Intestinal and Muscular Sarcocystis Infections

David S. Lindsay, Ph.D, Steve J. Upton, Ph.D., Louis M. Weiss M.D., M.P.H.

Parasitology

Life cycle

               Sarcocystis hominis and Sarcocystis suihominis infect the intestinal tract of humans. Sarcocystis species have 2-hosts in their life cycles. Humans are the definitive hosts for S. hominis and S. suihominis and cattle/water buffalos and pigs, respectively are intermediate hosts. Humans become infected by ingesting raw beef or water buffalo meat (S. hominis) or pork (S. suihominis) containing sarcocysts (tissue cysts) containing bradyzoites. Bradyzoites are digested out of the cysts in the stomach and duodenum and they enter the lamina propria and become sexual stages. The sexual stages consist of macrogamonts (female egg-like stages), microgamonts (male stages) that produce flagellated microgametes (sperm-like stages). The microgametes fertilize the macrogamonts and unsporulated oocysts are produced. The oocysts sporulate in the lamina propria and each contains 2-sporocysts each with 4 sporozoites. The oocyst wall is thin and often ruptures as the oocyst makes its way from the lamina propria to the intestinal lumen. As a result, individual sporocysts are often seen in the stool while true oocysts are usually not present. It takes about 10 to 14 days for the life cycle to be completed in the human intestinal tract. The sporocysts are not infective for humans. The sporocysts are infectious for the intermediate host, which are cattle or swine. In an appropriate host the sporocysts will excyst and the sporozoites will penetrate into the vascular system. They undergo 2 asexual divisions by schizogony in endothelial cells of various tissues and eventually produce merozoites that will invade striated and cardiac muscle cells. These merozoites become metrocytes (mother cells) and produce bradyzoites. Mature infective sarcocysts are present in muscle in about 2 to 3 months. Bradyzoites produce the sexual stages in the human intestinal tract.

               Humans are also accidental intermediate hosts for several species of Sarcocystis. These have been lumped under the name S. lindemanni but many structurally different species are involved and this name is no longer valid. It is assumed that humans acquire infections by ingesting sporocysts in contaminated water or on contaminated food. Asexual multiplication by schizogony occurs in unknown locations and eventually sarcocysts are produced in striated muscle.

Epidemiology

               Sarcocystis hominis and S. suihominis are present wherever human stool comes in contact with cattle or swine. Many reports of intestinal sarcocystosis are from Europe and in Asia (8). These parasites are not established in the United States or Canada. Little is known about the prevalence of intestinal sarcocystosis in Latin America. Sarcocystis hominis has been reported from Brazil (23). Fifty samples of raw kibbe from 25 Arabian restaurants in São Paulo, Brazil, were examined for the presence of bovine Sarcocystis species based on sarcocyst wall structure, S. hominis was present in 94% of the samples (23). Cultural habits of ingesting rare or undercooked meat help maintain human intestinal sarcocystosis where it is endemic. In Belgium, 97% of all macroscopic cysts in cattle were shown to be S. hominis using a PCR test (28). Most cases of human muscular Sarcocystis infection have been reported from the Far East (1) particularly Malaysia.

Clinical Manifestations

               Clinical Sarcocystis infections in humans can manifest primarily as intestinal disease if infected meat is ingested or muscular disease in sporocysts are ingested (8). Clinical cases in patients as young as 3 years have been reported (4). Intestinal disease can occur soon after consumption of infected meat (3 to 6 hr) and is characterized by nausea, abdominal pain, and diarrhea (25). Intestinal disease can be more severe in individuals that have additional enteropathogens present in the gut. Intestinal Sarcocystis infection combined with invasion by gram-positive bacteria has been associated with over 20 cases of segmental enterocolitis in Thailand (3,4). Experimental studies with human volunteers have produced more severe disease in those that have ingested pork containing S. suihominis than those that have ingested beef containing S. hominis; 8 however (23) reported diarrhea in 2 of 6 volunteers that ingested sarcocysts of S. hominis and Lian et al., (15) reported that a human volunteer developed clinical signs of intestinal sarcocystosis 3 days after ingesting 60 g of S. hominis infected beef. Some individuals can be infected and have no symptoms.

