six species of echinococcus have been recognized , but four of them are of public health concern ( 1 ) . the greatest prevalence of hydatidosis in human and animal hosts is found in sheep - raising countries , including north america and south america , the entire australia , new zealand , europe , central asia , china , and parts of africa ( 1 , 2 , 46 ) . various parts of body may be involved with hydatid cyst but the liver and lungs are the main locations ( 7 ) . skeletal muscle infection is rare , and reported 0.5%  4.7% of all cases of echinococcosis ( 8) . patients with hydatid cyst are asymptomatic and present at an advanced stage of hydatidosis , when lesions have become extensive ( 7 ) . here we report a case of this rare entity of an isolated hydatid cyst of the muscle of the thigh . a 70-years old housewife living in ardoghesh , a village in neyshabur , northeast of iran , was admitted to general surgery clinic in feb 2014 because of a painless mass in the back of her left thigh . she had a history of removing hydatid cyst surgery in her left thigh from three years ago . laminated layers of hydatid cysts were observed and two round masses was successfully removed ( fig . hydatid cyst with laminated layer ( original ) gross pathology showed two soft cystic masses ( 134/54/5 cm and other 1744 cm in diameter ) that contained gelatinous material and multiple daughter cysts . in histopathologic examination cross section of a hydatid cyst with laminated layer and germinal layer , brood capsules containing multiple protoscolices , hooklets are considered as diagnostic keys ( h&e ) . pathological examination confirmed diagnosis as hydatid cyst of thigh ( fig . 2 , 3 ) . hydatid cyst in muscle ( original ) hydatid cyst in muscle showing germinal layer ( original ) hydatid disease is a very serious problem in sheep - raising countries caused by tapeworms belonging to the class cestoda , in the family taeniidae , of the genus echinococcus . they measure 3 mm to 6 mm long when mature and lives in intestine of carnivores , particularly dogs and other canines , as definitive hosts . many mammals may serve as intermediate hosts , but herbivorous species are most likely to become infected by eating eggs on contaminated herbal material . humans are seldom involved as accidental intermediate hosts in these cases and infected by accidentally ingestion of echinococcus spp . hydatidosis is a serious public health problem in some parts of the world ( 4 , 10 ) . khorasan province , located in the northeastern part of iran had the highest incidence rate for hydatidosis ( 11 ) . although the incidence of hydatidosis has decreased because of education and control programs , there are still concerns in some parts of the world ( 12 ) . hydatid cyst is most commonly found in the liver and lung , while they can occur in other organs including muscle , brain , eye , spleen , kidney , orbit , lymphatic glands , myocardium , tonsil , pancreas , skin , ovary , uterus and parotid glands ( 2 , 1317 ) . hydatidosis are usually asymptomatic until adolescence due to the slow growing process of the parasite in tissues such as muscle and bone , although it can be acquired at any age ( 18 ) . incidence rate of musculoskeletal hydatidosis is not clear . some reports showed an incidence of musculoskeletal echinococcosis including involvement of subcutaneous tissue as 0.5%4.7% among all cases of hydatid disease and soft - tissue hydatid cysts occur in 2.3% of cases reported from endemic areas ( 8 , 19 ) . muscles provide a poor environment for the parasite because of the presence of lactic acid and mechanical factors , such as contractile activity , may make encystment less likely ( 1820 ) . it may mimic any soft tissue tumor such as abscess , synovial cyst , and malignant tumor . before biopsy of cyst , diagnosis of hydatidosis should be confirmed to avoid leakage of cyst contents and the accompanying risks of anaphylaxis ( 20 ) . hydatidosis in soft tissues may present with a variety of patterns and recognizing them is necessary in diagnosis ( 18 ) . in our presented case , preoperative diagnosis was malignant neoplasm and after surgery , hydatidosis was confirmed . rokni yazdi et al . noted left thigh hydatidosis in a 50-years - old housewife living in a village in zanjan , northwestern iran ( 18 ) . noted left thigh hydatidosis in a 50-year - old woman from rural area around rasht city with no history of trauma , fever , or weight loss ( 20 ) . in conclusion , the hydatid cyst can present in any part of the body and no part is protected . the infestation may resemble a soft tissue tumor in the muscle and therefore in endemic area of hydatidosis , hydatid cyst should be considered as differential diagnosis of any soft tissue mass . in this case ,
hydatid cyst is an important endemic zoonosis in iran . 
 it may be seen in various organs of body . 
 musculoskeletal system is rarely involved by hydatid cyst , the larval form of echinococcus granulosus . on clinical basis 
 , it may resemble any soft tissue tumor . 
 a 70-years old housewife living in ardoghesh , a village in neyshabur , northeast iran , was admitted to general surgery clinic because of a painless mass in the back of her left thigh . 
 this case emphasizes that hydatidosis should be included in differential diagnosis of any soft tissue mass especially in regions where hydatidosis is endemic .