Etiology, Epidemiology, and Pathogenesis of ARDS in Foals
Acute respiratory distress syndrome (ARDS), first described in humans, is recognized as a rare cause of respiratory failure in horses and other veterinary species. ARDS represents an overwhelming and profound inflammatory response in the lung triggered by a primary disease process (pneumonia, sepsis), trauma, or other external factors (smoke inhalation, near drowning). The hallmark features of ARDS include severe lung damage and pulmonary edema, profound hypoxemia, and respiratory failure. Isolated cases consistent with ARDS have been described as a complication of sepsis in neonatal foals. In older foals (1 week to 8 months) ARDS is recognized as a distinct clinical condition previously referred to as acute bronchointerstitial pneumonia.
The exact etiology of ARDS in older foals is not clear and is most likely multifactorial. Concurrent or recent history of pneumonia is the most commonly reported predisposing cause, with many foals having a history of antimicrobial treatment when clinical signs develop. A single bacterial pathogen has not been consistently identified. Rhodococcus equi Rhodococcus equi Pneumonia in Foals Rhodococcus equi is a gram-positive, intracellular bacteria that causes chronic suppurative bronchopneumonia with abscessation in foals 1–5 months of age. Clinical signs are mild and nonspecific... read more , Klebsiella spp, Streptococcus spp, Pseudomonas aeruginosa, and Pneumocystis jiroveci have been inconsistently cultured from the lungs of affected foals. Additional predisposing factors may include recent viral infection (no virus has been consistently isolated) and warm weather (> 85°F [29.4°C]) when clinical signs develop.
Clinical Findings and Lesions of ARDS in Foals
Older foals with ARDS present with acute or peracute onset of severe and rapidly progressive respiratory distress and, occasionally, sudden death due to fulminant respiratory failure. Clinicopathologic evaluation should include arterial blood gas, CBC, and serum chemistry analysis. Severe respiratory distress is the most striking clinical sign and foals are often unable or reluctant to move. Care should be taken to not stress the foal during examination. Nonspecific findings include fever, abnormal lung sounds, increased heart rate, depression, and rarely, cyanosis.
Diagnosis of ARDS in Foals
Sampling airway secretions
Consistent findings on arterial blood gas analysis include severe hypoxemia as well as hypercapnia and respiratory acidosis. These findings quantify the severity of respiratory impairment and are used to monitor response to treatment. Other laboratory findings vary but commonly include hyperfibrinogenemia and neutrophilic leukocytosis.
Radiographic or ultrasound examination of the lungs is recommended as clinical findings may appear similar to those of foals with severe R equi pneumonia Rhodococcus equi Pneumonia in Foals Rhodococcus equi is a gram-positive, intracellular bacteria that causes chronic suppurative bronchopneumonia with abscessation in foals 1–5 months of age. Clinical signs are mild and nonspecific... read more . Radiographically, ARDS appears as diffuse or caudodorsally distributed interstitial and bronchointerstitial pulmonary opacities. With advanced disease, the radiographic pattern progresses to include patches of a coalescing alveolar nodular pattern with air bronchograms.
Tracheobronchial aspiration can be performed to identify an underlying primary disease; however, it may be prohibitively dangerous to perform on a dyspneic foal. In stabilized foals, tracheobronchial aspirate (TBA) samples can be obtained for bacterial culture/sensitivity, cytologic evaluation, and virus isolation. Cytologic evaluation of tracheal aspirates reveals acute neutrophilic inflammation with or without evidence of sepsis. Various bacterial organisms are often recovered from TBA samples or at postmortem examination; however, no single organism is consistently recovered.
Postmortem examination reveals diffusely enlarged lungs that fail to deflate upon opening of the thoracic cavity, with rib impressions visible on the visceral pleural surface. The cut surface of lung is mottled, with dark red lung interspersed with more normal-appearing lung tissue and edematous separation of lobules. The most prominent histopathologic findings are severe, diffuse, necrotizing bronchiolitis, alveolar septal necrosis, and neutrophilic alveolitis. Surviving foals develop a proliferative epithelial and interstitial response, including bronchiolar and alveolar epithelial hyperplasia, type II cell hyperplasia, and hyaline membrane formation.
Treatment of ARDS in Foals
Foals with ARDS require prompt and intensive care. Because the cause of ARDS is rarely known, treatment is largely symptomatic and should include addressing primary disease (if identified), controlling inflammation, improving oxygenation, and supportive care.
Hypoxemia associated with ARDS is relatively resistant to supplemental oxygen treatment; nevertheless, intranasal oxygen supplementation should be provided. Bronchodilators may be useful but should be administered with caution as they may worsen V/Q mismatch associated with respiratory failure in these foals. Anti-inflammatory treatment with corticosteroids (eg, dexamethasone 0.1 mg/kg per day, IV) appears to improve survival. Febrile foals can be administered NSAIDs. Broad-spectrum antimicrobial treatment should be instituted to treat existing or secondary bacterial infections but typically has little bearing on the outcome in foals with ARDS.
Supportive treatment includes provision of a clean, comfortable environment and appropriate nutritional support. When needed, an alcohol bath, an air-conditioned stall, and/or a fan can be used in conjunction with NSAIDs to maintain rectal temperature < 102.5°F (39.1°C).
Although mortality is high, especially in neonatal foals with other comorbidities, older affected foals that receive aggressive medical care have a reasonably favorable prognosis for survival (60%–70%).(1 References Acute respiratory distress syndrome is a rare and rapidly progressive cause of respiratory failure in foals and is characterized by acute onset of respiratory distress and high mortality. A... read more ) Foals that survive will often stabilize or improve within a few days of receiving treatment. The longterm consequences after recovery from ARDS are variable, ranging from a successful athletic career to persistent exercise intolerance.
ARDS is recognized as a clinical syndrome in foals between 1 week and 8 months of age; it can also be a complication to sepsis in neonatal foals.
Affected foals present with acute onset respiratory distress; the most dramatic clinical finding is profound hypoxemia.
A specific underlying pathogen is rarely identified in foals diagnosed with ARDS.
Prognosis is poor for neonatal foals; in older foals, prompt and aggressive medical treatment can result in a fair prognosis for survival.
Dunkel B, Dolente B, Boston RC. Acute lung injury/acute respiratory distress syndrome in 15 foals. Equine Vet J, 2005;37:435–440.