Infestation with or disease caused by sporozoan protozoans of the genus Sarcocystis. Wildlife of many different species appear to be susceptible (mammals, birds and reptiles).
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. There are no recognizable signs of the 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.
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 (although the organism is not considered pathogenic for humans) or fed to cats and dogs.
There are numerous species of Sarcocystis in the literature, with most of them 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.
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. More recently, it has been determined that some species of Sarcocystis in cattle and sheep are the intermediate stage of coccidian parasites found in cats, dogs and humans.
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, now another indirect method of infection has been proven whereby carnivores and omnivores pass an infective stage of the parasite in their faeces. An animal is infected by ingesting material contaminated by the infected faeces.
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 the USA, sarcosporidiosis is a disease of aviary birds spread by opossum faeces, rats, cockroaches, and rainwater that washes over faeces that have been left on roof tops. Wild birds recorded with the infection are mallard duck, black duck, redhead duck, common golden-eye, blue-winged teal, Canada goose, ring-necked pheasant, moose, cottontail rabbit, red-tailed hawk, cooper's hawk, sharptailed grouse, American woodcock and morning dove. Mallards and black ducks are the ducks most commonly reported with the disease.