               Many of the over 50 reported cases of muscular Sarcocystis infection have been incidental findings or associated with eosinophilic myostitis. Clinical signs associated with an outbreak of human muscular Sarcocystis infection occurred in 7 of 15 members of a US combat troop in Malaysia (1). The signs developed about 3 weeks after the troops returned from the jungle and were fever, myalgias, bronchospasm, fleeting puritic rashes, transient lymphadenopathy, and subcutaneous nodules. Eosinophilia, elevated erythrocyte sedimentation rate, and elevated levels of muscle creatine kinase were present in these troops (1). The prevalence of muscular Sarcocystis species in tongues from 100 consecutive autopsies in Malaysia was 21% (31). The ages of subjects was 16 to 57 years (mean = 37.7 years) and no differences were observed due to race, sex or occupation (31).

               Seven different types of Sarcocystis were observed in a study of 35 cases and the structure of Sarcocystis observed in human muscle often resemble those found in primates (2).

LABORATORY DIAGNOSIS

               Examination of stool specimens will permit the detection of Sarcocystis infection. Universal precautions should be followed when handling fresh stool samples. Ten or 5% formalin solution are appropriate fixatives for stools suspected of containing intestinal coccidia including Sarcocystis. Formalin fixation does not interfere with some of the immunodetection methods currently employed to detect Cryptosporidium spp. and Giardia spp., which is a draw back of PVA fixative. Oocyst and sporocyst structure will last several months when stools are stored at 4C in formalin fixatives. A single negative stool specimen is not conclusive in the examination of stools for coccidial parasites, a total of 3 or more stool specimens collected on subsequent days need to be examined before Sarcocystis infection can be ruled out. Liquid stool samples can be concentrated by centrifugation and the pellet used for examination using wet mounts, concentration techniques, or in stained smears. Large numbers of sporocysts may make diagnosis less challenging but do not always translate directly to severity of clinical signs. Some individuals may excrete sporocysts and be asymptomatic.

               The sporocysts of human infective Sarcocystis species are elongate, and contain 4 sporozoites. The sporocysts of S. hominis average 15 x 9 µm and those of S. suihominis average 14 x 11 µm. If intact oocysts are present in the stool sample then it is easy to confuse them with those of Isospora belli. Oocysts of I. belli may have progressed to the 2-celled sporoblasts stage in fresh stools but they will not be fully sporulated as will be the oocysts of Sarcocystis species. Additionally the oocysts of I. belli are elongate and will average 32 x 14 µm in size. Cyclospora spp. and Cryptosporidium spp. oocysts are smaller and spherical in shape.

               Little has been published on the detection of Sarcocystis sporocysts in stool samples. Autofluorescence of Sarcocystis sporocysts has been reported (16) and this should be useful in identifying these parasites in fecal smears. As few as 5 sporocysts of S. neurona could be detected in mouse feces using a PCR test based on the ssurRNA (7). Several PCR methods have been developed to detect sarcocysts in the muscles of intermediate hosts (10, 28, 32). Sequence analysis of PCR products from thick-walled sarcocysts collected from minced beef in Belgium revealed that S. hominis was present in 97.4% of the samples. Sarcocystis suihominis has been reported from pigs from Spain based on a PCR test (10). It is likely that these PCR tests can be used to detect the species of human Sarcocystis sporocysts in feces.

               Muscle biopsy is presently used to diagnosis cases of myositis due to Sarcocystis spp. in humans. It is likely that PCR methods can be developed to identify the species of Sarcocystis present in human muscle. Molecular methods should help us better understand the taxonomy of the species of Sarcocystis infecting human muscle.

