Interstitial pneumonias of undetermined etiology in foals in California, 1990-2020.

Interstitial and bronchointerstitial pneumonias of unknown cause in foals were studied through retrospective autopsy analysis, examining clinical signs, histological lung changes, and potential infectious agents, but no definitive viral cause was found.

Study Background and Purpose

• Interstitial and bronchointerstitial pneumonias in young foals often present without a known cause, posing a diagnostic challenge in equine medicine.
• Various potential causes have been suggested, including unknown viral infections, toxins, hyperthermia, lung cell function deficiencies, antibiotics, and abnormal immune responses to bacterial infections.
• The study aimed to retrospectively analyze foal autopsy cases diagnosed with interstitial or bronchointerstitial pneumonia with undetermined etiology in California from 1990 to 2020.

Study Population and Methodology

• 41 foals were included, with a median age of 3 months.
• Most cases were received during summer (28 foals) and spring (10 foals), indicating possible seasonal trends.
• Clinical data were reviewed, including signs, antibiotic treatment, and lung pathology findings.
• Histological lung samples were evaluated for characteristic pathological changes.
• Special stains and microbiological cultures were conducted to detect microorganisms.
• Additional molecular methods, such as herpesvirus consensus PCR and viral metagenomics, were used on some cases to search for viral agents.

Clinical and Gross Pathological Findings

• The most frequent clinical signs reported were difficulty breathing (dyspnea/tachypnea) in 28 foals and fever in 19 foals.
• Half of the foals (21) received antibiotic treatment, primarily penicillin and gentamicin.
• On gross examination, the lungs were often diffusely firm and rubbery in consistency (35 cases) and failed to collapse upon removal (22 cases), suggesting substantial lung involvement.

Histological Lung Changes

• Lung tissue showed combinations of pathological phases commonly associated with interstitial pneumonia:
• Exudative phase (E) characterized by hyaline membrane formation.
• Proliferative phase (P) marked by type II pneumocyte hyperplasia.
• Fibrotic phase (F) indicated by fibroplasia and lung fibrosis.
• Most lungs showed combinations such as E+P (15 cases) or E+P+F (13 cases), showing progression or varying stages of lung injury.
• Necrosis of the small airway (bronchiolar) epithelium was uncommon (4 cases), suggesting the primary injury was within the interstitium rather than airways.
• Concomitant suppurative (pus-forming) bronchopneumonia was frequent (22 cases), indicating bacterial infection was common but likely secondary.
• Some foals (5 cases) had pulmonary pyogranulomas, which are localized inflammatory lesions often associated with chronic infection or foreign material.

Microbiological and Molecular Findings

• Using Grocott-Gomori methenamine silver stain, fungal or certain bacterial species (e.g., Pneumocystis spp.) were seen in 8 cases, showing possible opportunistic infections.
• Bacteria were cultured from the lung tissue in 22 cases, with Rhodococcus equi (7 cases) and Streptococcus spp. (6 cases) being the most common pathogens isolated.
• Despite these bacterial findings, no definitive viral agents were identified through routine diagnostics or specialized viral detection techniques (herpesvirus consensus PCR and viral metagenomics) in tested samples.

Conclusions and Implications

• Interstitial pneumonia of unknown cause in foals is associated with seasonal patterns and manifests with significant lung damage involving multiple pathological stages.
• Secondary bacterial infections are common but likely do not explain the primary lung lesions.
• No clear viral pathogen was detected, even after advanced molecular investigations, suggesting other non-viral causes or undiscovered agents may contribute.
• Findings highlight the need for continued research focusing on non-infectious causes, immune dysfunction, or novel pathogens to further understand these cases.