Bovine Histophilus somni Disease Complex – NAVLE Study Guide

Overview and Clinical Importance

Histophilus somni (formerly Haemophilus somnus) is a gram-negative, pleomorphic coccobacillus that causes a multisystemic disease complex in cattle known as histophilosis. This opportunistic pathogen is a normal commensal of bovine respiratory and reproductive tract mucous membranes but can cause severe, often fatal disease when host immunity is compromised.

H. somni is one of the four major bacterial pathogens in the Bovine Respiratory Disease (BRD) complex, alongside Mannheimia haemolytica, Pasteurella multocida, and Mycoplasma bovis. The disease is most common in North American feedlot cattle and represents a significant economic burden to the cattle industry.

Etiology

Organism Characteristics

Epidemiology

Geographic Distribution and Prevalence

Histophilosis is most common in North America (especially Canada and the United States) but occurs sporadically worldwide. Up to 25% of cattle may harbor serum antibodies to the organism, and up to 90% of bulls may carry the organism in their reproductive tract.