PARASITIC DISEASE

Sarcocystosis of the Skeletal and Cardiac Muscle, Mouse

l.P.Dubey

Synonym. Sarcosporidiosis.

Gross Appearance

Some Sarcocystis species form grossly visible sarcocysts (Greek: sarco = flesh, kystis = blad­der) in skeletal muscle of mice (Fig. 186). Gross­ly visible sarcocysts are dull-white, thread-like structures, replacing as much as 25% of the muscle mass. Heavily infected limb muscles bulge, and mice may have difficulty walking.

Microscopic Appearance

Sarcocysts are located within myocytes and usually cause no host reaction (Fig. 187). How­ever, some intact sarcocysts are surrounded by mononuclear cells. The rupture of sarcocysts may result in severe granulocytic to mononu­clear cell infiltrations (Ruiz and Frenkel 1976), followed by fibrosis (Figs. 188, 189). Sarcocysts may disappear completely within 3-6 months after infection.

Ultrastructure

Sarcocysts are located within myocytes at all stages of development. They are separated from the host cell cytoplasm by a parasitophorous va­cuole. The parasitophorous vacuolar membrane bears numerous small bleb-like protrusions into the cytoplasm of the host cell (Fig. 190). Imme­diately adjacent to this membrane is the ground substance, which is continued as septa inside the sarcocyst. Two types of zoites are found in the sarcocyst. The globular zoites at the periphery are metrocytes (mother cells), and they are the only zoites present in immature sarcocysts. As the sarcocyst matures metrocytes divide into

Fig.186. Thread-like sarcocysts in skeletal muscles, mouse. (Courtesy of E. C. Powell) Unstained

two, forming banana-shaped zoites called brady­zoites or cystozoites (Figs. 187, 190). Bradyzoites are about 15 x 3 11m and contain pe­riodic acid Schiff-positive granules. The parasitized myocyte surrounds sarcocysts that are up to 150 11m in diameter and several millimeters long. The host cell cytoplasm under­goes partial degeneration and is compressed by the sarcocyst. The myofibrils disintegrate and may become arranged in groups. Lines are dis-

T. C. Jones et al. (eds.), Cardiovascular and Musculoskeletal Systems© Springer-Verlag Berlin Heidelberg 1991

166 l.P.Dubey

Fig.1S7 Cross section of a sarcocyst in a myocyte, mouse. Note thin sarcocyst wall (SCW), numerous banana-shaped bradyzoites (B), and metrocytes (M). Giemsa, x 750

Table 9. Summary of Sarcocystis species in house mouse (Mus musculus). (Modified from Dubey et al. 1989)

Sarcocystis Intermediate host species

Maximum Schizonts in length of sarcocysts

S. muris Railliet 1886; Several centimeters Hepatocytes Labbe 1899

S. dispersa Cerna et al. 1978 Microscopic Hepatocytes

S. crotali Entzeroth et al. 1985 4mm ?

S. scotti Levine and Tadros 1980? ?

S. sebeki Tadros and Larrman 1976 Several centimeters ?

S. dirumpens Hafner and Matuschka 1984 25 mm ?

S. muriviperae Matuschka et al. 1987 8 mm Hepatocytes

?, unknown.

Definitive host

Cat (Felis catus), ferret (Putorius putorius Juro)

Barn owl (Tyto alba), masked owl (Tyto novae hollandiae)

Mojave rattlesnake (Crotalus scutylatus)

Tawny owl (Strix aluco)

Tawny owl (Strix aluco)

Snakes of the genus Bitis Palestinian viper (Vipera palaestinae)

rupted and mitochondria accumulate in the cy­toplasm around the parasitophorous vacuolar membrane (Sheffield et al. 1977; Viles and Po­well 1976, 1981).

guished from Sarcocystis spp. by its structure. The tissue cysts of T. gondii are always micro­scopic ( < 100 !lm), the cyst wall is smooth, and there are no septa. The predominant lesion in toxoplasmosis is necrosis and the lesion is more severe in the brain.

Differential Diagnosis

Myositis can also be caused by toxoplasmosis. However, Toxoplasma gondii is easily distin-

Sarcocystosis of the Skeletal and Cardiac Muscle, Mouse 167

Fig.188 (above). Longitudinal section of skeletal muscle, mouse. Numerous intact sarcocysts on top and degenerating sarcocysts below. H & E, x 75

Etiology and Frequency

The house mouse (Mus musculus) is host for seven speces of Sarcocystis (Table 9). Sarcocystis spp. have an obligatory prey-preda­tor two-host life cycle (Fig. 191). Asexual stages develop only in the intermediate host, which in

Fig.189 (below) . Cross section of skeletal muscle, mouse. Degenerating (arrow) and intact sarcocysts sur­rounded by granulocytes and mononuclear cells. H & E, x 150

nature is often a prey animal (mouse). Sexual stages develop only in the definitive host, which is carnivorous. Sarcocystis muris, which com­pletes its life cycle in the domestic cat (or ferret) and the house mouse (Ruiz and Frenkel 1976; Rommel 1979; Rommel et al. 9181), will be used as an example. The cat becomes infected by eat-

