Sarcocystosis: Introduction
(Sarcosporidiosis) |  |
| In sarcocystosis, the endothelium and muscles and other soft tissues are invaded by protozoans of the genus
Sarcocystis
. As the name implies,
Sarcocystis
spp
form cysts in various intermediate hosts—humans, horses, cattle, sheep, goats, pigs, birds, rodents, camelids, wildlife, and reptiles. The cysts vary in size from a few micrometers to several centimeters, depending on the host and species. |
| Etiology, Transmission, and Pathogenesis: |
|
Sarcocystis
spp
normally develop in 2-host cycles consisting of an intermediate host (prey) and the final host (predator). Species-specific prey-predator life cycles have been demonstrated for cattle-dog (
S
cruzi
), cattle-cat (
S
hirsuta
), cattle-human (
S
hominis
), sheep-dog (
S
capracanis
,
S
hircicanis
), sheep-cat (
S
gigantea
,
S
medusiformis
), goat-dog (
S
capracanis
,
S
hircicanis
), goat-cat (
S
moulei
), pig-dog (
S
meischeriana
), pig-human (
S
suihominis
), pig-cat (
S
porcifelis
), and others. Some wildlife may serve as intermediate hosts (such as raccoons) or final hosts (coyotes) for some species of
Sarcocystis
. |
| About 1 wk after ingesting muscle tissue that contains
Sarcocystis
cysts (sarcocysts), the final host begins to shed infective sporocysts in the feces; shedding continues for several months. After ingestion of sporocysts by a suitable intermediate host, sporozoites are liberated and initiate development of schizonts in vascular endothelia. Merozoites are liberated from the mature schizonts and produce a second generation of endothelial schizonts. Merozoites from this second generation subsequently invade the muscle fibers and develop
into the typical sarcocysts. Initially, sarcocysts contain only a few metrocytes—round, noninfective parasites that give rise to the banana-shaped infective zoites found in mature cysts beginning 2-3 mo after infection. Sarcocysts of some species grow so large that they are easily visible with the unaided eye. The presence of such sarcocysts can cause condemnation of the carcass during meat inspection.
S
cruzi
is probably most important in condemnation of beef cattle for human consumption. However,
S
hirsuta
has been primarily responsible for dairy cattle condemnation for visible sarcocysts.
S
meischeriana
is usually the species responsible for sarcocyst condemnation of pork and may affect meat quality. Sarcocysts are easily recovered from esophagus, diaphragm, and heart muscle. Sarcocysts of some species remain microscopic even though tremendous numbers of cysts may be present in the muscles. |
|
Sarcocystis
spp
were considered of doubtful pathogenicity until induced infection with
S
cruzi
sporocysts from canine feces caused acute disease in calves; eosinophilic myositis in cattle; and abortions, stillbirths, and deaths in pregnant cows. Two cases of necrotic encephalitis in heifers have been reported. Similar pathogenicity has been demonstrated for
S
tenella
in lambs and ewes. An outbreak of myositis affecting 20 ewes with flaccid paralysis was a result of heavy
Sarcocystis
infection. Immune status of the host and the dose of cysts may be the most important factors for the development of clinical disease. Immunosuppression may be responsible for tissue invasion of final hosts (eg, dog and cat). Pathologic changes in myocardium and skeletal muscles were more pronounced in cows with lymphatic leukemia. “Immunization” using small doses of sporocysts appears to prevent development or reduce severity of clinical disease in sheep when
challenged with large doses later (premunitive immunity). In dogs, a longer prepatent period and shortened patent period resulted after repeated infection. Pigs can also have persistent acquired immunity after immunization infections. |
| Humans may also serve as intermediate hosts and suffer myositis and vasculitis, but this tissue phase is rare, and the source of such human infection has never been determined. Human intestinal illness, with clinical signs of nausea, abdominal pain, and diarrhea that lasted up to 48 hr, has followed ingestion of sarcocysts of
S
suihominis
in uncooked pork and
S
hominis
in uncooked beef. The extent of human illness from ingestion of infected meat has not been documented. |
|  |
| Clinical Findings: |
|
Sarcocystis
spp
infections are quite prevalent in farm animals; however, there have been few outbreaks of clinical disease. Most animals are asymptomatic, and the parasite is discovered only at slaughter. In cattle severely affected by
S
cruzi
, the signs include fever, anorexia, cachexia, decreased milk yield, diarrhea, muscle spasms, anemia, hyperexcitability, weakness, prostration, and death. Cows infected in the last trimester of pregnancy may abort.
Sarcocystis
sp
infection was associated with the loss of tailswitch hair in a group of feedlot cattle. After recovery from acute illness, calves failed to grow well and eventually died in a cachectic state. Anemia, hepatitis, and myocarditis were the primary lesions in acute ovine sarcocystosis after experimental challenge with
S
tenella
sporocysts. Cases of encephalomyelitis in sheep were associated with a
Sarcocystis
sp
infection. After recovery from acute illness, some sheep may lose their wool. At necropsy, acutely affected animals have hemorrhage of the serous membranes of the viscera and myocardium.
Sarcocystis
spp
infections are probably most important in growing ruminants and swine, in which they can result in subclinical anemia and reduced weight gain.
Sarcocystis
spp
may also induce abortion in sheep. |
| Equine protozoal myeloencephalitis (EPM,
Equine Protozoal Myeloencephalitis: Introduction) is now considered to be caused by
S
neurona
. An experimental DNA probe appears promising as a diagnostic tool. Only asexual stages of this parasite have been found, and they may be located in neurons and leukocytes of the brain and spinal cord. Horses may also develop a myopathy. Multifocal myositis has been reported and is possibly due to
S
fayeri
. Clinical signs in horses include gait abnormalities such as ataxia, knuckling, and crossing over. Muscle atrophy, which is usually unilateral, is possible. The lesions are typically focal, and brain-stem involvement is common. Depression, weakness, head tilt, and dysphagia are other possible signs. EPM can mimic many neurologic diseases. |
| |
| Control: |
| Livestock become infected by sporocysts from the feces of carnivores. Because most adult cattle, sheep, and many pigs harbor cysts in their muscles, dogs and other carnivores should not be allowed to eat raw meat, offal, or dead animals. Supplies of grain and feed should be kept covered; dogs and cats should not be allowed in buildings used to store feed or house animals. Amprolium (100 mg/kg, sid for 30 days), fed prophylactically, reduced illness in cattle
inoculated with
S
cruzi
. Prophylactic administration of amprolium or salinomycin also protected experimentally infected sheep. Therapeutic treatment of cattle and sheep has been ineffective. Vaccines are not available. Experimental work demonstrated that infected pork could be made safe for consumption by cooking at 70°C for 15 min or by freezing at -4°C for 2 days or -20°C for 1 day. |
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