Equis ISSN 2398-2977

Lung: pleuropneumonia - bacterial (pleuritis)

Synonym(s): Pleuropneumonia, bacterial pneumonia, infectious pleural effusion, pleuritis

Contributor(s): Tim Brazil, Christopher Brown, Tim R C Greet, Geoff J Lane, Timothy Mair, Nicola Menzies-Gow, Vetstream Ltd


  • Cause: aerobic and anaerobic bacterial infection of the lungs and pleural space by oropharyngeal bacteria which overwhelm the normal respiratory defense mechanisms. May be uni- or bilateral. Typically associated with a predisposing significant event, eg transport, strenuous exercise, general anesthesia or other stressor.
  • Signs: "sick" horse; pyrexia, varying degrees of respiratory distress and often evidence of thoracic pain, shallow respirations, soft cough, sweating and reluctance to move; easily confused with colic.
  • Diagnosis: thoracentesis on both sides of the chest, ultrasonography.
  • Treatment: drainage of pleural effusion and broad spectrum antibiotic therapy.
  • Prognosis: guarded; in acute cases with minimal effusion prognosis is good. However, the more chronic cases with extensive pathology have a very poor prognosis.



  • Wide range of aerobic and anaerobic bacteria are involved, usually normal inhabitants of the oropharynx which get inhaled.
  • Aerobic organisms commonly involved:
  • Anaerobic organisms commonly involved (occur in 46% of cases):
    • Bacteroidesspp.
    • Clostridiumspp.
  • Mixed infections are common.
  • See bacterial infections of the lung: overview   Lung: pneumonia - bacterial  .
  • Viral and mycotic pathogens can be the primary etiological agent(s) which suppress the respiratory defense mechanisms allowing bacterial infection.

Predisposing factors


  • Stress   →   compromised pulmonary defenses:
    • Prolonged transport.
    • Previous respiratory viral infection.
    • General anesthesia.
    • Racing.
    • Aspiration of saliva or foreign material, eg after choke.


  • Trauma to the thorax and penetrating chest wounds.


  • Bacterial colonization of the pulmonary parenchyma   →   pneumonia +/- abscessation   →   visceral pleura and pleural space   →   pleural effusion   →   fibrin production and organization   →   limited expansion and pain.
  • Transport   →   increased environmental temperature and humidity; also possible decreased airway immunity   →   increased number of aerosol bacteria   →   predisposition to lower airway disease.

The single most important predisposing factor is restraint, such that the horse is unable to lower its head as horses are commonly transported with their head tied up. Putting the head down aids the mucociliary escalator which is one of the normal defense mechanisms.

  • Strenuous exercise, eg racing, hard training or breaking in, increases the depth of penetration of bacteria and debris into the respiratory tract.
  • Pleural inflammation   →   sterile pleural fluid   →   pleural space (exudate).
  • Pneumonia   →   increased permeability of visceral pleural capillaries   →   bacteria enter pleural fluid   →   fibropurulent response.
  • Neutrophils, bacteria and cell debris accumulate.
  • Extensive fibrin deposits form on the parietal and visceral surface   →   membrane formation   →   loculation   →   limits extension of empyema BUT impedes therapeutic drainage.
  • Visceral and parietal fibroblasts   →   inelastic membrane (= 'pleural peel')   →   prevents lung expansion and pain on respiration (= pleurodynia). Because movement is painful   →   stiff gait.


  • Rapid onset of primary signs, often in hours to days after the stress event.
  • If early signs are missed   →   disease present 2-3 weeks   →   weight loss, purulent nasal discharge, sweating with a pain on respiration and walking raised jugular pulse height and ventral edema.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Bodecek S et al (2011) Pleuropneumonia in two horses caused by a tracheobronchial foreign body. Equine Vet Educ 23 (6), 296-301 WileyOnline.
  • Rush B R & Davis E G (2011) Pleuropneumonia: when additional diagnostics are indicated. Equine Vet Educ 23 (6), 302-305 VetMedResource.
  • Copas V (2011) Diagnosis and treatment of equine pleuropneumonia. In Pract 33 (4), 155-162 VetMedResource
  • Hilton H & Pusterla N (2009) Intrapleural fibrinolytic therapy in the management of septic pleuropneumonia in a horse. Vet Rec 164 (18), 558-559 PubMed.
  • Ferrucci F, Zucca E, Croci C et al (2008) Bacterial pneumonia and pleuropneumonia in sport horses: 17 cases (2001-2003). Equine Vet Educ 20 (10), 526-531 VetMedResource.
  • Racklyeft D J, Raidal S & Love D N (2000) Towards an understanding of equine pleuropneumonia - factors relevant for control. Aust Vet J 78, 334-338 PubMed.
  • Hudson N P H et al (1999) Case of pleuropneumonia with complications in a Thoroughbred stallion. Equine Vet Educ 11 (6), 285-289 VetMedResource.
  • Sprayberry K A & Byars T D (1999) Equine pleuropneumonia. Equine Vet Educ 11 (6), 290-293 VetMedResource.
  • Chaffin M K (1994) Diagnostic assessment of pleural effusion in horses. Comp Cont Ed 16 (8), 1035-1039 VetMedResource.

Other sources of information

  • Swinney C R (1998) Pleuropneumonia - diagnosis and treatment. In: Proc of the World Equine Airway Symposium.
  • Rose R J & Hodgson D R (1993) Manual of Equine Practice. W B Saunders Company, USA. pp 163-165. ISBN 0 7216 3739 6 (Excellent section on this disease).