Streptococcus zoopepidemicus in companion animals

Dr Simon Priestnall BSc (Hons) BVSc PhD PGC(VetEd) FHEA DipACVP FRCPath MRCVS

Adapted from Small animal disease surveillance: respiratory disease 2017

Published May 2018

Reviewed November 2019 

The organism

Streptococcus equi subspecies zooepidemicus is a Gram-positive, beta-haemolytic bacterium historically associated with opportunistic infections, predominantly in the reproductive tract in horses and with lymphadenitis in guinea pigs.  In recent years the bacterium has emerged as a significant cause of pneumonia in dogs, notably those housed in kennels or rehoming centres, and particularly racing greyhounds [1].  Infection is usually sporadic, but can result in severe, and not infrequently fatal, pneumonia. The source of infection is usually unknown but presumed to be introduced by a carrier animal. Although transmission from horses to dogs may occur, it is now thought that transmission occurs mainly directly between dogs. Close contact is required to transmit infection and densely housed dogs or those with a weakened immune response due to concurrent respiratory viral infection appear to be most severely affected. The bacterium produces various exotoxins and the most serious disease manifestations are believed to result from the animal’s immune response to these so-called ‘superantigens’, resulting in a ‘cytokine storm’, similar to toxic shock syndrome in humans.   The bacterium is, as the name implies, zoonotic and has been isolated from a wide range of different species including humans where it has been linked, albeit rarely, with severe infections; in at least one case this has been linked directly back to contact with a dog with pneumonia [2].


A number of outbreaks of haemorrhagic pneumonia in dogs attributed to infection with S. zooepidemicus have been reported from around the world [3,4]. Affected dogs may initially show signs of relatively benign upper respiratory tract infection (nasal discharge, coughing); this initial presentation resembles other causes of ‘kennel cough’ and so the specific diagnosis may be missed. Affected dogs can rapidly progress to pyrexia, lethargy, inappetence and, if untreated, hypovolaemic shock. Key warning signs for pneumonia development would be pyrexia and marked lethargy.  


In confirmed cases, treatment is largely supportive, focusing on rapid intravenous broad-spectrum antibiotic administration and fluid therapy. Dogs can recover from infection if treated early. Clinicians should be alert to any fatality due to respiratory disease in groups of dogs, prompting swift investigation. 


At postmortem examination dogs have severe fibrinosuppurative, necrotising and haemorrhagic pneumonia with copious haemorrhagic pleural effusion and often petechial haemorrhages on the pleural surfaces [5]. Infection can be confirmed via routine bacterial culture from oropharyngeal, or preferably nasal, swabs. There are few differential diagnoses for haemorrhagic pneumonia in dogs, but recently, cases of extraintestinal Escherichia coli infection have produced similar rapidly progressing clinical signs and pneumonia in intensively housed young dogs.


Infected dogs should be quarantined and the environment thoroughly disinfected to prevent spread; the bacterium is relatively resistant within mucus from nasal secretions. Water and food bowls, bedding and leads can be a source of infection for other dogs. Infection is not always associated with clinical disease, and ‘carrier’ animals may occur, and thus there are likely to be as yet unknown host and environmental factors that are involved in the clinical expression of disease.

UK data

Data from SAVSNET between 2010 and 2017 record 144 canine samples from which S. zooepidemicus was cultured. Seventy-five (52.1 per cent) of the samples were from the respiratory tract (nose, trachea or oropharynx). The next most common site for S. zooepidemicus culture was abscesses (n=22; 15.3 per cent).

Read our surveillanc report with a focus on Streptococcus zooepidemicus here.



  1. PRIESTNALL S, ERLES K. Streptococcus zooepidemicus: an emerging canine pathogen. Vet J 2011;188:142–8
  2. ABBOTT Y, ACKE E, KHAN S, et al. Zoonotic transmission of Streptococcus equi subsp zooepidemicus from a dog to a handler. J Med Microbiol 2010;59:120–3
  3. FITZGERALD W, CROWE B, BRENNAN P, et al. Acute fatal haemorrhagic pneumonia caused by Streptococcus equi zooepidemicus in greyhounds in Ireland with subsequent typing of the isolates. Vet Rec 2017;doi: 10.1136/ vr104275
  4. PESAVENTO PA, HURLEY KF, BANNASCH MJ, et al. A clonal outbreak of acute fatal hemorrhagic pneumonia in intensively housed (shelter) dogs caused by Streptococcus equi subsp zooepidemicus. Vet Pathol 2008;45:51–3
  5. PRIESTNALL SL, ERLES K, BROOKS HW, et al. (2010) Characterization of pneumonia due to Streptococcus equi subsp zooepidemicus in dogs. Clin Vaccine Immunol 2010;17:1790–6