[4] Histophilus somni is also a known causative agent that is a part of the Bovine Respiratory Disease (BRD) complex,[4] which typically involves multiple pathogens residing together in biofilm environments.
[9] Histophilus somni is a gram-negative, rod or coccobacillus shaped bacteria that does not express pili or flagella making it non-motile.
[14] The biofilm protects H. somni from harmful substances while allowing co-existence with the host by down regulating virulence factor productions.
[18] Methods have been developed for the differentiation between ovine and bovine strains of H. somni, including restriction enzyme analysis.
[12] Due to the economic importance from production losses the genomes of both Histophilus somni pneumonia strain 2336 (2,263,857 base pairs with 1,980 protein coding genes) and preputial strain 129Pt (2,007,700 base pairs with 1,792 protein coding genes) have since been sequenced.
[12][20] The genome studies of Histophilus somni strains have identified specific markers encoding tetracycline resistance, and virulence factors, while allowing a better understanding of the role of horizontal gene transfer in the evolution of these strains[12] Plasmid-borne antimicrobial resistance is an important topic in modern microbiology and occurs commonly in members of the Pasteurellaceae family.
[21] After entry into the bloodstream, the bacteria can colonize other tissues around the body such as the heart and is involved in biofilm production on cardiac endothelium in bovine myocarditis.
In serum resistant virulent strains, outer membrane proteins such as a sialic acid-modified lipooligosaccharide (LOS) and immunoglobulin-binding protein-A (IbpA) were found to be important .
[21][22][23] H. somni LOS provides critical protection to the bacterium against host defences by undergoing phase variation both structurally and antigenically and acts as an endotoxin producing apoptotic activity in bovine epithelial cells, a classical sign of histophilosis.
[22][23] H. somni immunoglobulin-binding proteins have two repeat domains (DR1 and DR2) that have cytotoxic Fic motifs as well as an ability to bind to bovine IgG2.
[24] As previously mentioned, the bacteria is capable of producing biofilm in the heart of affected animals with clinical bovine myocarditis.
[7] Histophilosis may be present as a component of the Bovine Respiratory Disease Complex[4][5][7][18] which has a higher incidence in feedlot cattle.
[7] Histophilosis may also present as septucemia,[5] thrombotic meningoencephalitis-myelitis,[7] pneumonia, pleuritis, pericarditis, necrotizing myocarditis, and arthritis.
[12][4][5][6][7] Damage to these areas are caused by thrombus formation and thromboemboli in a septucemic animal followed by subsequent infarction and necrosis of tissue.
[27] When H. somni is detected in pneumonic lungs, it presents as fibrinosuppurative bronchopneumonia and/or severe, diffuse fibrinous pleuritis.
[30] A new line of research is treating susceptible populations with bacterial isolates from subclinical carriers of Histophilus somni to act like a probiotic bacteria for the respiratory tract.
A nasal inoculation of a nonpathogenic strain of H. somni could allow for the respiratory tract mucosa to be colonized with the commensal bacteria.
[28] H. somni vaccines are usually killed cells or specific outer membrane proteins but have not been proven to be effective at protecting cattle against disease.
[27] There are many constraints to vaccine use on top of ineffectiveness, including the timeliness of administration, adverse reactions from individuals in the herd, and research into the epidemiology of the disease.