Sarcocystis or rice breast disease is a nonfatal, usually asymptomatic infection that is caused by a parasitic protozoan. Various species of this parasite affect mammals, reptiles and birds. The most commonly reported species of the parasite in North America is Sarcocystis rileyi, the species most commonly found in waterfowl.

Birds ingest the eggs or oocyst of the mature parasite in food or water that
is contaminated by carnivore feces, which contain the oocysts. The oocysts
develop in the intestine of the bird into an intermediate form, the sporozoites, that enter the bird's bloodstream and infect specific cells of the blood vessels. Multiplication of these cells gives rise to a second intermediate form, merozoites, that are carried by the blood to the voluntary muscles, where elongated cysts or macrocysts are eventually
produced. The life cycle is completed when a carnivore ingests the infected
muscle tissue of a bird and the parasite reaches maturity and releases
oocysts in the intestines of the carnivore.

Sarcoystis is a common parasitic infection of some waterfowl species, and it is found throughout the geographic range of those species in North America. Usually, there is no externally visible sign of this disease nor is it recognized as a direct cause of migratory bird mortality. Severe infections can cause loss of muscle tissue and result in lameness, weakness, and even paralysis in rare cases. The debilitating effects of severe infections
could increase bird susceptibility to predation and to other causes of

Visible forms of infection are readily apparent when the skin is removed from the bird. In waterfowl and in many other species, infection appears as cream‑colored, cylindrical cysts (the macrocysts) that resemble grains of rice running in parallel streaks through the muscle tissue. The cysts are commonly found in the breast muscle, but they are also found in other skeletal and cardiac muscle.

There are no known control methods for this disease, nor do any seem to be needed or are any being developed as current knowledge of the disease does not indicate any evidence that bird health is often compromised by infection. Nevertheless, the role of carnivores in the life cycle of Sarcocystis sp. infections should be considered when feeding uncooked, infected waterfowl to house pets and to farm animals such as hogs.

Sarcocystis sp. presents no known health hazard to humans. The primary importance to humans of sarcocystis in waterfowl is the loss of infected birds for food; the unaesthetic appearance of parasitized muscle may prompt hunters to discard the carcass.