Histophilus somni

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Pneumonia in a calf due to Histophilus somni infection
Postmortem of a calf showing meningoencephalitis

Histophilus somni is a gram-negative pleomorphic coccobacillus that is an obligate parasite of bovine mucosal surfaces. H. somni is incriminated in causing bovine respiratory disease and thrombotic meningoencephalitis in cattle[1]. The respiratory, genital, nervous, circulatory, and musculoskeletal systems can be affected.

Infections with H. somni occur sporadically worldwide, and the organism often affects cattle which are immunocompromised due to stress or other illnesses[2].

Young cattle appear to be more prone to infections, and symptoms are often rapid in onset. Clinical signs include dyspnoea, stiffness, muscle weakness, ataxia, paralysis, and opisthotonos. Pneumonia and meningoencephalitis are common sequela and death occurs in a large number of affected weaned calves.

Diagnosis is usually confirmed on postmortem, where lesions including fibrinous pleuritis, fibrinous pericarditis and fibrinopurulent bronchopneumonia can be observed. in some calves, polyarthritis and abscessed laryngitis have also been reported.

Treatment can be problematic as most antibiotics appear to be ineffective. However, a commercially available, oral formulation of chlortetracycline and sulfamethazine fed for the first 56 days of the feeding period has significantly reduced overall mortality[3].

Recombinant subunit vaccines have been recently reported as being effective at minimising disease severity in calves[4][5].

References

  1. Larson RL & Step DL (2012) Evidence-based effectiveness of vaccination against Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni in feedlot cattle for mitigating the incidence and effect of bovine respiratory disease complex. Vet Clin North Am Food Anim Pract 28(1):97-106
  2. Elswaifi SF et al (2012) The role of lipooligosaccharide phosphorylcholine in colonization and pathogenesis of Histophilus somni in cattle. Vet Res 43(1):49
  3. Merck Vet Manual
  4. Lo KL et al (2012) Antibody responses of calves to Histophilus somni recombinant IbpA subunits. Comp Immunol Microbiol Infect Dis 35(5):453-459
  5. Geertsema RS et al (2011) IbpA DR2 subunit immunization protects calves against Histophilus somni pneumonia. Vaccine 29(29-30):4805-4812