Equine Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) causes rapidly progressive and severe respiratory failure and is associated with high mortality in affected patients. First described in critically ill humans more than 50 years ago, ARDS is now recognized as a rare cause of respiratory failure in several veterinary species including horses, where it is referred to as veterinary ARDS (VetARDS).

ARDS and VetARDS develop as a complication when a primary disease or injury triggers overwhelming and uncontrolled inflammation in the lung. This inflammatory response causes the hallmark features of ARDS: severe pulmonary damage and edema, respiratory dysfunction, and profound hypoxemia (low oxygen in the blood). Pneumonia and conditions associated with severe systemic inflammation, such as sepsis, represent the most common risk factors for ARDS and VetARDS. Other potential risk factors include smoke inhalation, severe trauma, aspiration of stomach contents, and near drowning.

In horses, VetARDS is a rare, but distinct, clinical condition in foals younger than nine months of age. Previously, this condition was referred to as acute interstitial or bronchointerstitial pneumonia. Bacterial pneumonia is the most common predisposing cause of VetARDS in these foals. There does not appear to be a single bacterial pathogen associated with VetARDS in foals but Rhodococcus equi, Klebsiella sp. and Streptococcus sp. have all been identified with VetARDS. Sepsis represents the most likely risk factor in neonatal foals and conditions consistent with VetARDS have been reported in adult horses after smoke inhalation or near drowning.

A definitive diagnosis of VetARDS can be challenging, and distinguishing VetARDS from severe bacterial pneumonia may not be possible. The specific criteria established for the diagnosis of ARDS in veterinary species include recent onset of clinical signs (< 72 hours), exposure to a known risk factor, radiographic changes consistent with widespread pulmonary damage and edema, evidence of severe hypoxemia on blood analysis, and if possible, inflammatory changes identified on respiratory fluid analysis. These diagnostic criteria apply to all veterinary species with minor modifications existing only for neonatal foals less than one week of age. Specific postmortem diagnostic criteria also exist for animals that succumb to VetARDS.

Foals with VetARDS present with a sudden onset of severe and rapidly progressive respiratory distress that may follow a recent history of pneumonia. Other nonspecific findings on physical examination can include fever, abnormal lung sounds, increased heart rate, and depression. Radiographic or ultrasound examination of the lungs is recommended. Blood laboratory analysis is useful to detect and monitor hypoxemia. Other laboratory findings vary but are typically consistent with inflammation or other disease process. Additional diagnostics should focus on identifying any underlying primary disease.

Regardless of age, the goals of treatment for horses with VetARDS include addressing any underlying primary disease, controlling inflammation, improving oxygenation, and providing supportive care. These cases require prompt and intense management. In humans with ARDS, lung-protective mechanical ventilation is the only therapy that has resulted in reduced mortality. Unfortunately, this is not feasible for the vast majority of horses with this condition. Intranasal oxygen supplementation, administration of anti-inflammatory medications (e.g. corticosteroids) and antibiotics, and appropriate nutritional and intravenous fluid support represent the mainstay of therapy for most horses with VetARDS.

The prognosis for recovery from VetARDS is guarded. Survival rates of 60% have been reported in older foals but are often lower in neonatal foals. With appropriate treatment, survivors typically stabilize or improve within a few days. While the impact of this condition on future athletic performance is unknown at this time, limited reports suggest that recovered foals can go on to have successful athletic careers.

CONTACT: Kara Lascola, DVM, MS, DACVIM—kml0068@auburn.edu—334/844-4490—Auburn University, College of Veterinary Medicine, Department of Clinical Sciences, Auburn, Alabama

This is an excerpt from Equine Disease QuarterlyVolume 29, Number 3, funded by underwriters at Lloyd’s, London.