Research in Veterinary Science 136 (2021) 151–157
Available online 19 February 2021
0034-5288/Published by Elsevier Ltd.
Zoonotic Sarcocystis
Benjamin M. Rosenthal *
Animal Parasitic Disease Laboratory, United States Department of Agriculture- Agricultural Research Service, 10300, Baltimore Avenue, Beltsville, MD 20705, United
States of America
A R T I C L E I N F O
Keywords:
Enteritis
Zoonoses
Sarcocystis
Sarcocystosis
Myositis
Food-borne illness
A B S T R A C T
Apicomplexan species in the genus Sarcocystis form tissue cysts, in their intermediate hosts, similar to those
established in chronic toxoplasmosis. More than 200 species are known, but just a few are known to threaten
human health owing to infection in livestock species. Intestinal sarcocystosis occurs when people consume raw or
undercooked beef contaminated with Sarcocystis hominis or S. heydorni or undercooked pork contaminated with
S. suihominis. Those infections may cause mild enteritis, but most infections are thought to be asymptomatic.
People also become dead-end (intermediate) hosts for non-human Sarcocystis spp. after accidentally ingesting
sporocysts, leading to extraintestinal sarcocystosis. The clinical spectrum may range from asymptomatic muscle
cysts to a severe, acute, eosinophilic myositis associated with systemic symptoms with peripheral eosinophilia.
Most human cases have been described from Southeast Asia, but Sarcocystis parasites have a worldwide distribution,
especially where livestock is raised, and human infections in other areas have been described but may be
underrecognized.
1. Introduction
Apicomplexan parasites in the genus Sarcocystis cycle between
predators and prey. Predators (and scavengers) become infected by
consuming tissues harboring intracellular tissue cysts (sarcocysts). Such
predators experience gastrointestinal (GI) infections and the development
of sexual stages which mate and produce oocysts which the host
then excretes. In so doing, these carnivores serve as “definitive hosts.”
Herbivorous “intermediate hosts” acquire infection by ingesting food or
water contaminated with the excreted oocysts. They support asexual
parasite reproduction, culminating in the formation of tissue cysts
(sarcocysts). More than 200 known species of Sarcocystis cycle in nonhuman
hosts (Dubey et al., 2016). Some cycle efficiently between
domesticated livestock (intermediate) hosts and in domestic canine or
feline (definitive) hosts. Only three are known to cycle between livestock
intermediate hosts and human definitive hosts: Sarcocystis hominis,
Sarcocystis heydorni, and Sarcocystis suihominis. Human beings also sustain
infections with parasites capable of causing tissue cysts, though the
range of species capable of doing so (and the cycles that sustain them)
remain less well understood. This review focuses on those species of
Sarcocystis established or suspected of causing human disease.
When people consume undercooked beef containing sarcocysts they
can become definitive hosts of Sarcocystis hominis (Ahmadi et al., 2015;
Chen et al., 1999; Chen et al., 2003; Domenis et al., 2011; Dubey et al.,
1988; Dubey et al., 1989; Hajimohammadi et al., 2014; Heydorn et al.,
1975; Lian et al., 1990; Nimri, 2014; Nourani et al., 2010; Saito et al.,
1999; Vangeel et al., 2007; Wouda et al., 2006) or S. heydorni (Fig. 1).
The former produces thick-walled cysts in cattle tissue and has a predilection
for heart muscle, whereas the latter forms thin-walled cysts
that express distinctive, short protrusions from the cyst wall (Dubey
et al., 2015a; Hu et al., 2016). Consuming undercooked pork can cause
human infections with S. suihominis (Calero-Bernal et al., 2016; Chauhan
et al., 2020; Coelho et al., 2015; Fayer et al., 1979; Gazzonis et al., 2019;
Heydorn, 1977; Heydorn and Mehlhorn, 1978; Heydorn and Weniger,
1988; Huang et al., 2019; Li et al., 2007; Li et al., 2004; Mehlhorn and
Heydorn, 1977, 1979; Saito et al., 1998). These generally result in
transient infections restricted to the gastrointestinal tract. Light microscopy
suffices to differentiate thick from thin-walled sarcocysts in a
given host, but further differential diagnosis relies on ultrastructural
differences in the sarcocyst wall only discernable at the greater magnification
afforded by electron microscopy (exemplified for two species
occurring in pork in Fig. 3) or using genetic differentiation (discussed
further, below).
