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Haemophilus
somnus is
the cause of a variety of systemic diseases in cattle, including thrombotic-meningoencephalitis,
pneumonia, abortion and other reproductive diseases, arthritis, myocarditis,
and septicemia. Diseases due to H. somnus account for millions of
dollars in losses to the cattle industry, and are particularly prevalent
in feedlots. However, this agent may also affect dairy cattle. H. somnus is
one of the three most common pathogens involved in shipping fever pneumonia,
and may be the most economically important pathogen of feedlot cattle.
Although bacterin vaccines are available, they are considered inadequate.
Optimum development of a vaccine requires a clear understanding of the
mechanisms of pathogenesis and the organism’s capability to resist
host defenses. H. somnus does not produce potent cytotoxins, although
a weak toxin is made. This organism has been reported to survive within
neutrophils and macrophages, and it is highly efficient at inhibiting the
oxidative burst by bovine phagocytic cells. However, the mechanism responsible
for intracellular survival, or if it is related to inhibition of the oxidative
burst, has not been determined. This bacterium can also bind immunoglobulins,
which may act as a protective mechanism against host defenses. Nonspecific
binding of IgG may also be responsible for the nonspecificity of serological
diagnostic tests. The lipooligosaccharide (LOS) is currently the only component
of the organism that has been documented to be capable of inducing an inflammatory
response and therefore lesions characteristic of the diseases. The oligosaccharide
of the LOS is known to undergo phase variation and sialylation, which act
to protect the bacterium from host defenses and immunity. It has been reported
that H. somnus does not produce a capsular polysaccharide (CP).
However, a novel exopolysaccharide (PS) from this bacterium has recently
been purified and immune serum raised to it.
Only a few of the genes responsible for the wide variety of virulence factors
in this bacterium have been identified. Thus, whole genome sequencing can rapidly
provide a great deal of information to facilitate the identity, cloning, and
manipulation of these genes to determine their role in virulence and immunoprotection.
Furthermore, because many of the genes and some virulence properties of H.
somnus are similar to those of H. influenzae, comparative genomics
between these related bacteria should prove highly informative. |
