brucellosis is one of the important multi - organ zoonotic diseases with annually more than 500,000 new cases worldwide ( 1 ) . the epidemiological zone of this infection includes the arabian peninsula , mediterranean basin , indochina , some parts of central asia and south america ( 2 ) . in the endemic regions , consumption of non - pasteurized dairy such as soft cheese , butter , ice cream is the most usual transmission manner of this infection ( 3 ) . the main complaints of the infected patients are fever , chills , night sweats , myalgia , anorexia , headache , joint pain and heart attacks ( 4 , 5 ) . the main symptom of brucellosis is fever with unknown origin ; therefore , it can be misdiagnosed with similar pathologies such as all of fevers of unknown origin that may be caused by infectious diseases , malignancies , collagen vascular diseases including tuberculosis , malaria , rheumatic fever and leishmaniasis ( 6 ) . the forms of the clinical course of brucellosis in humans are acute , sub - acute and chronic ( 7 ) . to detect patients with the disease , medical history should be taken , and biochemical , hematological and serological test should be performed . isolation , and molecular tests should also be done . among serologic tests , standard agglutination test ( sat ) , the main important purpose of brucellosis treatment was to decline the involvement and signs of the disease . irrespective of the suitable treatment , some patients have relapse of the disease symptoms since these bacteria are intracellular and can survive within macrophages . these confidants may cause chronic and relapse infection ( 8) . the present study aimed to retrospectively analyze the clinical characteristics and complications in the clinical forms of human brucellosis . the current study included patients diagnosed with brucellosis at the infectious diseases clinic affiliated to babol university of medical sciences , babol , iran , within the past two decades . all demographic characteristics such as age , gender , residency , risk factors and clinical manifestations of all admitted patients with brucellosis were exploited from their files and recorded in the questionnaires . the diagnosis of brucellosis was confirmed by representing standard agglutination test ( sat )  1.320 and 2-mercaptoethanol ( 2-me )  1.80 for the patients with clinical signs and symptoms compatible with those of brucellosis . patients were divided into three groups according to their history , symptoms and clinical presentation time : acute ( 0 - 2 months ) , sub - acute ( 3 - 12 months ) and chronic ( > 1 year ) ( 9 ) . diagnosis of musculoskeletal system complications was determined by the finding of swelling , effusion and limitation of motion in an involved joint and was confirmed by x - ray in the prone position . moreover , spondylitis was diagnosed using magnetic resonance imaging ( mri ) . genitourinary system involvement was diagnosed by finding swelling and tenderness of scrotal skin , testis and epididymis , with confirmation by sonography . endocarditis was diagnosed by elevation of anemia , cardiac murmur , and was confirmed by the detection of vegetations using echocardiography . central nervous system ( cns ) involvement was made by considering the presence of sat positivity and abnormal findings obtained from cerebrospinal fluid ( csf ) analyses ( > 10 leukocyte / mm , protein > 45 mg / dl , glucose < 2:3 of the blood glucose level ) . overall , 957 patients with brucellosis were included in the study , 706 ( 73.8% ) cases were acute , 216 ( 22.6% ) sub - acute , and 35 ( 3.7% ) were chronic . among the patients 535 ( 55.9% ) were male and 422 ( 44.1% ) were female with the mean age of 34  16.9 years ( rang : 1 - 90 ) . in terms of gender distribution , involvement with acute form of brucellosis regarding other clinical forms , occurs significantly in males 418 ( 59.2% ) more than females 288 ( 40.8% ) ( p = 0.003 ) . most of the patients , 421 ( 44.1% ) , were 21 - 40 years . while 238 ( 24.9% ) case were < 20 years , 221 ( 23.1% ) were 41 - 60 years and 77 ( 7.9% ) patients were older than 60 years ; therefore , age differences between the groups and all stages of disease were significant ( p = 0.001 ) . more patients were from rural , 705 ( 73.8% ) , than the urban , 250 ( 26.2% ) , areas . significant relationships were found between clinical forms of the disease and frequency of occupational exposure or risk factors in brucellosis ( p = 0.01 ) . the most common clinical manifestations were arthralgia , sweating , fever and backache in 679 ( 71% ) , 638 ( 66.7% ) , 547 ( 57.2% ) and 376 ( 39.3% ) cases , respectively . between clinical features , the most frequent involvement was arthritis with 126 cases ( 13.2% ) , gastrointestinal complications 57 ( 6% ) , splenomegaly 49 ( 5.1% ) , sacroiliitis 46 ( 4.8% ) and spondylitis with 44 ( 4.6% ) cases . complications were observed at all stages of infection but arthritis ; spondylitis were significantly observed in patients with acute