenzae infection, click here | Haemophilus influenzae type b (Hib) vaccine, click here.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] ; Associate Editor(s)-in-Chief: Aditya Ganti M.B.B.S. [2]
Synonyms and keywords: Hib disease


H.influenzae is a gram-negative, cocco-bacillary, facultatively anaerobic pathogenic bacterium that can cause infections in people of all ages ranging from mild, such as an ear infection, to severe, such as a bloodstream infection. It is a normal commensal of nose and throat and does not cause any infection under normal situations. But when the host defense mechanisms are weakened it invades the epithelium and disseminates to other parts of the body where it causes infection.[1]



H. influenzae, including Hib, can cause many different kinds of infections. These infections can range from mild ear infections to severe diseases, like bloodstream infections. When the bacteria invade parts of the body, like spinal fluid or blood, this is known as "invasive disease." Invasive disease is usually severe and can sometimes result in death.
The most common types of invasive disease caused by H.influenza are:

The most common types of non-invasive disease caused by H.influenza are:

H influenza infection
Infection due to capsulated H influenzae(Invasive)
Infection due to non-capsulated H influenzae(Non-invasive)
Septic arthritis
otitis media


The pathogenesis of H.influenzae infections can be described as follows:[2][3]


  • Transmission is by direct contact or by inhalation of respiratory tract droplets.
  • Neonates can acquire the infection by aspiration of amniotic fluid or contact with genital tract secretions containing the bacteria.

Incubation period

The incubation period (time between exposure and first symptoms) of H. influenzae disease is not certain but could be as short as 7 days.



  • The capsule of H influenza plays a key role in the pathogenesis of the all the capsulated H influenza infections.
  • The antiphagocytic nature of the Hib capsule makes it resistant to natural host phagocytic defense mechanisms and facilitating bacterial proliferation.
  • After proliferation, the bacterial load disseminates to various sites, including meninges, subcutaneous tissue, joints, pleura, pericardium, and lungs triggering an inflammatory response and subsequently activating the complement system.
  • Capsulated H influenza can penetrate the normal epithelium and are therefore responsible for invasive infections.
  • Non-capsulated are non-invasive but can still induce the inflammatory response similar to that of capsulated organisms
  • The Hib conjugate vaccine induces protection by inducing antibodies against the PRP capsule.
  • The Hib conjugate vaccine does not provide protection against Non-typable H influenza strains. Since the widespread use of the Hib conjugate vaccine, Non-typable H influenza strains has become more of a pathogen.


  1. "Pinkbook | Hib | Epidemiology of Vaccine Preventable Diseases | CDC".
  2. Duell BL, Su YC, Riesbeck K (2016). "Host-pathogen interactions of nontypeable Haemophilus influenzae: from commensal to pathogen". FEBS Lett. 590 (21): 3840–3853. doi:10.1002/1873-3468.12351. PMID 27508518.
  3. Close RM, Pearson C, Cohn J (2016). "Vaccine-preventable disease and the under-utilization of immunizations in complex humanitarian emergencies". Vaccine. 34 (39): 4649–55. doi:10.1016/j.vaccine.2016.08.025. PMID 27527818.

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