Sarcocystis species-pigs

Species of the intracellular, apicomplexan protozoan Sarcocystis occur in mammals, reptiles and birds around the world, including in Canada.  The taxonomy within the genus is uncertain.  The life cycle is indirect involving a carnivore definitive host - in which sexual reproduction (gametogony) occurs in enterocytes in the small intestine, and a herbivore or omnivore intermediate host, in which asexual reproduction (merogony or schizogony) occurs in the vascular endothelium and in other tissues.  The end point of parasite development in the definitive host is sporocysts in the faeces (which are released from oocysts which hatch in the GI lumen) which are immediately infective for the intermediate hosts, and in intermediate host is the sarcocyst, which is in some ways equivalent to the tissue cyst of Toxoplasma gondii.  Infection of the intermediate host is by ingestion of sporocysts in the faeces of a definitive host, and infection of the definitive host by ingestion of sarcocysts in the tissues of an intermediate host.

Many Sarcocystis infections, particularly in definitive hosts, are not associated with significant pathology or clinical signs.  Some species, however, can cause disease in intermediate hosts, for example S. bovicanis (aka S. cruzi) and Dalmeny disease in cattle, Sarcocystis species in the CNS of sheep causing neurological signs, and S. neurona and equine protozoal myeloencephalitis in horses.  For additional information on EPM, see the Sarcocystis neurona section.

Phylum: Apicomplexa
Class: Conoidasida
Subclass: Coccidiasina
Order: Eucoccidiorida
Suborder: Eimeriorina

Sarcocystis has many biological characteristics similar to those of Toxoplasma, Eimeria and Isospora, but there are important differences, particularly in the life cycle.  The taxonomy of species within the coccidial genus Sarcocystis is somewhat uncertain, but a number of distinct species have been identified in domestic and free-ranging animals and birds, in reptiles, and in people.  Currently the names of many species of Sarcocystis reflect the known definitive and intermediate hosts.  Sarcocystis species tend to be somewhat host specific, especially in their definitive hosts. 

The species names below are from: Taylor MA, Coop RL, Wall RL (2007) Veterinary Parasitology (Third edition). Oxford, Blackwell Publishing.

Cats - primarily definitive hosts

S. bovifelis (aka S. hirsuta) - cattle intermediate hosts
S. ovifelis (aka S. gigantea, S. medusiformis) - sheep intermediate hosts
S. porcifelis (aka S. suifelis) - pig intermediate hosts

Dogs - primarily definitive hosts

S. bovicanis (aka S. cruzi) - cattle intermediate hosts
S. ovicanis (aka S. tenella) - sheep intermediate hosts
S. canis - unknown intermediate hosts
S. fayeri - horse intermediate hosts
and many iothers

Cattle - intermediate hosts

S. bovicanis (aka S. cruzi) - dog and coyote definitive hosts
S. bovifelis (aka S. hirsuta) - cat definitive host
S. bovihominis (aka S. hominis) - human definitive hosts

Sheep - intermediate hosts

S. ovicanis (aka S. tenella) - dog definitive hosts
S. ovifelis (aka S. gigantea, S. medusiformis) - cat definitive hosts

Horses - intermediate hosts

S. equicanis (aka S. bertrami) - dog definitive host
S. fayeri - dog definitive host

Pigs - intermediate hosts

S. suicanis (aka S. porcicanis, S. miescheriana) - dog, wolf and fox definitive hosts
S. porcifelis (aka S. suifelis) - cat definitive hosts
S. suihominis - human definitive hosts

People - definitive hosts (and sometimes intermediate hosts for other species)

S. bovihominis (aka S. hominis) - cattle intermediate hosts
S. suihominis - pig intermediate hosts

The sporocysts of Sarcocystis species in the faeces of definitive hosts are oval, measure 10-15 µm by 7-10 µm, and each contains four sporozoites.  Identification to species is often difficult without specialized knowledge and experience.

The life cycle stages of Sarcocystis in the vascular endothelium of intermediate hosts can be detected only histologically, and identification to species is often difficult without specialized techniques.  The sarcocysts are usually linear in shape, and depending on the species and stage of development vary in size from less then a 1mm to a few centimetres in length.  Naked eye detection of the very small sarcocysts is often difficult or impossible.

Various species of Sarcocystis are found in domestic and free-ranging animals and birds around the world, including in Canada, sometimes at high prevalence and abundance .  The parasite occur in cats, dogs, cattle sheep, horses, pigs and people.  Some of these act as definitive hosts, some as intermediate hosts, and some as both.  Species of Sarcocystis show some host specificity.

The life cycle of Sarcocystis is indirect, with a carnivore definitive host and a herbivore or omnivore intermediate host.  Sometimes carnivores can also serve as intermediate hosts, or at least are infected with the life cycle stage that is infective for definitive hosts. In the definitive hosts the parasite undergoes development typical of the genus Sarcocystis, involving sexual reproduction (gametogony) in the enterocytes of the small intestine, which results in the production of oocysts.  These sporulate in the intestinal lumen  and the oocyst wall disintegrates, releasing  two sporocysts each containing four sporozoites.  These sporocysts leave the host in the faeces and are immediately infective. This development in the definitive host is different from that for Toxoplasma, Eimeria and Isospora, in which there is both asexual and sexual reproduction in the enterocytes.

Infection of intermediate hosts is by ingestion of the sporocysts.  In these hosts the sporozoites are released and invade the intestinal mucosa, then blood vessels, where they enter the endothelial cells and undergo one or more cycles of asexual reproduction (merogony or schizogony), each resulting in the production of meronts containing merozoites.  Finally the merozoites enter muscle cells and form sarcocysts containing large numbers of bradyzoites.  Ingestion of the sarcocysts by definitive hosts completes the life cycle.  Depending on the species, sarcocysts can be found in a range of organs and tissues, and clinical signs can occur if they develop in sensitive areas (for example the central nervous system).

