Omsk hemorrhagic fever (OHF) is an infectious, viral disease and is endemic to parts of Russia, where it was first documented in Omsk. The disorder is characterized by bleeding from multiple orifices and organs. It is primarily spread by tick bites or contact with infected muskrats.
Omsk hemorrhagic fever (OHF) is endemic to western Siberia, where the Omsk hemorrhagic fever virus (OHFV) is primarily spread through contact with infected muskrats, ticks and contaminated water. There is no male or female predominance, and no human to human transmission.
OHF is mainly characterized by the presence of a fever, as well as bleeding diathesis. From the time of infection, symptoms may take between 3 days and 1 week to appear, occasionally preceded by malaise.
Initially, patients experience high-grade fever for up to 2 weeks, often accompanied by chills. They may also experience a headache, cough, myalgia and gastrointestinal disturbance. Bleeding can occur from multiple sites, commonly the nose and mouth. Other organs such as the uterus, gastrointestinal tract, urinary tract, lungs and skin may also bleed, resulting in complaints that include hemoptysis, hematuria, petechiae and easy bruising, with an accompanying maculopapular rash on the torso and extremities in rare cases  .
There is widespread hyperemia often seen on the upper body, dry mucous membranes, halitosis, dehydration, as well as oropharyngeal and facial edema . Many individuals become hypotensive and bradycardic. Hepatomegaly may also be diagnosed. Leukopenia and thrombocytopenia are usually present. The clinical condition worsens during the course of a few days, with additional symptoms such as gingivitis, gingival bleeding, myalgia, lymphadenopathy and hyperesthesia.
OHF is divided into two phases. The second phase of infection is only experienced by some individuals, commencing 3 weeks after the onset of symptoms. It involves the central nervous system, causing encephalitis, meningism, brain edema, and disruption of the blood-brain barrier.
Despite the plethora of symptoms, the case fatality rate is low; literature states that it is between 0.4-10%, and full recovery is the norm. Patients may have lingering general body weakness. In a minority of cases, there is permanent hearing impairment, hair loss, and neurological sequelae such as memory impairment, psychiatric disorders, and behavioral changes. The most common causes of death are internal bleeding or severe sepsis, as OHF causes increased susceptibility to secondary infections .
The classic presentation, as described above, is seen in only one in five patients . There are reported cases where the only sign of the disease is abnormal bleeding  .
Diagnosis of Omsk hemorrhagic fever is clinical, coupled with the knowledge of the endemic locations of the infection. Laboratory tests include serology such as enzyme-linked immunosorbent assay (ELISA), neutralization tests (NTs), complement fixation (CF) tests and hemagglutination inhibition (HI) . NT has the highest specificity, while CF has low sensitivity and should be used in conjunction with other tests. It is often not possible to directly detect the virus through the polymerase chain reaction as the body clears the virus before the onset of symptoms. OHFV is part of the Flaviviridae family, thus other flaviviruses are able to cross-react and thus produce false positive results for OHF. In particular, OHFV is extremely similar to Tick-borne encephalitis virus (TBEV), both in presentation and biochemical findings. It may be difficult to detect OHFV antibodies in the elderly, as they have a less pronounced response to the virus.