| Abstract|| |
Sarcocystis is an apicomplexan protozoan belonging to same phylum as toxoplasma. The parasite encysts inside striated muscles of its intermediate host. Humans are accidental host infected by eating food or water contaminated with oocysts or sporocysts of an infected definitive host. The infection is increasing in Southeast Asia and may be overlooked in histological sections if one is not aware of the histomorphological features. The size and shape of the bradyzoites and the appearance of the cyst wall are the reliable features to distinguish this parasite from other parasites of the same phylum. The incidence of human infection is rising in Southeast Asia and histopathology is an important method for the diagnosis of muscular infection. It is important to recognize the histomorphology of this parasite and its differentiation from similar parasites.
Keywords: Anatomy and histology, humans, muscle, skeletal, sarcocystis
|How to cite this article:|
Makhija M. Histological identification of muscular sarcocystis: A report of two cases. Indian J Pathol Microbiol 2012;55:552-4
| Introduction|| |
Sarcocystis gets its name from 'sarcomere' as it was first reported as a thread like cyst in striated muscles of a house mouse and was initially named Miescher's tubules after its discoverer. , It has now been recognized as a protozoan of the phylum apicomplexa as the protozoa posses an apical complex structure involved in penetrating the host cells. Other apicomplexan protozoans include toxoplasma, Babesia More Details, plasmodium, Cryptosporidium parvum, Isospora belli., The parasite is present widely in livestock and occurs in a large number of mammals and birds.  There are a large number of morphologically similar species, which occur in range of intermediate and definitive hosts.  The incidence of human infection is rising in Southeast Asia and histopathology is important method for the diagnosis of muscular infection in a intermediate host. , Hence, the importance of recognizing the histomorphology of the parasite and its differentiation from similar parasites. The paucity of reports from India may be due to lack of recognition of the parasite on histological sections.
| Case Reports|| |
A 20-year-old boy from Patna (Bihar, India) presented with a lump in the left arm for about 2-3 months. The lump was, firm, mobile, slightly tender but not painful. The ill-defined lump measured up to 4 cm in greatest dimension. The clinical impression was that of an old rupture and healed muscle or tendon. No radiological study was performed and a muscle biopsy was done.
A 50-year-old man from Varanasi (Uttar Pradesh, India) underwent right hemimandibulectomy and modified neck dissection (type II) for ulcerated squamous cell carcinoma in the lower right alveolar sulcus. The patient had multiple small lymph nodes in the neck largest measuring 1 cm in diameter, however, never complained for pain or stiffness in the neck. He was asymptomatic prior to the oral complaints.
The parasite was not identified on gross examination, which showed unremarkable pieces of skeletal muscle.
Case 1 showed 4-5 encysted worms in one plane of section from the muscle and these were the only significant abnormality identified.
In case 2, the parasite was seen on microscopy as an additional incidental finding in one of the random sections from the middle one-third of sternocleidomastoid muscle. Sections from the skeletal muscle of both the cases show oval cysts measuring 1-2 mm in length. The cysts were lined by an outer wall, which on higher magnification showed characteristic striations and hair like projections or radiating processes known as cytophaneres [Figure 1]. The cyst wall, however, was thin and smooth in case 2 [Figure 2]. These striations could be appreciated in hematoxylin and eosin stained sections and did not require special histochemical stains.
|Figure 1: A cyst of Sarcocyst. This longitudinal section demonstrates the wall showing radial processes on the outer surfaces (cytophaneres). The internal mass is separated into many compartments. (Hematoxylin and Eosin stain, ×400)|
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|Figure 2: A Sarcocystis in cross section. The cyst is filled with bradyzoites inside the thin cyst wall. The bradyzoites showed cresentric shapes and appeared like a "swarm of worms". (Hematoxylin and Eosin stain; ×400)|
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The cyst typically showed internal septations in case 2. The septations were not clearly identified, however, the formation of groups by the bradyzoites suggested thin irregular septations not identified at this magnification [Figure 3] and [Figure 4].
|Figure 3: Encysted parasite in skeletal muscle showing internal bradyzoite nuclei separated by clear internal septations. (Hematoxylin and Eosin stain; ×100)|
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|Figure 4: Encysted oval shaped parasite occupying full thickness of the muscle fiber and showing internal septations. (Hematoxylin and Eosin, ×40)|
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Cysts from both the cases showed crowded hematoxylin stained organisms like a "swarm of worms" [Figure 2]. Although these bradyzoites were identifiable on high magnification, examination under oil immersion lens showed their 'banana shape', somewhat resembling gametes of plasmodium falciparum with which the organisms shares taxonomical phylum. Metrocysts, which appear larger than bradyzoites, were not seen in both the cases.  Typically both the cases did not show any myocyte necrosis or significant inflammation.
| Discussion|| |
The exact incidence of human infestation is not known but the prevalence was found to be 21% in an autopsy study from Southeast Asia. , Although these infections have been considered to be incidental findings, some investigators have suggested that Sarcocystosis may be emerging as a significant food-borne zoonosis in Southeast Asia. 
Rarity of detection in histological specimens, despite such high prevalence in Southeast Asia is probably due to the lack of clinical disorder (like in case 2) and consequently many infections go undetected. A similar association of squamous cell carcinoma in the head and neck region was observed by Larbcharoensub et al.  in a recent report.
Histopathological differentials include the commoner toxoplasma and newer recognized neospora species. Toxoplasma is also an apicomplexan encysted parasite found as an end stage accidental infestation of humans from feline definitive host. The parasite is smaller (<1 mm) than those of sarcocystis, which can even be larger enough to be noticed with the naked eye as a thread like organism.  The cyst wall of toxoplasma is thin and does not show the striations as seen in one of our cases. Although, this is a useful feature when present (like in case 1), variability in staining amongst various species is expected. Histochemical (periodic acid Schiff and phosphotungstic acid hematoxylin) stains can be helpful in cases where hematoxylin and eosin stained section do not reliable conclude the presence or absence of these striations.
The appearance of cyst wall, size, and appearance of the bradyzoites were found to be the most useful in differentiating the two organisms.  The bradyzoites of toxoplasma appear as dot like structures on light microscopy at ×400 magnification, while the bradyzoites of Sarcocystis are larger and appear as cresenteric 'banana' shaped structures on ×400 magnification like in our cases. , The cyst wall is also thicker in cases of sarcocystis and some species show striations (cytophaneres) while that of toxoplasma is thin and the details are not discernible on light microscopy. Neospora is a more recently described parasite and has been recognized in neural tissue. The encysted stage is rather indistinguishable from toxoplasma. 
Humans carrying muscular infections of Sarcocystis may be asymptomatic as in the case 2 where the parasite was detected as an incidental finding in a mandibulectomy done for squamous cell carcinoma of the lower alveolus.  However, patients may present with subcutaneous swellings like in case 1. Other clinical symptoms include musculoskeletal pain, fever, rash, cardiomyopathy and bronchospasm. 
Histological sections from both our cases did not show any inflammatory response as is often been observed in literature.  This is due to the encysted nature of the parasite.
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