Characterization of pneumonia due to Streptococcus equi subsp. zooepidemicus in dogs

Abstract

Streptococcus equi subsp. zooepidemicus has been linked to cases of acute fatal pneumonia in dogs in several countries. Outbreaks can occur in kenneled dog populations and result in significant levels of morbidity and mortality. This highly contagious disease is characterized by the sudden onset of clinical signs, including pyrexia, dyspnea, and hemorrhagic nasal discharge. The pathogenesis of S. equi subsp. zooepidemicus infection in dogs is poorly understood. This study systematically characterized the histopathological changes in the lungs of 39 dogs from a large rehoming shelter in London, United Kingdom; the dogs were infected with S. equi subsp. zooepidemicus. An objective scoring system demonstrated that S. equi subsp. zooepidemicus caused pneumonia in 26/39 (66.7%) dogs, and most of these dogs (17/26 [65.4%]) were classified as severe fibrino-suppurative, necrotizing, and hemorrhagic. Three recently described superantigen genes (szeF, szeN, and szeP) were detected by PCR in 17/47 (36.2%) of the S. equi subsp. zooepidemicus isolates; however, there was no association between the presence of these genes and the histopathological score. The lungs of S. equi subsp. zooepidemicus-infected dogs with severe respiratory signs and lung pathology did however have significantly higher mRNA levels of the proinflammatory cytokines tumor necrosis factor alpha (TNF-α), interleukin 6 (IL-6), and interleukin 8 (IL-8) than in uninfected controls, suggesting a role for an exuberant host immune response in the pathogenesis of this disease.

FIG. 1.

Canine lung sections. (A) Typical fibrino-suppurative, necrotizing, and hemorrhagic pneumonia due to S. equi subsp. zooepidemicus, characterized by diffuse filling and obliteration of alveolar spaces by numerous neutrophils, fibrin, and proteinaceous fluid. Bronchioles contain hemorrhaged blood, proteinaceous fluid, and sloughed necrotic cells. This section was stained with hematoxylin and eosin. Magnification, ×20. (B) Neutrophils frequently contain numerous intracellular Gram-positive cocci. This section was Gram stained. Magnification, ×40.

FIG. 2.

PCR amplification of S. equi subsp. zooepidemicus DNA using internal primers to the superantigen genes szeF, szeN, and szeP. Lanes: SZ+, S. equi subsp. zooepidemicus isolate SzBHS5; SC, S. canis control; H₂O, water control with no DNA added; SZ, S. equi subsp. zooepidemicus isolate from one dog (isolate SzBHS7).

FIG. 3.

Mean number of canine cytokine and chemokine mRNA copies in samples of lung tissue from control and S. equi subsp. zooepidemicus-infected dogs. Values that are statistically significantly different (P < 0.005) in the control lung and infected lung are indicated by an asterisk. The values are means plus 1 standard deviation (error bars).