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Histophilus somni

Bovine Respiratory Disease complex, bronchopneumonia, Thromoembolic meningoencephalitis (TEME)

Histophilus somni (Haemophilus somnus) is a common disease-causing bacterium of cattle, with a large proportion of cattle carrying antibodies to the organism. H. somni tends to be an opportunistic pathogen that complicates viral infection and increases the severity of infection with other bacterial agents. H. somni can infect several organ systems after it spreads through the bloodstream, giving rise to the term H. somni complex. Respiratory infection often precedes infection of other organ systems. The respiratory, genital, nervous (brainers, thromboembolic meningoencephalitis, TEME), circulatory, and musculoskeletal systems (joints) can be affected, either individually or together.

In some areas, H. somni is considered a major respiratory pathogen and one of the three major bacterial participants in the shipping fever complex, along with Mannheimia haemolytica and Pasteurella multocida. The incidence of any of the H. somni syndromes is not known with certainty, but when all the disease forms are considered together they make H. somni an economically significant pathogen.

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  • Histophilus somni (previously known as Haemophilus somnus) is a bacterium that lives in the nasal passages of cattle.  Generally speaking, H. somni infects vascular tissue (blood vessels) and endothelium of organs, causing inflammation, thrombosis (formation of a vascular obstruction) that interrupts the blood supply, and causes local cellular death. Pathogenic or nonpathogenic H. somni is normally present in the genital organs of both male and female cattle, and may colonize in both the upper and lower respiratory tract. If H. somni infects the lungs, pneumonia can result in rapid death.

    When H. somni is involved in pneumonia it is often overgrown by Pasteurella organisms. If H. somni gains access to the bloodstream, it spreads throughout the body, a condition known as septicemia. Involvement of the central nervous system, whereby blood flow to the spinal cord and brain is affected, results in a syndrome known as thromboembolic meningoencepahlitis (TEME). The respiratory syndrome is often preceded by primary infection with viral pathogens, with respiratory signs sometimes followed by TEME.

  • Animals of all ages can be infected with H. somni, but cattle 6 months to 2 years of age tend to be most frequently affected. The respiratory syndrome occurs most often in feedlot calves, where a high concentration of animals and stress conditions exist. Clinical signs include high fever, depression, dyspnea, nasal and ocular discharge, stiffness, prostration, and death, sometimes within a day after onset of signs.

  • The lesions from Histophilus somni infection in the lungs can not be distinguished from Mannheimia hemolytica or Pasteurella multocida. A diagnosis depends on isolation of Histophilus somni from lung tissues.

  • Because respiratory infection is transmitted by aerosols, sick and exposed cattle should be isolated and treated. Management practices that minimize overcrowding and stress should be implemented. Metaphylaxis with DRAXXIN or EXCEDE in newly arrived feedlot cattle may have benefit in controlling H. somni infection. Post arrival antibiotic treatment combined with H. somni vaccination may help reduce the occurrence of disease in endemic areas. Generally, 2 doses given 2 to 4 weeks apart are required to obtain protective effect.