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.