Article about sarcocystosis (sarcosporidiosis).
- Clinical features
- Further reading
Sarcocystosis is characterized by the invasion of muscles and sometimes other tissues by protozoa of the genus Sarcocystis, of which S. hominis (intermediate host domestic cattle) and S. suihominis (domestic pig) are the most significant to humans, to whom they are transmitted by ingestion of uncooked beef or pork. Humans serve as either intermediate or final host: (1) intermediate host—presence of cysts in muscle is usually asymptomatic, but may cause myositis or myopathy; detected on clinical examination or muscle biopsy; (2) final host—may be asymptomatic or cause fever and gastrointestinal upset; oocysts or sporocysts can be detected in faeces. There is no specific treatment. Prevention is by not eating uncooked meat from any animal.
Although often described as uncommon in humans, Sarcocystosis appears to be widespread but undetected. It has been reported from most continents but the exact distribution of the different species remains uncertain, largely on account of the absence of definitive clinical signs in many cases. Over the past decade, veterinary studies, especially serological surveys, have indicated that Sarcocystis species are present in a wide range of domesticated and wild mammals and other animals, often at a high prevalence. Snakes and their rodent prey are definitive and intermediate hosts for many species of Sarcocystis; there is evidence of coevolution of the parasites with their vertebrate hosts. Equine protozoal myeloencephalitis, a disease of domestic horses due to S. neurona, has prompted a considerable body of research on Sarcocystis in recent years because of its great economic importance.
Above: Sarcocyst of Sarcocystis sp. in muscle tissue, stained with H&E (magnification 500x).
Sarcocystosis presents both actual and perceived public health problems. Some species, such as Sarcocystis hominis and S. suihominis, can be transferred from animals to humans but others, while often causing alarm among those who encounter them, do not appear to be transmissible. For example, S. rileyi, which commonly affects ducks and geese in North America, presents with readily visible cream-coloured cysts generally running in parallel lines in the muscles of affected birds. This condition, often termed ‘rice breast disease’, is familiar to hunters and to those who skin waterfowl before they are cooked. Many affected carcasses are discarded, but meat containing the cysts presents no known hazard to people who eat it.
However, the role of host resistance in sarcocystosis has not been fully investigated and it is possible that immunosuppression may render humans susceptible to species of Sarcocystis that are primarily parasites of wild birds, reptiles, or mammals.
Above: Sarcocystosis Life Cycle
Worldwide, but more common in areas where livestock is raised.
In cases of intestinal sarcocystosis, when humans serve as the definitive hosts, infections are often asymptomatic and clear spontaneously. Occasionally, mild fever, diarrhea, chills, vomiting and respiratory problems may occur. When humans become infected with sarcocysts of non-human species, the infections are not intestinal but rather result in muscle cysts; symptoms such as myalgia, muscle weakness and transitory edema may occur. In these cases, humans are dead-end intermediate hosts.
Humans as the final host
Depending on the species of parasite and the previous health of the host, infection in humans who have ingested meat containing cysts of Sarcocystis can have effects that range from gastrointestinal disorders and pyrexia to an asymptomatic state.
Humans as the intermediate host
The presence of cysts in human skeletal, visceral, or cardiac muscle is usually not associated with symptoms or clinical signs but it is likely that large numbers may, as in animals, cause myositis or myopathy, especially if calcification occurs, sometimes with vasculitis.
Humans as the final host
Oocysts or sporocysts can be detected in faeces in smears (especially using Heine’s method), in wet saline preparations, or, better, using a sodium chloride or sucrose flotation method. The oocysts/sporocysts are usually readily recognized by an experienced parasitologist but can easily escape the attention of those who are less familiar with the organism. Sarcocystis must be distinguished from other sporozoal organisms that are either being produced in the intestine or are in transit in the lumen following ingestion.
Above: Sporulated oocyst of Sarcocystis sp. in unstained wet mounts, magnification 400x.
Humans as the intermediate host
Occasionally, tissue cysts are detected during routine clinical examination, especially if calcification has occurred. They may also be seen in muscle biopsies, either as an incidental finding or because samples have been taken specifically for diagnostic purposes. Calcified cysts found in biopsies or located at autopsy have a gritty texture when cut. Sarcocystosis of muscle must be differentiated from toxoplasmosis, in which tissue cysts can also be found. The morphology of the two protozoa differs. In particular, cysts of Sarcocystis have a distinct wall, which is thick and striated in some species, and do not stain with periodic acid–Schiff stain, which usually gives Toxoplasma cysts a magenta colour.
There is no specific therapy for sarcocystosis in humans or animals, although albendazole has been reported to ameleriorate symptoms in a human patient with skeletal cysts and ponazuril has been shown to prevent infection of the central nervous system of mice experimentally given sporocysts of S. neurona. When humans are the final host, symptomatic and supportive treatment is indicated.
Can I get sarcocystosis?
Yes. People can get sarcocystosis. They acquire the disease by ingesting (oral) the protozoan, most commonly through undercooked meat products. Disease in humans can involve either intestinal infection or muscular invasion by the parasite. Usually the disease resolves on its own with no signs of illness. When illness occurs, signs of muscular disease may include muscle tenderness or painful swelling, muscle weakness, headache, cough, transient itchy rashes. Intestinal symptoms may include fever, chills, sweating, abdominal pain, diarrhea, nausea, vomiting.
Sarcocystosis can be prevented by not eating uncooked meat from any animal. Vaccines, at present experimental, have been shown to produce cellular immunity to certain Sarcocystis species in horses.
Above: Esophogeal muscle with white oval multifocal cysts (sarcocytes) of sarcocystosis
Above: Cysts in the breast muscle of a mallard duck
How can I protect myself from sarcocystosis?
Avoid eating raw or undercooked meat. Cooking to a temperature of 150-160oF for 15 minutes can minimize risk. Freezing meat to -4oF can also help prevent transmission. Use good personal hygiene, such as hand washing, after contact with animals.
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