disease ( p = 0.03 , p = 0.001 ) respectively . focal organs involvement such as epididymo - orchitis and respiratory system were presented in 40 ( 4.2% ) and 22 ( 2.3% ) of the cases respectively . surprisingly , osteomyelitis , bursitis , cellulitis , cns and skin involvements were not observed in chronic form of the disease . brucellosis is a public health problem that can cause severe significant disability and complications ( 10 ) . according to the annual report of world health organization ( who ) brucellosis can occur in both genders and any age group ( 12 ) . in the current study , 535 ( 55.9% ) of the patients were male and 422 ( 44.1% ) were female with a mean age of 34  16.9 years which was similar to some other studies ( 13 ) . in some studies , the female ratio was reported more than that of the male ( 14 ) . overall , the prevalence of brucellosis can occur at any age but is common in adults and young people ( 15 ) . in the current study the most common age group was 21 - 40 years , 421 patients ( 44.1% ) , which was similar to other studies ( 16 ) . of this age group 312 ( 44.2% ) were acute , 95 ( 44.2% ) sub - acute and 14 ( 40% ) were chronic . these results show how age range reflects socio- economic and cultural status of this infection in an endemic area . some studies reported older mean age groups usually working on farms with dairy production ; therefore , brucellosis is more frequently occurs in this age group ( 17 ) . the current study results were similar to some studies presenting that brucellosis was still endemic in rural areas , but rare in urban areas ( 18 ) . in the north of iran , the rate of population involved in this infection in rural areas is more than that of the urban areas . in recent years human brucellosis cases a recent study found an overall 247 ( 25.8% ) of patients in high risk occupational groups . many previous studies overlooked some of these important at risk occupational groups ( 20 ) . the primary transmission route of brucellosis is by the ingestion of unpasteurized dairy products in the endemic countries ; whereas in the developed countries infection occurs mostly due to occupational exposure ( 21 ) . similar to other studies , the majority of patients , 439 ( 45.9% ) cases , had risk factors such as history of consuming unpasteurized milk or milk products ( 22 ) . in contrast to the current study results about residency and risk factors , some experts showed that chronic brucellosis were from rural areas because of consumption of unpasteurized dairy products or contact with animals ( 9 ) . brucellosis seems to affect human immune system and can cause acute , sub - acute and chronic clinical features . in the present study , 73.8% , 22.6% and 3.7% of the patients had the acute , sub - acute and chronic forms of the disease , respectively . but the study by eini et al . presented that 24% of the patients had acute brucellosis , 70.8% were in sub - acute stages and 5.2% had chronic brucellosis ( 23 ) . acute form in different genders was compared ( male : 59.2% vs. female : 40.8% ) . in another study by keramat et al . most patients had brucellosis with the main clinical symptoms of being arthralgia , sweating , fever and backache ; findings were similar to the results obtained by the others working in endemic regions ( 24 ) . moreover , typically cases with acute brucellosis present signs such as fever , fatigue , chills and sweating ( 25 ) . in this study , 71% of the patients had arthralgia and in terms of clinical type , 70.7% had the acute form . the results of the current study were similar to those of the studies in other countries ; and arthritis was the most common complications of brucellosis ( 22 ) . on physical examination , sweating ( 66.7% ) and fever ( 57.2% ) were mostly observed in the acute form the disease . another study identified a significant correlation between fever and the acute form of brucellosis ( 26 ) . in which the most frequent symptoms were fever ( 63.2% ) , sweating ( 62.7% ) , arthralgia ( 59.1% ) and back pain ( 58.5% ) ( 27 ) . brucellosis complications are a major medical problem in countries where the infection is still endemic . the prevalence of focal involvement has been reported to range from 20% to 40% in many studies ( 28 ) . arthritis involvement was observed in 14.7% , 9.7% and 2.9% of the patients with acute , sub - acute and chronic brucellosis , respectively . showed that sacroiliitis and polyarthritis were more frequent complications in acute cases ( 29 ) . in a recent study , spondylitis occurred in 2.8% , 10.2% and 5.7% of the patients with acute , sub - acute and chronic disease , respectively . according to bodur et al . spondylitis was observed in 12 ( 46% ) and 13 ( 50% ) patients with acute and sub - acute brucellosis ( 30 ) . , people should be taught to avoid consumption unpasteurized dairy products and contact with infected animals . therefore , due to the various clinical presentations and undiagnosed complications , diagnosis of brucellosis is difficult .