SUSCEPTIBILITY IN VITRO AND IN VIVO

               Bradyzoites of S. suihominis will undergo sexual development and produce oocysts in cell culture (20). No attempts at in vitro chemotherapy have been reported.

               Nonhuman primates are susceptible to meat induced intestinal infections with S. hominis and S. suihominis (9, 13,15, 24). No chemotherapy studies have been done using non-human primates. There are no in vitro or animal models for muscular Sarcocystis spp. infections in humans, however, fatal disseminated Sarcocystis infection was seen in a captive-born juvenile female rhesus macaque (Macaca mulatta) 8 days after it was brought to a new location (14). Schizonts were observed in endothelial cells in vessels of various tissues and sarcocysts were observed in skeletal muscle. The sarcocysts were examined by PCR of the 18s ribosomal RNA gene sequence and found to be 96% similar to S. hominis but disease was believed to be caused by an unidentified Sarcocystis species because their was not complete agreement in sequences (14). A 2-year-old, captive-born, clinically healthy male, rhesus macaque, was examined at necropsy as part of an unrelated study and found to have muscular Sarcocystis spp. infection (11).

ANTIPARASITIC THERAPY

Drug of Choice

               There is little known about treatment or prophylaxis for intestinal infection, myositis, vasculitis or related lesions in humans due to sarcocystosis. Drugs used to treat I. belli infections (19) may have efficacy against intestinal Sarcocystis species. Amprolium (Amprol, Corid) is an anticoccidial agent used in the poultry production industry and is believed to act on the asexual stages, sexual stages and on sporulating oocysts (17). The activity against sexual stages and sporulating oocysts is important because these are the only developmental stages that occur in the intestine in intestinal Sarcocystis infection. Amprolium is structurally related to the vitamin thiamine and it competitively inhibits the active transport of thiamine in coccidial parasites. Amprolium has been used in an AIDS patient suffering from severe diarrhea caused by I. belli infection (30). Amprolium was given orally beginning at 10 mg/kg and increased to 90 mg/kg and the frequency of diarrhea lessened after 6 days of treatment (30). Amprolium treatment was stopped on day 7 because of polyneuropathy but re-initiated on day 20 at a reduced dose of 30 mg/kg. The stool became normal by day 28 of treatment and no oocysts were present after day 35 (30). Sulfonamides were the first effective anticoccidials used in poultry production and they have some activity against sexual stages of intestinal coccidia (17). Sulfonamides are often used with dihydrofolate reductase/thymidylate synthase (DHFR/TS) inhibitors. Unlike other eukaryotic cells, the coccidial DHFR/TS enzyme has both dihydrofolate reductase and thymidylate synthase inhibitory activity (24). Pyrimethamine and trimethoprim are examples of DHFR/TS inhibitors that are often combined with sulfonamides because of the synergistic activity in the inhibition of folate biosynthesis when used in combination (17). Combination therapy with oral trimethoprim (160 mg)-sulfamethoxazole (800 mg) 4 times a day for 10 days resulted in a decrease in diarrhea and abdominal pain due to I. belli infection within 1 to 6 days (mean = 2.5 days) after treatment (21). Combination therapy with oral trimethoprim (320 mg)-sulfamethoxazole (1,600 mg) 2 times a day for 10 to 14 days is as effective and may be an easier course of therapy for some patients. Triazine anticoccidials used in the poultry production industry have activity against sexual stages of coccidia (17) indicating that they may have activity against intestinal Sarcocystis infection. Diclazuril (Clinicox) is a triazin anticoccidial and it has been used at 200-300 mg/kg for 7 days to treat Isospora belli infections in AIDS patients (19).

