Sarcocystis species are intracellular protozoan parasites that mainly infect mammals, and some birds and reptiles.
They exist in a predator-prey relationship where they depend on the host for survival.
For Sarcocystis species, the life cycle of the parasite typically involves two host species, a definitive host and an intermediate host.
The definitive host is often a predator, while the intermediate host is the prey.
The parasite undergoes sexual reproduction in the gut of the definitive host, which is passed out of the host as fecal matter and consumed by the intermediate host, where the oocysts enter the muscle tissue.
The cycle is complete when the definitive host eats the intermediate host.
Sarcocystis species are generally host-specific, infecting only a particular host or closely related host species.
For example, Sarcocystis hominis and S. suihominis affect mostly humans and some primates.
These species are acquired after eating raw meat from cattle and pigs, respectively.
However, the prevalence of intestinal sarcocystosis in humans is relatively low and is only seldom associated with illness, except in volunteers who ingest large numbers of sarcocysts.
Cases of infection where humans act as intermediate hosts, with intramuscular cysts, are less than 100 and are of unknown origin.
Livestock, however, is not immune to the effects of Sarcocystis.
The species are often characterized by acute debilitating infections, resulting in abortion of the fetus and death, or chronic infections with failure to grow or thrive.
There are about 130 species of Sarcocystis, some of which include:
- Sarcocystis accipitris
- Sarcocystis alces
- Sarcocystis alceslatrans
- Sarcocystis bovifelis
- Sarcocystis bovihominis
- Sarcocystis buffalonis
- Sarcocystis greineri
- Sarcocystis booliati
- Sarcocystis bovicanis
- Sarcocystis grueneri
- Sarcocystis hardangeri
- Sarcocystis hemioni
Sarcocystis was first recognized in a mouse in 1843 by Miescher, and his son, Johann Friedrich Miescher, a Swiss biologist, discovered similar structures in the muscles of pigs in 1865.
These structures were unnamed until the name Sarcocystis miescheriana was proposed for them in 1899.
Life Cycle of Sarcocystis
Sarcocystis species have an obligatory 2-host life cycle; the intermediate host is usually a herbivore or omnivore, and the definitive host is a carnivore.
The sexual reproductive stage of the life cycle begins when adult male and female protozoa sexually reproduce in the epithelial cells of the definitive host. The newly created oocysts are sporulated in the host.
When passing out of the host, the oocysts usually rupture, releasing infective sporocysts.
Then the host sheds both sporocysts and sporulated oocysts in the feces.
The next stage of the cycle involves the intermediate host ingesting infective sporocysts in the fecal matter that has contaminated food or water.
In asexual reproduction, Sporozoites leave the sporocysts and invade the intestinal mucosa to reach the endothelial cells of the host.
An asexual cycle begins, whereby initial reproduction forms schizonts.
Signs and Symptoms of Sarcocystis
Symptoms of sarcocystosis in human definitive hosts include:
- Fever
- Rash
- Subcutaneous swelling
- Musculoskeletal pain
According to the American Society for Microbiology, human volunteers in Germany who ate raw beef containing S. hominis became infected and shed oocysts in their feces.
One person became ill. Symptoms that manifested at about 3 to 6 hours after consuming the beef included nausea, stomach ache, and diarrhea; the total illness lasted for about 36 hours.
Transmission
The consumption of raw or undercooked beef and pork that contains mature sarcocysts of S. hominis and S. suihominis, respectively, has resulted in intestinal sarcocystosis of humans.
This is based on histologic examination of intestinal lesions from infected persons in Thailand who had eaten undercooked meat from Bos indicus cattle and possibly other animals.
S. cruzi, the species most commonly found in cattle muscle, infects dogs but not humans, but several species of domesticated meat animals contain sarcocysts infective for unknown definitive hosts.
Diagnosis
Diagnosis of Sarcocystis by biopsy of an infected muscle.
Sarcocysts can be identified using eosin and hematoxylin.
Inflammatory cells may be found along with Sarcocystis.
Other findings include myositis, perivascular and interstitial inflammation, myonecrosis, eosinophilic myositis, vasculitis,
Treatment
There is no known treatment for intestinal sarcocystosis. Infections occur within a short time and are often asymptomatic.
Because the infection is rarely symptomatic, treatment is not often required. No trials have been published, so treatment remains empirical.
Agents that have been used for the treatment of myositis include albendazole, metronidazole, and cotrimoxazole for myositis.
Corticosteroids have also been used to provide symptomatic relief.
Amprolium and salinomycin, which have been effective in preventing severe illness and death in experimentally infected calves and lambs, have not been tried on humans to date.
Prevention
Sarcocystis infection can be prevented by cooking the meat thoroughly before eating.
Also, freezing the meat below freezing level (−5°C) for several days before ingestion kills the sporocysts.
