Here’s the lowdown on this infection caused by a parasitic protozoan
The greenwing would have been lunch. The protozoans had other plans.
Seconds after starting to skin the plump drake, I noticed its dark breast meat was littered with small white granules. Disappointed, I recalled some college zoology lessons and realized I’d be eating a sandwich that noon.
Many hunters have encountered ducks in similar condition. Most call this phenomenon rice breast. And although it isn’t an everyday happening, rice breast occurs often enough to remind us that ducks and hunters are really only small players in nature’s vast interwoven web.
Rice breast is actually a parasitic infection caused by a single-celled organism in the genus Sarcocystis. And unfortunately, you cannot determine if a duck has it until after you’ve killed the bird.
“You won’t usually see external evidence of [a] Sarcocystis infection,” Thomas J. Rolfe, supervisory veterinary medical officer with the U.S. Fish and Wildlife Service’s National Wildlife Health Research Center, and Scott Craven, professor of wildlife ecology at the University of Wisconsin-Madison, wrote in a paper for the National Wildlife Health Research Center. “However, once you skin the bird, you can easily recognize the visible form of the disease — you will see cream-colored cylindrical cysts running in parallel lines throughout the muscles.”
This visible form of the infection most commonly affects dabbling ducks, the paper said, with cysts typically appearing in the breast and thigh muscles. Diving ducks are only occasionally infected. A microscopic form of the disease, which produces no visible evidence, often occurs in snow geese, Canada geese and various ducks.
Rolfe and Craven wrote that waterfowl become infected with Sarcocystis by consuming the parasite’s eggs in food or water.
“The parasite requires a primary host (carnivore) and a secondary host (waterfowl and other herbivorous animals) to complete its life cycle,” the paper said. “In the primary host’s intestine, the parasite matures and produces microscopic eggs. The eggs pass out in the carnivore’s feces, contaminating the environment. Waterfowl ingest the eggs while feeding. When the eggs hatch, the parasites move through body tissues to the skeletal muscles, where they form cysts. The cycle is completed when a carnivore consumes prey infected with Sarcocystis.”
Waterfowl affected by Sarcocystis typically don’t look or act sick, the paper said. The condition is rarely fatal, although severe infections can cause muscle loss, making the bird lame or weak.
Rice breast also doesn’t pose a danger to hunters, Rolfe and Craven wrote, as the parasite isn’t known to transmit to humans, and proper cooking destroys Sarcocystis. Still, most hunters discard affected birds. And Sarcocystis species common in waterfowl haven’t been documented to infect dogs.
“However, because dogs are susceptible to at least some species of Sarcocystis, we don’t recommend feeding uncooked infected waterfowl to domestic animals,” Rolfe and Craven wrote.
I didn’t encounter rice breast for several years after cleaning that teal. But after returning home from a Louisiana hunt during Winter 2020, I carved the breasts off a young pintail and again noticed those telltale white granules (as seen in the accompanying photo).
Sigh. Sandwiches don’t make a bad lunch. They just don’t compare with greenwings and pintails.
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Realtree waterfowl editor Brian Lovett has been an obsessive duck and goose hunter for more than 30 years, chasing his passion on the Dakota prairies and the marshes and open water of his home state of Wisconsin. He's been a writer and editor in the outdoors industry since 1991.