               A case of eosinophilic myositis was successfully treated with trimethoprim-sulfamethoxazole (27). Given the efficacy of trimethoprim-sulfamethoxazole against other coccidian infections and its activity in vitro against asexual stages of S. neurona (18), it is reasonable to assume that this drug may have some efficacy against asexual stages that precede tissue cyst formation by Sarcocystis. Bradyzoites in sarcocysts are dormant stages that are resistant to treatment. There is a case report of albendazole having efficacy in a patient that was in an outbreak of eosinophilic myositis due to Sarcocystis sp. (1). It is likely that steroids have a role in decreasing the inflammatory response in cases of myositis and vasculitis due to Sarcocystis spp. infection, but this has never been evaluated in a controlled trial.

Special Situations

Unusual sites:  The asexual stages of a Sarcocystis sp. were reported from the liver of a patient with AIDS in Argentina (29). Sexual stages were observed in the duodenum of the patient. Figure 1 in their paper is an oocyst of I. belli and not Sarcocystis sp. as reported in the paper. It is likely that this patient had biliary and intestinal I. belli infection.

Immunosuppressed Hosts: Muscular Sarcocystis infection was reported in the heart of a patient with Hodgkin's disease (12). Eight of 11 cases of muscular sarcocystosis in Malaysia were associated with malignancies, especially of the tongue and nasopharynx (22). Disseminated muscular sarcocystosis was seen in a cat with lymphosarcoma indicating that immunosuppression might make a patient more susceptible to muscular sarcocystosis (5).

Pregnancy: There are no published reports of abortion due to Sarcocystis spp. infection in humans. Abortions due to Sarcocystis spp. have been documented in several species of domestic and wild animals but these animals are the intermediate hosts and abortions are caused by asexual stages (6). Abortions associated with intestinal Sarcocystis spp. infection (sexual stages) have not been reported in animals.

ADJUNCTIVE THERAPY

               Dehydration can occur in patients with intestinal Sarcocystis infections and intravenous or oral rehydration is appropriate in these patients. Folinic acid 5 to 10 mg per day can be used to prevent adverse effects of trimethoprim-sulfonamide or pyrimethamine administration. Antimotility drugs can be used to control diarrhea.  Myositis caused by muscular Sarcocystis infection may benefit from steroid therapy.

ENDPOINTS FOR MONITORING THERAPY

               Examination of stool samples will indicate when sporocysts are no longer present. We suggest repeated fecal samples be examined, as is done for Giardia duodenalis and Entamoeba histolytica, be negative before a patient is considered clear of the infection.

VACCINES

               There are no vaccines for Sarcocystis infection in humans.

INFECTION CONTROL MEASURES

Overview

               Ingesting only meat that has been thoroughly cooked or meat that has been frozen can prevent intestinal Sarcocystis spp. infections. Washing of hands, cutting boards, and utensils that come in contact with meat with warm soapy water will kill any bradyzoites that might be present. Cases of muscular Sarcocystis spp. infection are acquired by the ingestion of sporocysts from the environment. Contaminated water was the cause of 1 outbreak of muscular sarcocystosis (1). Sporocysts of Sarcocystis are resistant to most disinfectants that can be used in water. Boiling water will kill the sporocysts.

Hospital Infection Control Measures

               Human to human Sarcocystis spp. infection has not been reported. Normal procedures are all that is needed to assure hospital staff do not come in contact with feces containing sporocysts.

Prophylaxis

               No prophylaxis is needed for intestinal infections. Cases of human muscular Sarcocystis spp. infection are rare. Individuals that travel to Malaysia, where most cases of muscular Sarcocystis spp. infection have been reported, may consider taking trimethoprim-sulfamathoxazole to prevent muscular sarcocystosis.

COMMENTS

               The life cycle of S. hominis and S. suihominis does not occur in the United States. Intestinal infection with these parasites is acquired by the ingestion of raw or rare beef or pork. Patients with intestinal infection due to Sarcocystis species should have a history of travel to endemic areas. Muscular sarcocystosis should be considered in patients with a history of travel to Malaysia or the Far East and muscular pain.

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