Haemophilus. influenzae was early described by Richard Pfeiffer in 1893, all alone during an influenza pandemic. H. influenzae is accomplished only in human beings; it is a facultative anaerobic bacterium. 

The animal reservoir has just been sourced for H. influenzae. These organisms spread from one person to another by direct contact, secretions, and via aerosol, which they colonize through the nasopharynx. Infected individuals, specifically those left untreated and with infections with poor immunity, spread the disease by respiratory droplets with H. influenzae. 

Exposure criteria to H. influenzae are more in children. According to the 2017 annual report, the occurrence rate was 1.7 for non-b H. influenzae and 0.18 for Hib, and 1.6 for non-typeable H. influenzae, which affected 100,000 children under five years old. The carriage criteria are approximately 20% in juveniles and 1-year-old babies, increasing to about 50% in 5 years old children. 

Healthy adults are also infected by Haemophilus influenzae, which spreads and colonizes by aerosol, direct and venereal contact, or through secretions. The colonization rate is higher in nontyable H. influenzae compared to type b H. influenzae. But still, both types multiply quickly and maintain a higher growth rate. 

Scientific classification:  

Kingdom: Bacteria  

Phylum: Proteobacteria  

Class: Gammaproteobacteria  

Order: Pasteurellales  

Family: Pasteurellaceae  

Genus: Haemophilus  

Species: Haemophilus influenzae  

Structure: 

• Haemophilus influenzae (also Pfeiffer’s bacillus) is a Gram-negative coccobacillus that resembles other pathogenic bacilli in ultrastructural features. It is a capnophilic pathogenic bacteria. 
• H. influenzae is a mesophilic, non-motile, and facultatively anaerobe which grows at 37°C optimum temperature. 
• Size varies from the length of 1.0 to 1.5 microns and width of 0.3 microns, often encapsulated. The strains are subdivided into serovars a-f based on encapsulation and the definite structure of capsule polysaccharides. 
• Their cell walls are made of lipooligosaccharides, like Gram-negative bacilli’s lipopolysaccharide but differ in short side chains. 

Six known antigenic types of Haemophilus influenzae are identified by their capsular polysaccharides, which are important virulence factors. The six types are: 

• Haemophilus influenzae type a (Hia) 
• Haemophilus influenzae type b (Hib) 
• Haemophilus influenzae type c (Hic) 
• Haemophilus influenzae type d (Hid) 
• Haemophilus influenzae type e (Hie) 
• Non-typeable Haemophilus influenzae (NTHi)

Among these types, H. influenzae type b (Hib) is the most common cause of invasive diseases, such as meningitis and bacteremia, especially in young children. The other types are less commonly associated with disease but can cause respiratory, ear, and sinus infections. Non-typeable H. influenzae (NTHi) lacks a capsule, is a periodic colonizer of the human respiratory tract, and can cause various infections. 

H. influenzae is a mucosal parasite that causes disease in the upper respiratory system of healthy persons. Outer cell membrane consisting of Lip oligosaccharide in H. influenzae is majorly responsible for skin inflammation.

The strains generally identified are non-encapsulated. They are hardly virulent. IgA1 protease is produced in the meninges and respiratory tract mucosa as an affinity by H. influenzae. Pathogenicity can be determined by the primary determinant in which cell phagocytosis is prevented due to capsule protection. 

 H. influenzae pathogenesis is not understood, although encapsulated type b (Hib) has a poly ribosyl ribitol phosphate capsule within them, these serotype causes conditions known as epiglottitis, which is meant to be a primary virulence factor.

The capsule of type b H. influenzae allows them to fight against complement-mediated lysis and phagocytosis in the host with nonimmunity. The unencapsulated strains are always less invasive. However, they can launch an inflammatory response in human subjects, which leads to various symptoms.  

