What is Haemophilus?

Pathogenicity and Clinical Significance

H. influenzae was named when it was isolated in the 1890’s from
persons suffering from influenza. It was later shown to be a
secondary bacterial infection and not the causative agent of that
disease. It is similar to many other members of this genus. In fact, H.
aegyptius, which causes conjunctivitis and Brazilian purpuric
fever, has been reclassified as a subtype of H. influenzae rather
than a separate species. Natural infections occur only in humans, although
infection can be artificially induced in a few animal species.

Both encapsulated and nonencapsulated strains exist. The encapsulated strains
show higher degrees of pathogenicity, most likely because the
capsule offers some protection against the host’s immune system and possibly
increases the bacteria’s virulence. Encapsulated strains are
divided into six serotypes (a-f) with H. influenzae serotype B
(Hib) being the most common pathogenic group. Before the widespread use of Hib
vaccine, approximately 95 percent of all invasive Haemophilus
infections in children, including 75 percent of meningitis cases, and 50 percent
of Haemophilus infections in adults, were caused by Hib. Hib
commonly causes meningitis, pneumonia, bacteremia, cellulitis, epiglottitis, and
septic arthritis. It also can cause osteomyelitis and endocarditis. In developed
countries, Hib infections in children have markedly decreased since the early
1990’s, when the
Hib vaccine became widely used. In the United States, for
example, Hib infections in children decreased 99 percent between 1990 and
2000.

The percentage of infections caused by nonencapsulated H. influenzae (NTHi) has risen markedly since the introduction of the Hib vaccine. NTHi strains are present in the nasopharynx of 80 percent of the adult population and, because the strains lack capsules, are not affected by the vaccines that target capsular antigens. Migration of the NTHi bacteria from the nasopharynx can lead to otitis media (middle-ear infection), sinusitis, bronchitis, and pneumonia. Many of these infections are self-limiting because the immune system recognizes nonencapsulated strains more readily than those that are encapsulated. NTHi can also lead, more rarely, to disseminated systemic disease. Smoking, viral infections, chronic lung disease, and immunodeficiency can make NTHi infections much more likely. In 2006, NTHi accounted for almost two thirds of all H. influenzae infections in the United States.

Ampicillin has been the drug of choice for treating H. influenzae, but many strains have developed resistance to the penicillin family of antibiotics. Chloramphinicol has also been used, but chloramphenicol resistance is also on the rise. Second and third generation cephalosporins, fluoroquinolines, and clarithromycin are good alternatives. In severe cases, the cyclosporins cefotaxime and ceftriaxone can be administered intravenously.

H. ducreyi was first isolated in 1899. It is most commonly isolated from the urogenital mucosa of humans, the bacterium’s only natural host. Like most members of its genus, H. ducreyi is a fastidious bacterium that requires enriched chocolate agar for growth. Genetic testing of H. ducreyi has shown it to be genetically related (albeit distantly) to other Haemophilus spp. and even to other members of Pasteurellaceae, although it has nutritional requirements similar to other members of this family. Some bacteriologists have suggested that H. ducreyi be placed as a monotypic genus in its own family.

H. ducreyi infection leads to chancroid
(soft chancre), a common sexually transmitted disease in less developed countries
in tropical and subtropical regions. The disease causes ulceration of the
genitalia and is endemic to sub-Saharan Africa, especially among men who have sex
with sex workers, who often are reservoirs for H. ducreyi.
H. ducreyi infection increases the likelihood of
human
immunodeficiency virus (HIV) transmission ten to one hundred times. Chancroid is uncommon in
the United States, with the last major outbreak in the 1980’s. Azythomycin is the
drug of choice for treating H. ducreyi infections. Erythromycin,
ciprofloxacin, and in severe cases, ceftriaxone are also used.

Other Haemophilus spp. that are commensal in humans only
rarely cause opportunistic infections. H. haemolyticus,
H. parahaemolyticus, and H. parainfluenzae
are commonly found in the nasopharynx and oral cavities but are seen associated
only with pharyngitis and other conditions in debilitated persons. It
has been suggested that H. avium and H. agni be
placed within other genera in the Pasteurellaceae family because they are
genetically distant from all other Haemophilus spp. Other
species, such as H. paracuniculus and H.
parasuis, are somewhat genetically closer to the
Haemophilus spp. that affect humans, but their taxonomy is
under scientific review.

Albritton, W. L. “Biology of Haemophilus ducreyi.” Microbiological Reviews 53 (1989): 377-389.

Garrity, George M., ed. The Proteobacteria. Vol. 2 in Bergey’s Manual of Systematic Bacteriology. 2d ed. New York: Springer, 2005.

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

Spinola, Stanley M., Margaret E. Bauer, and Robert S. Munson, Jr. “Immunopathenogenesis of Haemophilus ducreyi Infection (Chancroid).” Infection and Immunity 70 (2002): 1667-1676.