A 41-year-old man who recently retired from active duty overseas in the US Navy presented to his primary care physician with a 3-month history of worsening shortness of breath and a non-productive cough.
He has not had fever, weight loss, or fatigue, and he has not had a prior diagnosis of asthma, chronic bronchitis, reactive airway disease, atopy, or allergic diseases.
He has mild hypertension that is well controlled on lisinopril, and he is otherwise healthy. He has never smoked cigarettes. On physical examination, vital signs are normal, and the lungs are clear.
There is no lymphadenopathy or hepatosplenomegaly. Chest radiography is normal.
Laboratory testing shows mild anemia (Hb, 11.5 g/dL) and an elevated WBC count (23.2K/mm3 [54% eosinophils]).
Blood and stool cultures are negative. There is mild, non-reversible airflow obstruction on spirometry. A high-resolution CT of the chest reveals multiple miliary nodules and ground-glass opacities.
What is the most likely cause of eosinophilia in this patient?