What is a Haemophilus influenzae infection?

Definition

Haemophilus influenzae is a small gram-negative bacterium that
causes a variety of infections, primarily in young children. The species
designation influenzae reflects the earlier misdiagnosis of this
bacterium as the cause of influenza, which is a viral disease.

Causes

Strains of H. influenzae may be either encapsulated or
nonencapsulated, and six distinct types of capsules are recognized. Most cases of
serious invasive disease are caused by type B encapsulated strains (Hib), but
bacteria of any subtype may be present as part of the normal respiratory tract
flora in healthy persons. Humans are the only natural host of the
organism, and disease is spread from person to person by inhalation of respiratory
droplets or by direct contact with respiratory secretions.

Risk Factors

Young children who are not immunized are at high risk for contracting
H. influenzae infections, particularly if contact has been
established with a child with invasive Hib disease. Others at risk include people
with human
immunodeficiency virus (HIV) infection, sickle cell
disease, asplenia (absent or nonfunctioning
spleen), or malignant neoplasms.

Symptoms

H. influenzae infections can result in a range of symptoms. Most are respiratory tract infections such as pneumonia, bronchitis, sinusitis, and otitis media and thus cause symptoms associated with those diseases, such as coughing, sneezing, and pain. Some strains cause invasive diseases and their accompanying symptoms, such as meningitis, epiglottitis, cellulitis, and septic arthritis.

Screening and Diagnosis

Persons with a suspected H. influenzae infection may have a sample taken for analysis. In the laboratory, H. influenzae cells are most easily cultured on chocolate agar, a rich growth medium containing essential growth factors from hemolyzed red blood cells. Colonies appear gray, with a diameter of 0.5 to 0.8 millimeters and usually with rough edges. Strains surrounded by a polysaccharide capsule usually produce larger colonies that are somewhat mucoid in appearance.

Prevention and Outcomes

Until 1988, when the effective Hib conjugate vaccines were first introduced, Hib was the most common
cause of bacterial meningitis in children in the United States. Acute
epiglottitis caused by massive Hib colonization had a high
mortality in children age two to four years. In the years since the widespread use
of these vaccines, the incidence of invasive Hib disease in infants and young
children has decreased by almost 99 percent.

Three different Hib conjugate vaccines are commercially available in the United States, and all show excellent effectiveness with minimal adverse reactions. Children as young as two months of age can be immunized. Either a two- or three-dose regimen is administered, depending on the vaccine prescribed, and a booster is recommended at age twelve to fifteen months.

Treatment and Therapy

Invasive Hib infections are most commonly treated with a third-generation cephalosporin antibiotic, such as cefotaxime or ceftriaxone. Meropenem or the combination of chloramphenicol and ampicillin have also been used effectively. For localized respiratory tract infections such as otitis media, oral amoxicillin is usually prescribed. Because as many as 5 to 50 percent of isolates from around the world are resistant to ampicillin, an oral cephalosporin may be required as well. Rifampin has proved to be successful as a chemoprophylaxis agent in households with a minimum of one contact younger than four years of age because it eliminates Hib from the pharynx in the vast majority of carriers.

Bibliography

Brooks, George F., et al. Jawetz, Melnick, and Adelberg’s Medical Microbiology. 25th ed. New York: McGraw-Hill Medical, 2010.

Madigan, Michael T., and John M. Martinko. Brock Biology of Microorganisms. 12th ed. Upper Saddle River, N.J.: Pearson/Prentice Hall, 2010.

Pan American Health Organization. World Health Organization. Control of Diphtheria, Pertussis, Tetanus, “Haemophilus influenzae” Type B, and Hepatitis B Field Guide. Washington, D.C.: Author, 2005.

Southwick, Frederick. Infectious Diseases: A Clinical Short Course. 2d ed. New York: McGraw-Hill, 2007.