Haemophilus somnus 129PT
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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.