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Sarcocystosis is a disease caused by organisms of the genus Sarcocystis. Members of this genus are unicellular parasites found in muscles and other tissues of mammals, birds and reptiles.
Uncertainty as to the exact classification of Sarcocystis has existed in the past with it being identified as a protozoan by some authorities and a fungus by others. In recent experiments it has been demonstrated that some species of Sarcocystis in cattle and sheep are the intermediate stage of coccidian parasites found in cats, dogs and humans.
There are some morphological differences between the Sarcocystis cysts of different animals. The sarcocysts found in sheep, rabbits, mice and ducks are macroscopic in size, whereas other mammalian forms (deer, moose and elk) are microscopic. In ducks, the cysts are whitish streaks which resemble grains of rice (hence the common name rice grain disease), while the cysts in rabbits are thinner and more elongate.
There are numerous species of Sarcocystis in the literature with most of them being named with respect to the host in which they are found, i.e. S. rileyi (duck), S. cuniculi (rabbit), S. tenella (sheep) and S. miescheriana (pig). These parasites are not always host specific and it is possible that all represent a single species, S. miescheriana, the organism first found in a mouse by Miescher in 1843. Accurate classification will require further knowledge of the complex and varied life cycles of these parasites.
Sarcocystis is worldwide in distribution. It is found in many species, including sheep, cattle, horses, swine, dogs, cats, rabbits, mice, chickens, and humans. Many wildlife species have been found to be infected, including deer, moose, elk, caribou, ducks, seals, and many others.
In Michigan, species identified with sarcocystis are mallard, black duck, redhead, common golden-eye, American widgeon, blue-winged teal, Canada goose, ring-necked pheasant, moose, elk, white-tailed deer, raccoon, cottontail rabbit, bald eagle, red-tailed hawk, cooper's hawk, sharptailed grouse, American woodcock, mourning dove, and common grackle. Each year several ducks are diagnosed with the disease, with mallards and black ducks being the most common. Occasionally rabbits are identified with the organism. A survey of 208 white-tailed deer from Ontario, Texas and Wisconsin showed 80% to be infected. Four of 16 white-tailed deer were found infected in a small survey in Michigan.
Transmission and Development
The mode of transmission from animal to animal is incompletely understood. For many years it was believed Sarcocystis was transmitted by ingestion of flesh containing sarcocysts. However, another indirect method of infection has been proven whereby carnivores and omnivores pass an infective stage of the parasite in their feces. An animal is infected by ingesting material contaminated by the infected feces.
Clinical Signs and Pathology
In most animals Sarcocystis infections are not considered to be of any serious pathogenic significance. However, heavy infections have caused mortality in sheep, pigs and mice. An Oregon study reported infection and death in mule deer fawns experimentally inoculated with sporocysts of S. hemionilatrantis.
There are no recognizable signs of an infection in most living animals, and a diagnosis of Sarcocystis is almost always made after death. In heavy infections, lameness, weakness, and paralysis have been reported.
A diagnosis is usually made by finding the cysts in striated muscle after the animal's death. The large cysts found in ducks, sheep, rabbits, and mice are easily seen with the unaided dye as grayish to whitish streaks, 1-10 mm in length, running lengthwise with the muscle fibers. In other animals the cysts are microscopic and can only be found by histological examination.
Other tests used in the diagnosis of Sarcocystis are complement-fixation and dermal sensitivity tests.
Treatment and Control
No effective treatment is known. Since the disease can be transmitted by the ingestion of feces containing sporocysts, good sanitation and hygiene are important in preventing the disease.
Domestic animals that are heavily infected may be condemned as unfit for human consumption. Ducks and rabbits are the species of Michigan wildlife that hunters and wildlife biologists are most likely to find infected with Sarcocystis. At this time so much is unknown about Sarcocystis that it is recommended that infected meat from ducks and rabbits not be used for human consumption or fed to cats and dogs.