Hib conjugate vaccine is productive in inhibiting Hib infection, but it doesn’t prevent infections caused by NTHi strains. The non-encapsulated H. influenzae strain can also cause upper and lower respiratory tract infections, even in immunocompromised adults, 

• Host’s Serum antibody attacks the capsule in the case of typable H. influenzae or somatic antigens; it is bactericidal and promotes phagocytosis. 
• In the last ten years, it was trusted that bactericidal antibodies directed against PRP capsules of type b H. influenzae were entirely responsible for host resistance against infection. Bactericidal antibodies that react with individual outer membrane proteins or lipo oligosaccharide constituents have been identified. 
• The host’s primary immune response consists of native T-cells and antigen-presenting cells (APCs) from specialized skin-fatiguing lymph nodes. Its serum anti-cps antibodies confer immunity against invasive Hib disease, a short time immunity protection counter to invasive Hib diseases. 

• Haemophilus influenzae directly causes respiratory tract infections, including pneumonia, eye infections, otitis media (where the throat swells), meningitis, and bloodstream infections. It also causes infectious like Infectious arthritis (redness of the joint) and cellulitis (damaged rough skin). 
• Sinusitis, otitis media (middle ear infection with fever in children), pneumonia, and tracheobronchitis are caused by Non-typable H. influenzae. 
• Non-typable H. influenzae strains cause chronic or acute sinusitis in patients of all ages. 
• Cellulitis and Epiglottitis: A skin infection later becomes a syndrome caused by Type B H. influenzae. Swelling and inflammation of epiglottitis, upper airway closing condition. Difficulty in breathing progresses from a sore throat quickly, stridor, and airway obstruction, which results in respiratory arrest. 
• These organisms also cause respiratory infections like otitis media, pneumonia, acute tracheobronchitis, and sinusitis. 
• H. influenzae causes bacteremia with successive seeding of vascular choroid plexus in the meninges or causes meningitis via lymphatic drainage directly by reaching the meninges. 
• Brazilian Purpuric Fever is a hardcore disease caused by H. influenzae, which ends as a syndrome with substantial mortality. Primary symptoms with skin lesions, vomiting, and nausea, later show up with hemorrhagic fever, shock, and prostration. 

• The identification method involves a cerebrospinal fluid test for a pathogen.  
• The capsular antigen may be detected in an antigen test. 
• Purulent sputum, pus, and blood samples are used for Microbial assay, microscopy, and culture assays.  
• V factor requirement is indicated by satellite on the blood agar.  
• The porphyrin test readily confirms the X factor requirement in the presence of H. influenzae, which shows a positive result.
• Many cases are diagnosed after showing chest infections that do not respond to first-generation cephalosporins or penicillin, or Alveolar consolidation can be identified by chest X-ray. 
• Clinical diagnosis of invasive H. influenzae infection is typically determined by bacterial culture, latex granule agglutination tests, or polymerase chain reaction tests on clinical samples; respiratory secretions and cerebrospinal fluid can be cultured on chocolate agar. 

• The Hib vaccine is recommended for all infants and young children, and the pneumococcal vaccine is recommended for adults who are at increased risk of pneumococcal disease, which some strains of H. influenzae can cause. 
• Practicing good hygiene, like washing hands regularly with soap and water, Masking the mouth when coughing, and restricting close contact with sick people, can help reduce the pathogen’s spread. 
• Antibiotics may be prescribed to treat Haemophilus influenzae infections, especially for severe or invasive infections, such as meningitis and pneumonia. 
• Patients with severe or invasive H. influenzae infections may need to be isolated to prevent the spread of the bacteria. Healthcare workers should also use appropriate contact precautions, like wearing gloves and gowns, when caring for patients with H. influenzae infections. 
• Public health authorities may conduct surveillance for H. influenzae infections to monitor the incidence and prevalence of the disease, identify outbreaks, and develop strategies for controlling the spread of the bacteria. 

https://en.wikipedia.org/wiki/Haemophilus_influenzae 
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