Although parasites in the genus Sarcocystis have a worldwide distribution,
our understanding of the epidemiology of human sarcocystosis
relies primarily on case reports and occasional outbreaks from
* Corresponding author.
E-mail address: benjamin.rosenthal@usda.gov.
Contents lists available at ScienceDirect
Research in Veterinary Science
journal homepage: www.elsevier.com/locate/rvsc
https://doi.org/10.1016/j.rvsc.2021.02.008
Received 10 June 2020; Received in revised form 25 January 2021; Accepted 4 February 2021
Research in Veterinary Science 136 (2021) 151–157
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Southeast Asia. Malaysia and Thailand are known foci of transmission;
there, limited seroprevalence studies and stool surveys confirm widespread
distribution and exposures, and suggest human infections are
underreported (Wilairatana et al., 1996; Wong and Pathmanathan,
1992). Incidental findings at autopsy suggest widespread infection in
endemic areas. In one study of routine autopsy material, infection
prevalence was estimated at more than 20% (Wong and Pathmanathan,
1992). In Iran, sporocysts of Sarcocystis were identified (alone or in
conjunction with other parasites) in 4 of over 2000 undiagnosed cases of
abdominal distress (Agholi et al., 2016). This underscores the effort that
may need to be devoted to truly defining prevalence in a population.
Sarcocystis nesbitti, a parasite initially described from tissue cysts in
rhesus macaques (Mandour, 1969; Yang et al., 2005), was suspected on
phylogenetic grounds as having a predatory snake as its definitive hosts
(Tian et al., 2012). That suggestion proved helpful when seeking the
source of large outbreaks of human infections attributed to S. nesbitti
(Abubakar et al., 2013; Lau et al., 2014; Shahari et al., 2016). A toxin
isolated from S. fayeri appears responsible for food poisoning in people
who have consumed raw horsemeat (Kamata et al., 2014). Fig. 4 depicts
sarcocysts obtained from biopsies from three human subjects infected
with parasites attributed to S. nesbitti.
By domesticating livestock, human beings have created conditions
that favor the evolution of parasites exploiting our proximity to cattle
and pigs and our consumption of their meat. The likeliest sources of
exposure to tissue cysts of other, as-yet undiscovered species of Sarcocystis
would seem to be other domesticated livestock (such as poultry,
goats, sheep, and reindeer). People do eat a wide range of other animals;
wildlife, too, may harbor as-yet undiscovered zoonotic species for which
people might serve as definitive hosts. Accidental human infection with
parasites that naturally cycle between our prey and their natural
Fig. 1. Schematic representation of the life cycle of Sarcocystis hominis and S. suihominis. (Reproduced with permission from Murrell KD, Fayer R, Dubey JP. Parasitic
organisms. Advances in meat research. West Port, CT: AVI Publishing Co. 1986.) Like S. hominis, S. heydorni cycles between people and cattle. Predatory snakes are
definitive hosts for S. nesbitti, excreting oocysts that engender sarcocysts in human tissues.
B.M. Rosenthal
Research in Veterinary Science 136 (2021) 151–157
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predators cannot be ruled out. Nor can we rule out the possibility that
other forms of Sarcocystis may imperil human health when we ingest
sporcysts excreted by other carnivores. Such concern is underscored by
documented illness suffered by aberrant infections in hosts believed not
to naturally sustain cycles of a given parasite’s transmission (for
example, equine parasitic myeloencephalitis, which horses contract by
grazing on pastures contaminated with sporocysts of Sarcocyistis neurona
excreted by opossums; disease in horses can be severe, but raccoons,
armadillos, and other naturally-infected mammals more typically harbor
tissue cysts infectious for opossums).
