Sarcocystis sp. are intracellular, dixenous coccidian
protozoa that belong to the phylum Apicomplexa, which parasitize many animals
and occasionally man. For their life cycle, all species of Sarcocystis require two hosts:
an intermediate omnivore (prey) and a definitive carnivore (predator).
Sarcocystosis
is mainly a veterinary problem, since more than 50% of cattle, pigs and sheep
are infected by various Sarcocystis
species. However, at least two species
are pathogenic to man, namely Sarcocystis hominis and Sarcocystis
suihominis. Man is the definitive host (predator) that
is infected by the ingestion of raw or undercooked meat from intermediate
hosts, which are cattle for Sarcocystis
hominis and pigs for Sarcocystis
suihominis. Besides being the
definitive host, man can also act as an intermediate host, through the
ingestion of water or food contaminated with oocysts eliminated in the feces of
an infected predator.
Once Sarcocystis has been ingested together
with contaminated meat, its walls rupture and release bradyzoites that enter
the lamina propria of the intestinal epithelium. Here, they start sexual reproduction leading
to the formation of a mature oocyst containing two sporocysts which are excreted
in feces. Intact oocysts can be observed
in stool samples only in the first few days after excretion. The cyst wall is thin, barely visible, with
the two adjacent sporocysts containing four sporozoites each. Very often the
wall ruptures, releasing the sporocyst: this is often the only stage detected
in stool examination.
The
sporozoites released from the oocysts are mobile: they migrate through the
intestinal epithelium and enter endothelial cells of small arteries where the
first of four asexual stages occurs, leading to the formation of many
merozoites. Subsequent generations of merozoites develop downstream in the
direction of blood flow and then develop the final asexual generation in
muscles with sarcocyst formation and maturation. Sarcocysts can form in
virtually all striated muscles, including tongue, esophagus, diaphragm, heart
and, to a lesser extent, smooth muscle. For microscopic diagnosis, histological
examination of muscular biopsy specimens is advocated.
Cases
of human muscular sarcocystosis, although described in the literature, are
quite rare and mostly reported from Asian countries. Sarcocystosis in man is in
fact very frequently asymptomatic but it can cause intestinal symptoms
including nausea, stomachache, and diarrhea.
Data on the prevalence of human intestinal sarcocystosis is limited but
the infection is certainly more frequent in Europe than on other continents.
The prevalence of sarcocystosis is probably underestimated since oocysts are
hardly detected in stool examination because present in small numbers and only
for few days. Infection can be
prevented by thoroughly cooking or freezing meat to kill bradyzoites in the
sarcocysts.
Size: 15-19 μm long, 15-20 μm wide.
Morphology: the intact oocyst has a very fragile, thin
wall, containing two strictly paired ellipsoidal sporocysts (15-19 μm
long, 8-10 μm wide), each containing four sausage-shaped sporozoites and a
refractile residual body. The prepatent period ranges from 14 to 18 days for Sarcocystis hominis and from 11 to 13
days for Sarcocystis suihominis.
Stool examination should be performed using a density-flotation concentration
method (e.g. on sodium chloride, zinc sulfate, or sucrose gradients), rather
than sedimentation (e.g. with formalin-ether). Sarcocystis autofluoresces under UV light. Because sporocysts of different Sarcocystis species overlap in shape and
size, the species cannot be distinguished solely by microscopy. Molecular
biological methods involving the PCR amplification of species-specific rRNA may
be used for this purpose.