An epizootic among knots (Calidris canutus) in Florida. II. Ultrastructure of the causative agent, a Besnoitia-like organism.

Multinucleated cysts near the luminal surface of the thoracic aortas of diseased knots (Calidris canutus) were similar to besnoitia cysts. Ultrastructurally, the cyst had four distinct layers. The central area included a vacuole that contained a sporozoan with a conoid, polar ring, micronemes, rhoptries, nucleus, mitochondria, dense bodies, a lipid-like vacuole and endoplasmic reticulum. External to the vacuole was a layer with organelles typical of vertebrate cells. The wall of the cyst was irregular in thickness and was bound by a strongly osmiophilic membrane. There was a loose, acellular area of intertwined strands between the cysts wall and layer of organelles.