2. Pathogenesis
Sarcocystis parasites are intra-cellular coccidian parasites. S. hominis,
S. heydorni, and S. suihominis use humans as definitive hosts and are
responsible for intestinal sarcocystosis in the human host. Humans may
also become dead-end hosts for non-human Sarcocystis spp. after the
accidental ingestion of oocysts.
Individual sporocysts, and sporulated oocysts (each containing two
sporocysts) are excreted. Each sporocyst contains four sporozoites and a
refractile residual body. Sporocysts ingested by the intermediate host
(cattle for S. hominis and S. heydorni and pigs for S. suihominis) rupture,
releasing sporozoites. Sporozoites enter endothelial cells of blood vessels
and undergo schizogony, resulting in first-generation schizonts. Merozoites
derived from the first generation invade small capillaries and
blood vessels, becoming second-generation schizonts. Secondgeneration
merozoites invade skeletal and heart myocytes, as well as
neurons, and develop into metrocytes and undergo a series of internal
mitotic divisions (a process termed endodyogeny). When filled with
bradyzoites, the metrocytes becomes a sarcocyst which can withstand
digestion (Fig. 2).
Pathogenesis of intestinal human infections has been described in
human volunteers who were infected by arguably abnormally large
numbers of sarcocysts. After several days, and for several days thereafter,
infected persons excreted oocysts and infective sporocysts.
Although people can experience lengthy, indeed lifelong, infection with
tissue stages (sarcocysts), the identity and origins of those parasites is
unclear. Excellent, illustrated reviews of zoonotic sarcocystosis are
available (Dubey et al., 2016; Dubey and Fayer, 1983; Fayer, 2004;
Fayer et al., 2015). The complete genome of a single enzootic species
(S. neurona) serves as a model to explore biology and pathogenesis
(Blazejewski et al., 2015; Murungi and Kariithi, 2017).
3. Clinical manifestations
3.1. Enteric infection
Most individuals with intestinal sarcocystosis remain asymptomatic.
Symptoms induced by experimental infections include nausea, abdominal
discomfort, and self-limited diarrhea, with symptom severity
varying with the amount of meat consumed (Beaver et al., 1979; Fayer,
2004; Jeffrey, 1974). The onset of diarrhea is typically sudden (in some
subjects 3–6 h post-ingestion; usually within 48 h). Symptoms usually
resolve within 36 h. Segmental, eosinophilic, necrotizing enteritis
attributed to sexual forms of Sarcocystis has been reported; however,
Sarcocystis was not definitely established as the cause for these symptoms
(Bunyaratvej et al., 2007).
3.2. Muscle infection
As with gastrointestinal infection, cases of extraintestinal sarcocystosis
typically lack symptoms. When people ingest sporocysts, the
release of meronts to the endothelium and subsequent merozoite invasion
of striated muscle can induce vasculitic and/or musculoskeletal
symptoms. In an outbreak of eosinophilic myositis in a U.S. military unit
operating in Malaysia, 7 of 10 exposed personnel reported an acute
illness consisting of fever, myalgia, bronchospasm, fleeting pruritic
rashes, transient lymphadenopathy, and subcutaneous nodules associated
with eosinophilia, elevated erythrocyte sedimentation rate, and
elevated hepatic enzymes and muscle creatinine kinase (Arness et al.,
1999). One patient had serious, chronic sequelae; the other team
members had milder, self-limited illnesses. A muscle biopsy of the index
case showed an unidentified Sarcocystis species. In a recent outbreak in
Tioman Island, Malaysia involving 93 suspected cases, including 2 cases
confirmed as harboring tissue cysts of S. nesbitti, the most frequent
symptoms were fever, myalgia, headache, and cough (Abubakar et al.,
2013; Lau et al., 2014; Shahari et al., 2016).
