Loeffler endocarditis is a form of restrictive cardiomyopathy which affects the endocardium and occurs with white blood cell proliferation, specifically of eosinophils. Restrictive cardiomyopathy is defined as a disease of the heart muscle which results in impaired filling of the heart ventricles during diastole.
Eosinophilic states that may occur in association with Loeffler endocarditis include hypereosinophilic syndrome, eosinophilic leukemia, carcinoma, lymphoma, drug reactions or parasites, as reported in multiple case series.  Hypereosinophilia can be caused by a worm (helminth) that invokes the chronic persistence of these eosinophils, resulting in a condition known as hypereosinophilic syndrome.
The eosinophilia and eosinophilic penetration of the cardiac myocytes leads to a fibrotic thickening of portions of the heart (similar to that of endomyocardial fibrosis). Commonly the heart will develop large mural thrombi (thrombi which lay against ventricle walls) due to the deterioration of left ventricular wall muscle. Symptoms include edema and breathlessness. The disease is commonly contracted in temperate climates (due to the favorable conditions for parasites), and is rapidly fatal.