Eosinophilic Pneumonia in a Traveller Returning from Mexico

A previously healthy 45-year-old Korean-born man presented with pleuritic chest pain and a dry cough in February 2005. His symptoms began one week after returning home to Toronto, Ontario, following a trip to northern Mexico, where he had spent several days in a large city attending meetings. He denied having fever, rash or arthralgia. His chest pain and cough were initially managed symptomatically with an analgesic and a cough supressant. However, the symptoms persisted and after one week, he was referred to a respirologist for further investigation and management. His physical examination was normal. His complete blood count revealed marked eosinophilia (6.0×109/L) and his chest radiograph documented a right lung infiltrate. A computed tomography scan confirmed consolidation in the right upper lobe and revealed centrilobular nodules in a tree-in-bud pattern, right hilar and mediastinal lymphadenopathy and small nodules (less than 0.5 mm) scattered throughout both lungs. He underwent bronchoscopic alveolar lavage and transbronchial biopsy. Cytology of the bronchoscopic alveolar lavage fluid revealed marked eosinophilia. Gram stain and calcofluor white (fungal) stain were negative. Histopathology of the biopsy documented nonspecific eosinophilic bronchitis. There were no identifiable organisms with hematoxylin and eosin, periodic acid-Schiff or GrocottGomori methenamine-silver stains of the tissue. A microbiological test was performed. What was the diagnosis and how was it made?