Summary
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.
Taxonomy
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
Note: Our understanding of the taxonomy of helminth, arthropod, and particularly protozoan parasites is constantly evolving. The taxonomy described in wcvmlearnaboutparasites is based on that in the seventh edition of Foundations of Parasitology by Larry S Roberts and John Janovy Jr., McGraw Hill Higher Education, Boston, 2005.
Morphology
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.
Host range and geographic distribution
Life cycle - indirect
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).
Epidemiology
Pathology 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.
Diagnosis
Treatment and control
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.