Symptomatic cases can experience painful muscle swelling,
measuring 1 to 3 cm in diameter and initially associated with erythema
of the overlying skin; these occur episodically and last from 2 days to 4
weeks. Sometimes, lesions are accompanied by fever, diffuse myalgia,
muscle tenderness, weakness, eosinophilia, and bronchospasm. A discharging
sinus, painful swelling with erythema, an ill-defined mass, and
chronic pain with ulceration have been reported from New Zealand
(Mehrotra et al., 1996).
Fig. 2. Photomicrographs of Sarcocystis muris, an
enzootic species of Sarcocystis that exemplifies features
also found in zoonotic forms. (A) Developing
sarcocyst in muscle tissue. Immature metrocytes (mc)
and mature bradyzoites (bz) are enclosed by the sarcocyst
wall (scw). (B) Two unsporulated oocysts (top
left) and two sporulated oocysts (bottom right). In
sporulated oocysts, sporozoites (sz) and residuum
bodies (rb) are enclosed by the prominent sporocysts
wall (spw), which, in turn, are paired within the
oocyst wall (ow). In many instances, unpaired sporocysts
will be recovered from ruptured oocysts. The
scale bar denotes 20 μm.
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3.3. Diagnosis
Visible spooocysts (each containing four sporozoites) and sporulated
oocysts (each containing two sporocysts) appear in microscopically in
freshly voided stool specimens with the aid of flotation procedures. The
acid-fast oocysts of Sarcocystis resemble those of Isospora. Oocysts lack
features enabling their differential diagnosis via microscopy; they typically
measure 15 × 20 μm, and sporocysts measure 12 × 6 μm. Visible
trophozoites or bradyzoites in biopsy tissue also confirm diagnosis.
Genetic variation provides a basis to discriminate among excreted
parasites (Xiang et al., 2009). The 18S rDNA was the first locus
sequenced for species in the genus and affords modest resolution for
differentially diagnosing members of the genus and for reconstructing
their phylogeny. More recently, more variable markers such as ribosomal
ITS-1 and mitochondrial cytochrome oxidase have enriched the
understanding of parasite diversity, relationships, and transmission cycles
(Abe et al., 2019; Antunes Murata et al., 2018; Cerqueira-Cezar
et al., 2018; Cerqueira-Cezar et al., 2017; Dahlgren et al., 2008; Dubey
et al., 2015b; Gjerde, 2012, 2013; Gjerde et al., 2020; Gjerde et al.,
2017a; Gjerde et al., 2017b; Rubiola et al., 2018; Valadas et al., 2016;
Zeng et al., 2018).
Muscle biopsy can definitively attribute myositis to Sarcocystis
infection after excluding other muscle cyst-forming organisms, such as
Toxoplasma gondii and Trypanosoma cruzi. Sarcocysts are septate and
contain ultrastructurally distinct cyst walls. Bradyzoites of Sarcocystis
and T. gondii stain with periodic acid–Schiff, but T. cruzi does not. Intact
sarcocysts can measure up to 100 μm in diameter and 325 μm in length,
but are more often in the 20 to 60 μm range and are do not generally
induce inflammatory reactions in the surrounding tissue. However, mild
inflammatory reactions can cause pain (Mehrotra et al., 1996). In the
absence of muscle biopsy, eosinophilia in the presence of a compatible
clinical syndrome and epidemiologic exposure support a probable
diagnosis, especially if other diagnoses can be excluded. The chronic
nature of sarcocyst infections and the cross-reactivity of sarcocyst antigens
with those of related species renders serology of uncertain diagnostic
value (Garcia-Lunar et al., 2015; Gondim et al., 2017;
O’Donoghue and Weyreter, 1984; Weber et al., 1983; Weyreter and
O’Donoghue, 1982). Studies of adults in rural areas of Laos and Tibet
document seroprevalence of 10% - 20% (Xiang et al., 2009).
