Figure 1

Figure 1. Oocysts of Sarcocystisspp. from patients with human intestinal sarcocystosis, France, 2021–2024. A) Concentrated stool smear stained using the merthiolate-iodine-formaldehyde method. Sporulated oocysts are colorless and contain 2 elongated...

The patients included in this study underwent testing for intestinal parasites during October 2021–July 2024 because of gastrointestinal disorders or for systematic screening. Testing was performed in medical analysis laboratories by microscopic examination of fresh homogenized stool samples highlighting Sarcocystis spp. oocysts, sporocysts, or both (Figure 1). Oocysts/sporocysts were observed in 19 patients (Table), 8 women and 11 men, ranging in age from 19 to 94 years, all living in France. Of the 19 patients, 17 had reported acute, chronic, or occasional diarrhea lasting up to several months; the remaining 2 patients (case identification nos. S01-05 and S01-14) had infection diagnosed during systematic screening. No apparent cause other than Sarcocystis infection has been reported to explain the gastrointestinal disorders, except in 2 patients, 1 with concomitant Salmonella infection (case identification no. S01-03) and 1 with concomitant Taenia saginata infection (case identification no. S01-19). Some other symptoms were occasionally observed, such as abdominal pain, constipation, weight loss, nausea, ileitis, eosinophilia, or blood in stool (Table).

Figure 2

Figure 2. Phylogenetic tree for human Sarcocystisspp. from human intestinal sarcocystosis, France, 2021–2024. Tree is based on 54 partial mitochondrial cytochrome c oxidase subunit I gene sequences from patients compared...

A total of 23 stool samples from the 19 patients were prospectively sent to the French National Reference Center for Cryptosporidiosis, Microsporidia and Other Digestive Protozoa for further molecular analysis (Appendix). In brief, high-throughput sequencing (HTS) was performed on a 332-bp region of the mitochondrial cytochrome c oxidase subunit I gene. We constructed a phylogenetic tree on the basis of the partial gene sequence from the 41 characterized Sarcocystis spp. isolates and reference sequences from GenBank by using the neighbor-joining method (Figure 2). The different Sarcocystis spp. contigs clustered with the reference sequences with a maximum variation of 2 nt over the 332-bp sequence analyzed (Table). Most (11/19) patients were co-infected with multiple Sarcocystis species (S. hominis was most frequently detected); S. sigmoideus infection was detected in 9 patients and S. heydorni infection in 5 patients.

Among the Sarcocystis parasite species present in beef meat (i.e., cattle as intermediate host), S. hominis was the first species reported to infect humans as the definitive host (8). Then, S. heydorni was indirectly considered to infect humans after it was observed in calves fed with sporocysts from the feces of a human volunteer (7). Our results expand on that previous report of human S. heydorni infection by identifying 5 more human cases. Recently, S. sigmoideus was described as a novel species in bovine carcasses. Felids were hypothesized to be the definitive hosts for this species, whereas identical samples harboring both S. sigmoideus and S. hominis suggested a potential zoonotic role (9). Here, we confirm at least the second hypothesis by reporting that humans are a definitive host of S. sigmoideus.

During the study period, we performed molecular analyses on microscopically positive fecal samples (i.e., detection of oocysts/sporocysts) from 19 patients and detected S. sigmoideus parasites in 9 of them. An association with S. hominis parasites was detected in 6 patients and with S. hominis plus S. heydorni parasites in 3 patients.

A limitation of our study is that we could not microscopically distinguish between sporocysts of different species because most patients were co-infected. We also had to consider that, after ingestion of infected meat, some transient Sarcocystis DNA resulted in HTS reads that were not associated with the sporocysts seen in the fecal samples. However, we excluded that possibility because repeated stools spaced over 3 to 24 days for 3 co-infected patients showed the same species in the same proportions (data not shown). Also, in the 19 cases analyzed, we never detected reads from S. cruzi, which is highly prevalent in beef meat but does not infect humans. To definitively confirm that hypothesis, future attempts should be made to perform single-cell sequencing on sporocysts/oocysts isolated from microscopy or by species-specific labeling with species-specific hybridization probes.

The prevalence of S. sigmoideus parasites in cattle has been reported to be low, but it is likely to be underestimated, as suggested by the number of infected patients (9 of 19) in our study (9). Further molecular studies in cattle and human stool are needed to better estimate the real prevalence of S. sigmoideus parasites.

We used HTS for molecular analysis of Sarcocystis spp. parasites in human stools and found that human intestinal sarcocystosis is mainly caused by multiple species simultaneously (11 of 19 patients were co-infected). That finding is in accordance with recent veterinary data that detected Sarcocystis spp. parasites in 64% of randomly sampled cattle carcasses and mixed infections of up to 3 species simultaneously (including S. sigmoideus and S. hominis) in 25% of intralesional samples and in 5.8% of extralesional samples from carcasses condemned because of the presence of bovine eosinophilic myositis (9,10).

S. hominis parasites are considered mildly pathogenic in humans, whereas S. suihominis infection is more virulent (1). However, data about Sarcocystis pathogenicity are scarce, outdated, and mostly derived from volunteers who ingested experimentally infected meat (1). The pathogenicity of S. heydorni and S. sigmoideus parasites is unknown, and further studies are required to address that issue. In conclusion, our data demonstrate that humans are a definitive host for S. sigmoideus parasites and that intestinal sarcocystosis frequently results from infection with multiple species.