Sarcocystis spp. have two-host life cycle in which herbivores act as intermediate hosts that are infected after ingesting sporocysts or oocysts in the feces of the definitive hosts, which in turn acquire the infection by ingesting sarcocysts in the tissues of intermediate hosts (Dubey, 1976; Fayer, 1977; Dubey et al., 1982; Saito and Itagaki 1994). The parasite is not only pathogenic for calves but can cause abortion, clinical illness, and even death in experimentally infected cattle (Johnson et al., 1975; Hong et al., 1982; Frelier and Lewis, 1984). Bovine sarcocystosis is caused by S. cruzi and is known to cause considerable morbidity and mortality in cattle (Dubey, 1976; Frelier and Lewis, 1984; Carrigan, 1986). Surveys of Sarcocystis infection among slaughtered cattle in Korea indicated that the infection rate ranged from 29.1 to 43.6%, and a higher infection rate was observed from older groups of cattle (Kang et al., 1988; Jang et al., 1990; Yang et al., 1990; Park et al., 1994). Even though Sarcocystis infection has been established in experimental dogs fed with bovine cardiac muscle (Park et al., 1994), there have been no attempts to establish muscle cyst in cattle through reinfection of sarcocysts isolated from the infected dogs in Korea. The objective of this investigation was to identify domestic dogs as the definitive hosts of Sarcocystis in Korea, and to demonstrate the life cycle of Sarcocystis through re-infection of cattle with sarcocysts excreted through the feces of the definitive host.

All dogs fed with Sarcosystis-infected cardiac muscle of cattle produced oocysts or sporocysts in their feces. The presence of Sarcocystis in the feces of the two uninfected control dogs and four infected cats was not detected. The infected dogs commenced sporocysts (sometimes oocysts) in the feces from day 10 to day 12 post inoculation (p.i.) and continued during 20 to 35 days. The number of discharged sporocysts peaked on day 20 to 25 p.i. However, two dogs (No. 2 and 11) discharged sporocysts invisibly (Fig. 1). When three of the eight infected dogs were again fed with Sarcosystis-infected cardiac muscle 50 days after the first infection, they began to discharge sporocysts in the feces on day 12 to 13, and continued during 20 days with respect to the second feeding of infected cardiac muscle. The number of sporocysts discharged peaked on day 18 to 24 p.i. (Fig. 2).

During the experiment, all of the calves remained clinically healthy and retained normal appetites. The rectal temperature of calf 1 became elevated on day 30 p.i. and persisted until euthanatized, but the rectal temperature of calf 2 was similar to that of negative control (Fig. 3). Packed cell volume (PCV) levels of calf 1 and calf 2 began to decrease on 30 day p.i. (Fig. 4). Muscular cysts were found from both calves which were euthanatized on day 40 p.i. Proliferative forms of sarcocysts were detected in 9 of 31 muscle samples collected of which high concentrations were found in the heart muscle, esophagus, and muscle of thoracic limb (Table 1).

S. cruzi has been identified as the most common sarcocysts around the world, and the cattle is known to serve as the intermediate host (Fayer, 1977; Carrigan, 1986; Odening et al., 1995). In Korea, the identification of S. cruzi was reported by Park et al. (1994), who demonstrated that dogs were successfully infected after feeding of cardiac muscle of slaughtered cattle originated from Kangwon-do. The results of this study agreed with previous studies that sporocysts were observed only in dogs experimentally fed with cardiac muscle from infected cattle with sarcocysts and sporocysts appeared in the feces on day 12 to 13 p.i. with a peaked production of sporocysts during day 20 to 25 p.i. We further demonstrated the encystations of sarcocysts in the muscle of cattle by infecting cattle with sporocysts collected from the feces of infected dogs. Advanced initiation of sporocyst production on day 1 to 2 p.i. with a peak production of sporocysts on 18 to 24 days after reinfection in the three reinfected dogs indicate the fact that the immunity against S. cruzi was not fully established in the infected dogs.

The previous reports indicated that the infection rate of Sarcocystis among Korean cattle was as high as 29.1-43.6% (Kang et al., 1988; Jang et al., 1990; Yang et al., 1990), which suggested the existence of the associated life cycle of S. cruzi between domestic cattle and dogs. However, there has not been any reported case of naturally-acquired S. cruzi infection in domestic dogs in Korea, although several investigators have attempted to establish such findings. This study may indicate that naturally occurring infected cases are hard to detect since low number of sporocysts were detected even in the experimentally infected dogs. It is nearly impossible to detect sporocysts from the feces without a concentration method with samples collected during the peak secretion period.

Naturally and experimentally infected cattle with S. cruzi became severely ill or some even died with abortion and lesions similar to those in dalmeny disease (Fayer et al., 1976; Frelier and Lewis, 1984). Furthermore, acute anemia developed which primarily was the result of acute extravascular hemolysis. The most severe pathologic change occurred during day 26 and 33 p.i. (Johnson et al., 1975). In this report, the fluctuation of rectal temperatures and PCV levels were observed, which supported the findings described by above mentioned groups. Our results also suggest that more attention should be given to the pathogenicity of cattle in the field. In conclusion, these results confirmed that the S. cruzi found in Korean native cattle has a 2-host life cycle, with dogs as the definitive host and Korean native calves as the intermediate host.