3.4. Treatment
The terminally differentiated organisms in muscle sarcocysts do not
spread to new cells; treatments directed toward replicating stages do not
harm these mature stages. Although various treatments have been used
for symptomatic muscular infection, none have demonstrated efficacy,
and no specific treatment for Sarcocystis infection has been described.
Palliative treatment for symptoms can include corticosteroids, to reduce
episodic allergic inflammatory reactions after cyst rupture or to alleviate
more chronic, systemic symptoms. Cotrimoxazole treatment, especially
when initiated early, shortened the duration of symptoms in a small
cohort (Slesak et al., 2015). Because the intra-cystic bradyzoites do not
spread, there seems no risk of recrudescence.
3.5. Prevention
Presumably, muscle infections could be prevented by avoiding food
contaminated with feces of predatory carnivores and boiling water so
contaminated. Avoiding raw or undercooked beef and pork prevents
enteric infection with any of the three known species for which people
serve as definitive hosts. Sarcocysts in pork can be destroyed at 60 ◦
C for
20 min, 70 ◦
C for 15 min or 100 ◦
C for 5 min, or by freezing it at − 4 ◦
C
for 2 days or − 20 ◦
C for 24 h. This is in keeping with general CDC
guidelines which recommend cooking all non-poultry meat to an internal
temperature of 71 ◦
C. Sporocysts and oocysts die when heated to 60
◦
C for 1 min, 55 ◦
C for 15 min, or 50 ◦
C for one hour, but can survive
freezing. Commonly used disinfectants (e.g., 1% iodine, 10% formalin,
12% phenol, 2% chlorhexidine) fail to kill S. neurona sporocysts, but
5.25% sodium hydroxide (bleach) suffices (Dubey et al., 2002).
4. Public health burden
The full extent of the public health burden of human sarcocystosis
remains unclear. Although veterinary data implicate immune suppression
as a clinical risk factor for sarcocystosis, and although pregnancy
and AIDS markedly exacerbate toxoplasmosis (caused by a closely
related zoonotic parasite), less is known about how vulnerability to
sarcocystosis may vary in human populations. A clinical report
described marked symptoms and documented various life stages of a 31
year old AIDS patient, recommending sarcocystosis be considered as an
opportunistic pathogen in such persons (Velasquez et al., 2008).
Those who routinely consume uncooked meat and reside where
untreated human waste contaminates pastures are at greatest risk
(Bunyaratvej et al., 2007). However, the burden of these infectious in
such impoverished populations has remains ill-defined. Surprisingly
prevalent S. hominis in European cattle (Vangeel et al., 2007), but not in
American cattle (Pritt et al., 2008), suggests that even among developed
countries, variation in risk may occur. The true extent of infection
Fig. 3. The structure of the sarcocyst wall aids differential diagnosis of co-infecting species. Here, two sarcocysts in pig muscles are depicted. On the left,
S. miescheriana for which dogs serve as the definitive host. On the right, S. suihominis for which people serve as the definitive host. Note the villar protrusions on the
sarcocyst wall are thinner and longer for S. suihominis. (Dubey et al., 2016).
B.M. Rosenthal
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anywhere remains unclear, in part owing to the ubiquity of enzootic
infections that pose no known risk to human health. Veterinary and
human infections may exceed, in prevalence and consequence, that
which is presently recognize, but the actual extent of public health
burden remains ill-defined (Harris et al., 2015).
Declaration of Competing Interest
I have prepared this manuscript in the public interest with scientific
integrity and effective open communication as my governing values. I
have performed this work as a public servant and have no competing
financial interests that influence what I have written or what I have
omitted.
Acknowledgements
This work was funded by USDA Project "Detection and Control of
Foodborne Parasites for Food Safety" 8042-32420-007-00-D.
Fig. 4. Human muscular sarcocystosis attributable to S. nesbitti. A-C: light microscopy of cysts stained with hematoxalin and eosin; arrows indicate the thin sarcocyst
wall. D: transmission electron micrograph illustrating small blebs and thin ground substance comprising the sarcocyst wall. (Dubey, 2015).
B.M. Rosenthal
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