Haemophilus Influenzae: Diagnosis, Treatment and Prevention | Hib Treatment

 Haemophilus Influenzae

Haemophilus Influenzae is a small, fastidious Gram-negative coccobacillus, belonging to the family pasteurellacege. It is highly adapted to humans and found in 75% of healthy children and adults in the nasopharynx.

Epidemiology

Polysaccharide encapsulated ( serotypes a-f) and non-encapsulated " nontypeable" strains. Droplet transmission, H. influenzae serotype b (Hib), is a common cause of invasive infections in children, including meningitis, septic arthritis, and epiglottitis. The annual incidence of invasive Hib disease dropped dramatically after the introduction of the Hib conjugate vaccine in 1993. The lack of a toddler booster saw the incidence rising due to waning immunity and rising case from 1999. Booster vaccination to those under 4 years old from 2004 saw a reduction in invasive disease across all age groups, as carriage fell.

Pathogenesis

H.influenzae inhabits the upper respiratory tract of humans; 25-80% of healthy people carry non-capsulated organisms, while 5-19% carry capsulated strains (~50% of which are capsular type b). In addition to the polysaccharide fimbriae), ( involved in attaching to epithelial cells), IgA proteases (aid colonization), and OMPs ( involved in the invasion). There is evidence that simultaneous viral infection may initiate an invasion. Other serotypes and unencapsulated strains are rarely invasive but can cause pneumonia or severe disease in high-risk groups.

Clinical Features

• Invasive infections, e.g. meningitis epiglottitis, bacteraemia with no clear focus, septic arthritis, pneumonia, cellulitis. Mostly caused by capsular type b. Types E and F .and non-capsulated strains may also cause serious disease. Infections generally occur between 2 months and 2 years of age, as babies <2 months are protected by maternal antibody.
• Non-invasive infections, e.g. otitis media, sinusitis, endometritis, purulent exacerbations of COPD. These local infections are usually associated with non-capsulated organisms. There may be an underlying abnormality (anatomical or physiological). Intercurrent viral infection may precipitate an infection.

Diagnosis

• Culture-growth in the presence of X (haemin) and V ( nicotinamide adenine dinucleotide (phosphate), NAD (P) factors, X is needed to synthesize certain respiratory enzymes that contain iron ( e.g. cytochrome c, cytochrome oxidase, catalase, peroxidase). V is required for oxidation-reduction processes in metabolism. Blood agar (BA) contains both X and V, but H. influenzae grows poorly. NAD supplementation improves growth on BA, as will streaking an organism that excretes NAD, e.g. aureus-this phenomenon is called satellitism. H. influenzae grows well on chocolate agar, which is made by heating BA at 70-80०C for a few minutes to inactive the NADase which normally limits utilization of V factor. Growth is also better in CO2- enriched conditions. Antibiotic susceptibility testing with discs may be unreliable-nitrocefin strips are recommended to test for β-lactamases. Oxidase-positive.
• Antigen detection, e.g. latex agglutination. Beware of cross-reactions with S. pneumonia and E. coli. Culture is needed for confirmation. Molecular tests, e.g. PCR, are available.
• Capsule detection-encapsulated strains of H. influenzae are responsible for most invasive infections ( e.g. meningitis and epiglottitis), with non-encapsulated strains. The polysaccharide capsule can be demonstrated by the Quellung reaction with type-specific antisera.
• Antigen type-there are six antigen types (a-f). Hib causes the most severe invasive infections.
• Biotypes-there are eight biotypes of H. influenzae (I-VII), based on indole, ornithine decarboxylase, and urease reactions. The commonest are biotypes I-III, and the most invasive (type-b) organisms are biotype I.

Treatment

• Severe infection-first line: third-generation cephalosporine, e.g. ceftriaxone. Bactericidal, penetrate the CSF and are clinically effective. Alternatives include co-trimoxazole, ampicillin or chloramphenicol.
• Non-invasive infection-PO ampicillin, co-amoxiclav, or azithromycin.
• BLNAR ( β-lactamase-negative ampicillin-resistant) H.influenzae are becoming increasingly recognized worldwide. The mechanism of resistance is altered PBPs.  β-lactamase is absent phenotypically, but the ampicillin MIC ≥4mg/L. They are usually sensitive to ceftriaxone.

Prevention

• Vaccine-Hib conjugate vaccine (capsular polysaccharide and protein) was introduced in the UK in 1992. Now given at 2,3,4, and 12 months. The impact of vaccination far exceeded that predicted, due to the added benefit of herd immunity. Give to unimmunized contacts of index cases
• Chemoprophylaxis-only required if the index case is confirmed or probable and under 10 years of age, or there is a vulnerable individual in the household (ideally within 48h, but up to 4 weeks after index diagnosis). Preferred agent is rifampicin. Reduces carriage and onward transmission. May be considered for close contacts in school outbreaks.

Other Haemophilus Species

Haemophilus spp, other than H.influenzae, have been considered rare causes of human disease in the past. However, they may cause infections more commonly than previously believed. Most are normal flora of the human mouth and upper respiratory tract. They may be associated with infections such as endocarditis, respiratory tract infections, septicaemia, brain abscess, meningitis and soft tissue infections.

Haemophilus parainfluenzae

H. parainfluenzae is increasingly recognized as a cause of human infection. Clinical infections are similar to those caused by H. influenzae, but H. parainfluenzae tends to be less virulent. H. parainfluenzae has been reported as a cause of pharyngitis, epiglottitis, otitis media, conjunctivitis, dental abscess, pneumonia, empyema, septicaemia, endocarditis, septic arthritis, osteomyelitis, meningitis and urinary and genital tract infections. H.inluenzae differs from H. influenza, in that it is V factor-dependent only.

Haemophilus haemolyticus and haemophilus parahaemolyticus

It is commonly thought that these species rarely cause human disease. However, standard methods do not reliably distinguish H. haemolyticus from H. influenzae, so it may be common than previously considered.

Aggregatibacter aphrophilus

The species Haemophilus aphrophilus, Haemophilus paraphrophilus, and Haemophilus segnis have been reclassified as a single species Aggregatibacter aphrophilus-together with Actinovacillus actinomycetemcomitans. Organisms require CO2 for growth, and are independent of X factor and variably dependent on V factor for growth. They cause a variety of infections, including sinusitis, otitis media, pneumonia, empyema, bacteraemia, endocarditis, septic arthritis, osteomyelitis, meningitis, and wound infections.

Haemophilus ducreyi

This causes chancroid, an STI, common in Africa and South East Asia. It presents as a painful penile ulcer associated with inguinal lymphadenopathy. Microbiological diagnosis may be made when Gram-negative coccobacilli are isolated from a lymph node aspirate or from ulcer swabs. Microscopy is classically described as a "shoal of fish" appearance. Treatment options include tetracyclines, erythromycin, and c0-amoxiclav.

Haemophilus influenzae biogroup aegyptius

H. influenzae biogroup aegyptius was previously known as Haemophilus aegyptius or the Koch-Weeks bacillus. It is very similar biochemically to H.influenzae biotype III but can be differentiated by PCR. It causes Brazillian purpuric fever ( conjunctivitis leading to fulminant septicaemia, with a high mortality) and epidemic purulent conjunctivitis. Combination therapy with ampicillin and chloramphenicol is usually recommended.