Sarcocystis is ubiquitous worldwide. The protozoon commonly and characteristically causes a chronic, subclinical infection in the cardiac and/or skeletal muscles of livestock, but, on rare occasions, Sarcocystis does give rise to disseminated acute disease, encephalitis (which can occur in the absence of disseminated infection), abortion or premature birth and, possibly, eosinophilic myositis. In southern Africa, cases of acute (pre-muscular) sarcocystosis have so far been seen in goats and cattle142 (Figure 20.1 to Figure 20.3).

Aetiology and life cycle

The coccidial nature of Sarcocystis first became apparent approximately 30 years ago as a result of work by Fayer in the USA and by Heydorn and Rommel in Germany. The classification, history and nomenclature of Sarcocystis have been discussed elsewhere.31, 40, 79, 115, 135, 137 The organism is categorized as follows: phylum Apicomplexa (Sporozoa); class Sporozoasida; subclass Coccidiasina; order Eucoccidiorida; suborder Eimeriorina; family Sarcocystidae; subfamily Sarcocystinae; genus Sarcocystis. This generic name is derived from the cyst stage (Figure 20.4) in muscle (‘sarco’ = muscle, from the Greek word for flesh). Sarcocystis spp. have the organelles that are characteristic of the phylum Apicomplexa such as apical rings (also called conoidal or preconoidal rings), polar rings, conoid, pellicle, subpellicular microtubules, micropores and micronemes.

Typically, multiple species of Sarcocystis are found in domestic animals (Table 20.1). A two-host life cycle is involved (Figure 20.5). The intermediate hosts — water buffalo (Bubalus bubalis), cattle, goats, pigs, sheep, or equine hosts — harbour the asexual stages of the parasite, whereas a sexual cycle takes place in the definitive hosts, such as cats, dogs or humans (Table 20.1). The period of time required for completion of the life cycle varies considerably, depending upon the species of Sarcocystis.40 In the following description, the life cycle of S. cruzi will serve as an example.29

Dogs, coyotes, foxes and possibly wolves, jackals, and raccoons are the definitive hosts of S. cruzi, whereas bison (Bison bison) and cattle are the intermediate hosts.29 The definitive host becomes infected by ingesting muscular or neural tissue containing mature sarcocysts. Bradyzoites (synonymous with cystozoites or cystozoic merozoites88) liberated from the sarcocyst by digestion in the stomach and gut, penetrate the mucosa of the small intestine and transform into male (micro-) and female (macro-) gamonts (gametocytes). Within six hours of ingestion of infected tissue, gamonts are found within a parasitophorous vacuole (PV) in goblet cells near the tips of villi. Macrogamonts are ovoid to round, 10 to 20 μm in diameter, and contain a single nucleus. Microgamonts are ovoid to elongated and contain one or more nuclei. The microgamont’s nucleus divides and, as the microgamont matures, the daughter nuclei migrate towards the periphery of the gamont. Mature microgamonts of S. cruzi measure about 7 × 5 μm and contain 3 to 11 slender gametes. The microgametes are about 4 × 0,5 μm in size and have a compact nucleus and two flagella. Microgametes liberated from the microgamont actively move to the periphery of the macrogamont. After fertilization, a wall develops around the zygote and the oocyst is formed. The entire process of gametogony and fertilization can be completed within 24 hours and gamonts and oocysts may be found at the same time. The location of gametogony and the type of cell parasitized, vary with the species of Sarcocystis and stage of gametogenesis. Sarcocystis differs from other coccidia in that no asexual multiplication takes place in the intestine prior to sexual reproduction.

In contrast to other coccidia, Sarcocystis oocysts sporulate in the lamina propria of the gut. The inner mass of the oocyst (sporont) divides into two sporocysts. Four sporozoites are formed in each sporocyst. Sporulated oocysts are generally colourless and thin-walled (Figure 20.6). Occasionally, unsporulated or partially sporulated oocysts are shed in the faeces. The prepatent and patent periods vary (the latter is usually long), but oocysts of most Sarcocystis species first appear in faeces between 7 and 14 days after ingestion of sarcocysts. Curiously, both sporulating and fully sporulated oocysts of Sarcocystis have been seen in skeletal muscles of baboons in South Africa, which are frequently both intermediate and definitive hosts in terms of the life cycle.84

Figure 20.1a  Acute sarcocystosis in a goat: natural infection. Schizont in a vascular endothelial cell in the brain

Figure 20.1b  Natural infection: glomerulus in the kidney of a goat containing schizonts in vascular endothelial cells. (Tissue by courtesy of Dr J.A. Neser, Onderstepoort Veterinary Institute, South Africa)

The intermediate host becomes infected by ingesting sporocysts. Sporozoites excyst from sporocysts in the small intestine. The fate of the sporozoite from the time of ingestion of the sporocyst until initial development in arteries in the mesenteric lymph nodes, is not known. First-generation schizogony (merogony) begins in endothelial cells of arteries as early as seven days post-inoculation29 (DPI) and may be completed as early as 15 DPI. Second-generation schizonts (meronts) have been seen in vascular endothelium...

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