Maintenance of Sarcocystis within an ecosystem requires contacts between definitive and intermediate hosts.  The former must consume the latter and the latter must ingest sporocysts from the faeces of the former.  Other than that, little is known about the epidemiology of the parasite.

The development and sexual reproduction of Sarcocystis in the enterocytes of the carnivore definitive host are rarely if ever associated with clinical signs, unless the parasites are present in very large numbers or there is concommitant disease. On the other hand, the asexual reproduction in endothelial cells and the development of sarcocysts in a variety of organs and tissues in the intermediate hosts can cause significant pathology and clinical signs, the severity of which vary with host and parasite species, the location(s) of the parasites within the hosts, and parasite abundance.  The ongoing uncertainties concerning the taxonomy within the genus Sarcocystis means that it is often difficult to identify which species is doing what.  It can also be problematic to determine the relative roles of the various life cycle stages of the parasite in its intermediate hosts in the generation of lesions and clinical signs.

Cats - primarily definitive hosts

No reports of pathology or clinical signs associated with extra-intestinal parasite development of Sarcocystis.

Dogs - primarily definitive hosts

Very rare reports of pathology and clinical signs associated with extra-intestinal development of Sarcocystis species, possibly S. canis,  in the central nervous system, liver, skin and skeletal muscle.

Cattle - intermediate hosts

Infrequent reports of pathology and clinical signs associated with the extra-intestinal development of Sarcocystis bovicanis (aka S. cruzi), and possibly other species, in vascular endothelium in many organs and tissues.  Clinical signs include fever, emaciation, anaemia and abortion, with high morbidity and some mortality.  This is Dalmeny disease, first described in Dalmeny, Ontario, in 1963. 

Common reports of lesions visible to the naked eye associated with the sarcocysts of S. cruzi, and possibly other species, in skeletal and cardiac muscle.  This is eosinophilic myositis which is rarely if ever a cause of clinical signs, but which is commonly seen in cattle at slaughter and can lead to downgrading of the affected carcasses.

Sheep - intermediate hosts

Rare reports of  generalized disease and sometimes abortions associated with the development of Sarcocystis species in vascular endothelium.  Rare reports of pathology and neurological clinical signs associated with the development of sarcocysts of Sarcocystis species in the central nervous system.  More common reports of Sarcocystis species in skeletal muscle, leading to carcass downgrading.

Horses - intermediate hosts

Other than S. neurona, for which horses are an intermediate (or perhaps a dead-end) host, little is known about Sarcocystis species in horses.

Pigs - intermediate hosts

Sarcocystis suicanis is usually asymptomatic, but it can cause problems in skeletal muscle during all stages of its extra-intestinal development, leading to carcass downgrading.  Sarcocystis porcifelis is believed to be relatively non-pathogenic in pigs.  Sarcocystis suihominis can cause significant pathology in the liver of pigs associated with the extra-intestinal development of the parasites in vascular epithelium.

People - definitive hosts (and sometimes intermediate hosts for other species)

Sarcocystis hominis and S. suihominis are rarely if ever associated with pathology or clinical signs in their human definitive hosts.  Several other as yet unidentified species of Sarcocystis have been associated with pathology and clinical signs caused by extra-intestinal life cycle stages of the parasite, particularly sarcocysts, in skeletal muscles and elsewhere.

Many but not all species of Sarcocystis can be identified on the basis of sporocyst structure, but it is not easy and is beyond the capability of many diagnostic parasitology laboratories.  The sarcocysts of many but not all species can be identified using light and/or electron microscopy to examine structural details, but some of them are very small and so initial detection with the naked eye can be a problem..  As with the sporocysts, identifying sporocysts to species is not often a routine procedure for many  diagnostic laboratories.  Immunohistochemistry is useful for histological sections of sarcocysts and other extra-intestinal life cycle stages, but most of the antibodies currently available cannot distinguish beyond the genus Sarcocystis.  PCR is useful for some species of Sarcocystis, but the lack of clarity in the taxonomy within the genus means that this approach is of limited value, except for S. neurona and other well-established species.

There is very rarely a need for specific treatment of either the intestinal stages of Sarcocystis in the carnivore definitive hosts or of the extra-intestinal stages in the herbivore or omnivore intermediate hosts, except for S. neurona in horses, and sometimes other intermediate hosts, that are showing clinical signs.  Supportive treatment, however, can be very helpful when dealing with clinical sarcocystosis in intermediate hosts, for example Dalmeny disease in cattle.

Control of Sarcocystis depends on limiting access by intermediate hosts to sporocysts in the faeces of definitive hosts, and especially not feeding definitive hosts on raw, incompletely cooked, or incompletely frozen meat and other tissues from infected intermediate hosts.  As mentioned in the Diagnosis section, it can sometimes be difficult to detect Sarcocystis in its intermediate hosts and, if detected, to identify the species and thereby the risk to animals fed the meat.

The two species of Sarcocystis for which people are known definitive hosts are zoonoses: S. hominis - acquired by ingesting sporocysts in meat and other tissues from infected cattle, and S. suihominis - acquired by ingesting sporocysts in meat and other tissues from pigs.  It is possible that other species of Sarcocystis will be found that use people as definitive hosts, and that the species that develop in the extra-intestinal tissues of people will be more completely identified.

Fayer R. (2004) Sarcocystis spp. in human infections. Clinical Microbiology Reviews 17: 894-902.