Haemophilus

(microbiology) A genus of gram-negative coccobacilli or rod-shaped bacteria of uncertain affiliation; cells may form threads and filaments and are aerobic or facultatively anaerobic; strictly blood parasites.

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A genus of gram-negative, pleomorphic bacteria that are facultative anaerobes and are nonmotile and non-spore-forming.

Haemophilus influenzae was the first of the species to be isolated and is considered the type species. It was originally recovered during the influenza pandemic of 1889 and for atime was believed to be the causative agent of influenza; thus it was called the influenza bacillus. However, when this fallacy became apparent, the organism was renamed, still reflecting the historical association with influenza.

Haemophilus species are distinguished by a number of criteria. Strains of H. influenzae can be separated into encapsulated and nonencapsulated forms. Encapsulated strains express one of six biochemically and antigenically distinct capsular polysaccharides that are designated serotypes a through f. Nonencapsulated strains are referred to as nontypable. See also Influenza; Meningitis.

Haemophilus influenzae is a human-specific pathogen that inhabits the upper respiratory tract and is acquired by exposure to airborne droplets or contact with respiratory secretions. Non-typable strains can be isolated from the nasopharynx of up to 80% of normal children and adults at any given time, usually in association with asymptomatic colonization. Overall, these organisms are the leading cause of exacerbations of chronic bronchitis, and the second most common etiology of acute otitis media and sinusitis. On occasion, nontypable H. influenzae causes invasive disease such as meningitis, septicemia, endocarditis, epiglottitis, or septic arthritis. Invasive disease occurs most often in neonates and in patients with underlying immunodeficiency, especially when abnormalities in humoral immunity are present.

Encapsulated strains of H. influenzae are present in the nasopharynx of only 2–5% of children and an even smaller percentage of adults. Historically, H. influenzae type b strains were the primary cause of childhood bacterial meningitis and a majorityof other bacteremic diseases in children. However, in recent years the incidence of disease dueto H. influenzae type b has plummeted in the United States and other developed countries, reflecting the routine use of H. influenzae conjugate vaccines. These vaccines provide effective protection against disease due to H. influenzae type b but fail to protect against non-type b strains.

Haemophilus aphrophilus, H. haemolyticus, H. parahaemolyticus, H. parainfluenzae, and H. segnis are members of the normal flora in the human oral cavity and oropharynx and have low pathogenic potential. Among these species, H. parainfluenzae is the most common pathogen and has been reported in association with a variety of diseases.

Strains of H. influenzae are increasingly resistant to a wide variety of antibiotics. Accordingly, an extended-spectrum cephalosporin is generally recommended for empiric treatment of serious disease. See also Antibiotic; Drug resistance; Medical bacteriology.

A genus of gramnegative pathogenic bacteria, frequently found in the respiratory tract of humans and other animals. Hae-mophilus species are generally sensitive to cephalosporins, tetracyclines, and sulfonamides.

A genus of hemophilic gram-negative coccobacilli or rod-shaped bacteria.

• H. agni — see H. somnus now called Histophilus somni.
• H. avium — now called Avibacterium avium.
• H. bovis — see moraxella bovis.
• H. equigenitalis — see taylorella equigenitalis.
• H. gallinarum — now classified as Avibacterium paragallinarum.
• H. haemoglobinophilus — found on canine genitalia; sometimes linked to puppy mortality but not often a cause of disease.
• H. influenzaemurium — the cause of respiratory disease and conjunctivitis in mice.
• H. ovis — now called Histophilus somni.
• H. paracuniculus — may be associated with mucoid enteropathy in rabbits.
• H. paragallinarum — now called Avibacterium paragallinarum.
• H. parahemolyticus (syn. H. pleuropneumoniae) — see actinobacillus pleuropneumoniae.
• H. parainfluenzae — reputed to cause a syndrome in pigs similar to Glasser's disease (H. suis, H. parasuis).
• H. parasuis — a common concurrent infection with swine influenza virus and causes glasser's disease of swine.
• H. piscium — a cause of ulceration of the gills and mouth of trout.
• H. somnus — now called Histophilus somni.
• H. suis — now classified as H. parasuis (above).

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