- Caudodorsally distributed within the right lung, a well-defined, wedge-shaped soft tissue opacity is present.
- The ventral aspect of the left caudal lung lobe and the right caudal lung lobes have a faint, unstructured interstitial pattern.
- · The pulmonary vasculature, pleural space, and cardiovascular structures are normal.
Conclusions
- Right caudodorsal alveolar pattern and mild left and right cranioventral unstructured interstitial pattern.
Primary consideration is given to a pulmonary thromboembolism from underlying, severe pneumonia (e.g., actinobacillus).
Ultrasound:
Follow-up:
- Transtracheal was confirmed bacterial pneumonia due to Actinobacillus equuli subsp haemolyticus
- He was initially treated with IV broad-spectrum antibiotics, and transitioned to oral Trimethoprim/ Sulfamethoxazole on based on culture and sensitivity results
- A recheck ultrasound 2 weeks later showed improvement of the pulmonary changes. The patient’s fever and respiratory signs have resolved. Given the subtotal resolution of the pulmonary changes, continued antibiotic therapy and another recheck ultrasound in 2 weeks were recommended .
A little bit more…
- Actinobacillus equuli is a Gram‑negative, pleomorphic bacterium traditionally associated with neonatal septicemia but increasingly recognized as a cause of disease in adult horses. Two subspecies—A. equuli subsp. equuli and A. equuli subsp. haemolyticus—are most commonly isolated from equine clinical specimens. When equine actinobacillosis cases are reviewed broadly, A. equuli subsp. equuli is typically the most frequently recovered isolate across all ages, followed by hemolytic Actinobacillus species. A substantial proportion of equine infections involve mixed bacterial populations, reflecting the opportunistic behavior of the organism and its ability to cause disease when mucosal barriers or pulmonary defenses are compromised.
- In adult horses, A. equuli may be isolated from respiratory infections, septicemic cases, pleuropneumonia, synovitis, peritonitis, and occasionally reproductive or cutaneous lesions. While the organism is often considered a commensal of the oral cavity and gastrointestinal tract, it can function as a primary pathogen under the right conditions or act as a secondary invader following aspiration, viral respiratory disease, gastrointestinal mucosal breach, or systemic stress.
Clinical Signs
- The clinical presentation in adult horses varies depending on the body systems involved:
- Respiratory System
- Fever, lethargy, anorexia
- Tachypnea, cough, nasal discharge
- Abnormal lung sounds (increased bronchovesicular sounds, crackles)
- Pleurodynia or reduced tolerance to exercise
- Possible foul odor with abscessation or necrosis
- Systemic / Septicemic Disease
- Depression or obtundation
- Tachycardia, weak pulses, injected mucous membranes
- Colic-like discomfort
- Diarrhea or signs of endotoxemia
- Laminitis as a secondary consequence
- Neurologic abnormalities when meningoencephalitis develops
- Evidence of multiorgan involvement (renal, hepatic, adrenal, lymphoid)
- Musculoskeletal / Other Tissues
- Joint effusion or lameness with septic synovitis
- Localized abscesses (subcutaneous, tendon sheath, guttural pouch)
- Reproductive tract infections or, less commonly, abortion
Clinical and Pathologic Findings
- Adult horses with actinobacillosis frequently exhibit lesions similar to those observed in neonatal cases:
- Suppurative or embolic pneumonia
- Fibrinous pleuritis or peritonitis
- Lymphoid necrosis of spleen, thymus, and lymph nodes
- Multifocal hepatic and adrenal cortical suppuration
- Embolic nephritis, often with intravascular bacterial colonies
- Necrotizing vasculitis, a hallmark of systemic disease
- Valvular endocarditis in some chronic cases
- The organism shows strong affinity for vascular endothelium, promoting microthrombosis, bacterial embolization, and necrotizing vasculitis throughout the body.
Diagnostic Approach
- Physical Examination and Laboratory Findings
- Laboratory abnormalities often include leukocytosis or leukopenia, left shift with toxic neutrophils, hyperfibrinogenemia, azotemia, elevated liver enzymes, and other indicators of systemic inflammation or sepsis. Blood gas alterations may be present with severe pneumonia.
- Culture and Identification
- Blood culture remains an important diagnostic tool in systemic disease.
- Transtracheal wash or bronchoalveolar lavage samples frequently yield A. equuli in respiratory cases.
- Culture of abscesses or synovial fluid may confirm localized infections.
- Mixed infections are common, so isolation patterns must be interpreted within clinical context.
- Radiography
- Thoracic imaging is an essential component of diagnosis:
- Radiographic Patterns
- Cranioventral or caudodorsal alveolar pattern typical of bacterial pneumonia
- Intermixed interstitial and alveolar patterns in diffuse disease
- Cavitary masses with gas‑fluid interfaces representing pulmonary abscesses
- Unilateral or bilateral pleural effusion with scalloping of lung lobes
- Wedge‑shaped soft tissue opacities when pulmonary infarction or thromboembolism occurs
- These findings reflect underlying suppurative bronchopneumonia, fibrinous pleuritis, and embolic phenomena commonly associated with actinobacillosis.
- Ultrasound
- Thoracic ultrasound complements radiographs by identifying regions of lung consolidation, abscesses near the pleural surface, comet‑tail artifacts, fibrin strands, and/or pleural fluid.
Pulmonary Thromboembolism Associated With Actinobacillus
- Pulmonary thromboembolism (PTE) is a significant complication of systemic actinobacillosis. The organism’s ability to colonize and damage vascular endothelium promotes the formation of bacterial microthrombi that embolize to the lungs. Features of PTE in horses include:
- Multifocal unstructured interstitial/alveolar patterns on imaging
- Sharp‑margined or wedge‑shaped lesions suggestive of infarction
- Severe, often disproportionate respiratory distress
- Concurrent systemic signs (tachycardia, endotoxemia, renal involvement)
- On gross and histologic examination, affected lungs reveal dense intravascular bacterial colonies, hemorrhage, necrosis, and arterial or venous thrombi. This embolic pattern is one of the most distinctive features of A. equuli septicemia.
Treatment
- Antimicrobial Therapy
- Treatment should begin promptly and typically involves:
- Broad‑spectrum bactericidal agents effective against Gram‑negative organisms
- Common empirical choices include combinations such as penicillin plus gentamicin, or third‑generation cephalosporins
- Therapy refined based on culture and susceptibility when available
- Prolonged treatment (weeks) may be necessary for abscesses or endocarditis
- Supportive and Adjunctive Care
- NSAIDs for inflammation and endotoxemia
- IV fluids and electrolyte support
- Plasma transfusion in severe systemic disease
- Drainage of pleural effusion or abscesses
- Oxygen therapy for respiratory compromise
Prognosis
- Prognosis varies widely. Horses with localized respiratory disease may respond favorably, whereas those with widespread septicemia, embolic pneumonia, or multiorgan dysfunction carry a guarded to poor prognosis. Early recognition and aggressive therapy significantly improve outcomes.
References:
- Layman, Quinci D., et al. “A retrospective study of equine actinobacillosis cases: 1999–2011.” Journal of Veterinary Diagnostic Investigation 26.3 (2014): 365-375.
- Stewart, Allison J., et al. “Actinobacillus sp. bacteremia in foals: clinical signs and prognosis.” Journal of Veterinary Internal Medicine 16.4 (2002): 464-471.
- Carvallo, Francisco R., et al. “Retrospective study of fatal pneumonia in racehorses.” Journal of veterinary diagnostic investigation 29.4 (2017): 450-456.
