An epizootic among knots (Calidris canutus) in Florida. I. Disease syndrome, histology and transmission studies.

About 150 knots found sick or dead had distention of serosal blood vessels, and small, raised hemorrhagic foci on the serosal surface of the jejunum and ileum. The principal finding was endaortitis with large intimal multinucleated cysts that contained a protozoan parasite within a cytoplasmic vacuole. There was endarteritis of mesenteric arteries and muscular arteries of the external layers of the intestinal tract. Single or multiple schizont-like stages containing zoites were in the wall of the small intestine and close to affected muscular arteries. The more usual intestinal lesion was an aneurysm of the inflamed muscular artery. It was concluded that endarteritis led to colonic infarction. Schizonts in medullary renal tubular cells were smaller than stages in the intestine. There was necrosis of epithelial cells of the medullary collecting ducts and inflammatory and regenerative changes. Schizonts and zoites were free in collecting ducts and ureters.