Isolation of influenza A virus from a 7-day-old foal with bronchointerstitial pneumonia.

This article chronicles a case study in 2001 where a week-old foal, infected with Influenza A virus, was found to have bronchointerstitial pneumonia, marking the first time such a case has been documented. The study aims to understand the causes of this disease, particularly in young horses, and investigate the role of the influenza A virus in equine respiratory disease.

Case Examination and Necropsy

• The seven-day-old foal showed symptoms of progressive respiratory disease four days after birth, which revealed atypical given the usual occurrence in 1.5- to 2.5-month-old foals.
• Upon examination, abnormalities were observed in the lower respiratory tract. The lungs were inflated, indicating accumulation of fluid, and uneven in color. There was also an enlargement of bronchial lymph nodes.

Microscopic Analysis

• The foal’s bronchiolar epithelium exhibited significant necrosis. The alveolar epithelial cells near infected bronchioles also exhibited mild necrosis, mixed with hemorrhage and mild fibrin exudation.
• The researchers found small numbers of multinucleated cells in the alveolar and bronchiolar lumina, alongside pneumocyte type 2 proliferation and hyaline membrane formation.

Etiology and Tests

• The pattern of necrosis suggested a viral cause, a supposition confirmed when tests for other equine viruses returned negative, while Influenza A virus was detected in lung tissue and allantoic fluid.
• The detection of Influenza A virus antigens in bronchiolar epithelium further supported the assumption of the virus being the cause.
• The researchers noted that detecting the virus in later stages of the disease when lung lesions have heightened to hyperplasia is hard; hence the unhindered discovery in this case as the bronchiolar epithelium necrosis was still ongoing.

Implications and Future Studies

• This case, marking the first known instance of a fatal bronchointerstitial pneumonia from which Influenza A virus has been isolated, opens up new questions and resistance towards previous findings.
• Investigators have initiated further studies to identify the strain of the Influenza A virus and determine its role in equine upper respiratory disease.
• No factors were identified that would explain why this particular foal contracted the bronchointerstitial pneumonia especially given the sufficient levels of serum immunoglobulin and normal birth conditions. The mother was also uninfected, theoretically offering colostral protection to the foal.
• These findings suggest a need for further research on bronchointerstitial pneumonia in foals, particularly its early onset and the role of Influenza A virus in its etiology.