168 l.P.Dubey

Fig.190. Sarcocyst in skeletal muscle, mouse. Note host cell nucleus (HN), parasitophorous vacuolar membrane (PVM), septa (S), metrocytes (M), and bradyzoites (B).

ing infected mice containing mature sarcocysts. Bradyzoites liberated from the sarcocyst pene­trate cells of the mucosa of the small intestine of the cat and transform these into either female (macro-) or male (micro-) gamonts. The micro­gamonts divide into several biflagellated micro­gametes. These microgametes are motile and penetrate the female gamont. A wall is formed around the resulting zygote to form the oocyst. Oocysts spo­rulate in the lamina propria of the small intes-

TEM, x 6273. Inset shows a high magnification of PVM. Note bleb-like protrusions (arrow). TEM, x 29573

tine. Sporulated oocysts contain two sporocysts, each with four sporozoites. Sporulated oocysts, or sporocysts, measuring about 10 x 8 ~m, are excreted in the feces. Mice become infected by ingesting food contami­nated with sporocysts. The sporozoites excyst in the intestinal lumen and reach the liver via the circulation. The sporozoite penetrates a hepato­cyte and the nucleus of the parasite then divides into several nuclei. Finally each nucleus is sur­rounded by cytoplasm which becomes confined

DEFINITIVE HOST

C.t~

"n" ••• r." by e., ", ill' ' " " I" ~ II I

'''',~~7~~J' fIr\~ ' )t:;~~:.-:~.11 1 1fI1 I . f""I , ., " , . ,I '"" , . ,.' , Ie. "" . ,11'" I I" "U",'II,

CYSTS In muscle

SCHIZONT in Liver

Sarcocystosis of the Skeletal and Cardiac Muscle, Mouse 169

SPOROCYST

INTERMEDIA TE HOST Mouse

Fig.191. Life cycle of Sarcocystis muris

by membranes to form merozoites. Merozoites are often arranged in a rosette within a schizont. Schizonts are seen in the liver 11-17 days after in­fection of the mouse. Unlike sarcocysts, schi­zonts locate directly in the host cell cytoplasm without a parasitophorous vacuole. Merozoites liberated from schizonts enter the circulation and penetrate myocytes to form sarcocysts. Earliest sarcocysts are seen around the 28th day after ino­culation. Sarcocysts become infections for the definitive host (cat) between 65 and 76 days fol­lowing inoculation (Ruiz and Frenkel 1976). Sar­cocysts begin to be seen with the naked eye at 80 days and are easily visible at 100 days after in­oculation. They start degenerating during the 4th month postinfection and may completely disap­pear by 6 months postinfection in some mice. However, some mice remain infected for more than 12 months, nearly for the life of the mouse. Sarcocysts are found only in skeletal muscles and not in the myocardium of mice. Sarcocystis muris infections are rare in mice raised under modern controlled conditions and their presence indicates contamination of bed­ding or food by feline feces.

Comparison with Other Species

More than 100 species of Sarcocystis have been described and they are generally host specific for their intermediate hosts (Dubey et al. 1989; Le-

vine 1986). Certain species of Sarcocystis can cause serious disease in cattle, sheep, goats, pig, and mule deer. Although all species of Sarcocys­tis have an obligatory two-host life cycle, Sarco­cystis species in livestock form schizonts in endo­thelial cells of blood vessels whereas in murine species schizonts are formed in hepatocytes.

References

Dubey JP, Speer CA, Fayer R (1989) Sarcocystosis of animals and man. CRC, Boca Raton

Levine ND (1986) The taxonomy of Sarcocystis (proto­zoa, apicomplexa) species. J Parasitol72: 372-382

Rommel M (1979) Das Frettchen (Putorius putorius Juro), ein zusatzlicher Endwirt flir Sarcocystis muris. Z Parasitenkd 58: 187-188

Rommel M, Schwerdtfeger A, Blewaska S (1981) The Sarcocystis muris infection as a model for research on the chemotherapy of acute sarcocystosis of domestic animals. Zentralbl Bakteriol Mikrobiol Hyg [A] 250: 268-276

Ruiz A, Frenkel JK (1976) Recognition of cyclic trans­mission of Sacocystis muris by cats. J Infect Dis 133: 409-418

Sheffield HG, Frenkel JK, Ruiz A (1977) Ultrastructure of the cyst of Sarcocystis muris. J Parasitol63: 629-641

Viles JM, Powell EC (1976) The ultrastructure of the cyst wall of a murine Sarcocystis. Z Parasitenkd 49: 127-132

Viles JM, Powell EC (1981) Myofiber damage accompa­nying intramuscular parasitism by Sarcocystis muris. Tissue Cell 13: 45-60