Omsk hemorrhagic fever is a naturally focal transmissible viral infection characterized by hemorrhagic reactions, predominant damage to the respiratory system and the central nervous system. Disease occurs with high fever, intense headache, myalgia, increased bleeding, frequent development of bronchitis, pneumonia, cerebral and meningeal phenomena. Diagnosis is based on clinical and epidemiological history, laboratory and instrumental data. Treatment includes detoxification, hemostatic therapy, hemotransfusion.
A98.1 Omsk hemorrhagic fever
Omsk hemorrhagic fever (spring-summer fever) is an endemic zoonosis of viral nature, occurring with fever, moderate hemorrhagic phenomena, predominant damage to the central nervous system, lungs and kidneys. Natural foci are found in the steppe and forest-steppe regions of Western Siberia (Omsk, Tyumen, Novosibirsk, Orenburg, Kurgan regions) and in the north of Kazakhstan. The rise in the incidence is recorded in the spring-summer period (May-June and to a lesser extent – in August-September). Disease is more often detected among people engaged in field work, hunters and fishermen, loggers, geologists. High susceptibility is noted in all age groups, but mainly in people from 20 to 40 years old. Currently, cases among the population of endemic areas are quite rare.
Omsk hemorrhagic fever is caused by an RNA-containing arbovirus of the Togaviridae family, the genus Flavivirus, similar in antigenic structure to the tick-borne encephalitis virus. The virus is highly sensitive to conventional disinfectant solutions and fat solvents, quickly dies at room temperature and instantly – when boiled, but for a long time (up to 4 years) it is preserved in a lyophilized state. The virus belongs to the 4th class of biological safety with high individual and public risk.
The main source of the virus in natural foci are the water rat, muskrat and small rodents (voles, shrews). Hares, ground squirrels, hedgehogs, frogs, and waterfowl that are susceptible to the pathogen also participate in the circulation of the virus. In rodents, the disease more often occurs in an erased form with prolonged asymptomatic viral transmission and excretion of the pathogen into the environment with urine and feces. The vector of arbovirus is the ixodic mites Dermacentor pictus and D. marginatus, parasitic on mammals. Transovarial and transphase (trans-stage) transmission of the Omsk hemorrhagic fever virus among ticks has been established.
Infection of a person occurs mainly by transmissive means – through the bites of nymphs and adult mites-carriers; often by air-dust when inhaling dust from the dry secretions of sick rodents; contact – when cutting and processing carcasses and skins of muskrats. Alimentary infection is possible when using raw lake water and food products contaminated with the secretions of sick animals.
The virus penetrates through damaged skin and mucous membranes of the respiratory and digestive tracts, spreads in the body by hematogenic dissemination. It is characterized by damage to the endothelium of small blood vessels, leading to the development of hemorrhagic reactions and abrupt changes in the brain and spinal cord, lungs, kidneys, hematopoietic system, stomach and intestines. After suffering from Omsk hemorrhagic fever, long-term immunity develops.
The incubation period of Omsk hemorrhagic fever lasts on average 3-7 days. According to the nature of clinical manifestations, acute non-recurrent and acute recurrent, typical (hemorrhagic) and atypical (without hemorrhagic syndrome) forms of Omsk hemorrhagic fever are distinguished, having a mild, moderate or severe course.
The initial period of Omsk hemorrhagic fever is acute with a high temperature (39-40C), chills, intense headache, myalgia, dyspeptic syndrome. Patients develop swelling and severe hyperemia of the face, neck, mucous membranes of the oropharynx, injection of vessels of the sclera and conjunctiva. From the first days of the disease, a hemorrhagic rash appears on the trunk and extremities, the mucous membrane of the throat and tongue. With Omsk hemorrhagic fever, nasal, pulmonary, gastrointestinal and uterine bleeding is relatively rare; they are usually not abundant and not long-lasting.
At the height of the disease (on day 3-5), 30% of patients with Omsk hemorrhagic fever develop catarrhal inflammation of the respiratory tract, cough, bronchitis, small-focal pneumonia. Typically, the defeat of the nervous system with the development of cerebral and meningeal phenomena, paresis. Hepatomegaly, bradycardia and arterial hypotension are detected. There may be signs of kidney damage (proteinuria, hematuria). The febrile syndrome in Omsk hemorrhagic fever in half of the cases has a two-wave character with a repeat at 2-3 weeks of the disease.
The total duration of Omsk hemorrhagic fever ranges from 15 to 40 days. In the post-infectious period, prolonged asthenia usually persists. Complications of Omsk hemorrhagic fever can be late atypical pneumonia, meningoencephalitis, otitis, pyelitis, neurological and mental disorders. In severe cases, a fatal outcome from infectious and toxic shock, blood loss or sepsis is possible.
Diagnosis and treatment
When verifying the diagnosis of Omsk hemorrhagic fever, the characteristic clinical data and epidemiological prerequisites, the results of laboratory diagnostics are taken into account: non-specific (general blood and urine analysis) and specific (ELISA, IFT, RPH, NR, HI, PCR). In the blood of patients, pronounced leukopenia, erythrocytosis, thrombocytopenia, decreased ESR is determined; in the urine – proteinuria, microhematuria, cylindruria, the presence of renal epithelium. The PCR method allows detecting the Omsk hemorrhagic fever virus in the blood during the first week of the disease. The presence of specific antibodies to arbovirus or a 4-fold increase in the titer of antibodies in the paired sera of the patient confirms the diagnosis of Omsk hemorrhagic fever.
In complicated cases, diffuse myocardial changes are recorded on the ECG, and signs of interstitial pneumonia are detected on chest radiography. As part of the differential diagnosis, other hemorrhagic fevers, tick-borne encephalitis, leptospirosis, meningococcemia, sepsis are excluded.
Patients with rhis disease are hospitalized with the appointment of strict bed rest during the febrile period. Symptomatic treatment of Omsk hemorrhagic fever includes detoxification therapy with an abundant drinking regime, infusions of water-salt solutions; relief of hemorrhagic syndrome by hemotransfusions, the use of blood and plasma substitutes, hemostatic and vasoconstrictors, the use of antipyretics. In severe cases of Omsk hemorrhagic fever, corticosteroids, antishock, cardiac, thrombolytic drugs, antibiotics are prescribed.
Prognosis and prevention
The prognosis is favorable: complications are quite rare; the mortality rate is 1%. Prolonged convalescence is characteristic (from 1-2 to 5-10 months). Prevention is reduced to carrying out health-improving, anti-epidemic measures in natural foci, protecting people from tick attacks, passive and active immunoprophylaxis.
The systematic suppression of the migration activity of the virus-carrying animals (water rats, small rodents) and their destruction is carried out. The organization of muskrat breeding provides for seasonal harvesting of muskrat skins only after a preliminary zoological and virological examination of the animals for the presence of this disease. In natural foci of infection, the use of protective clothing and repellents, regular examinations of clothing and body for the purpose of early detection of ticks, boiling of lake and river water are mandatory. In virological laboratories, strict compliance with the instructions for working with biologically hazardous material, vaccination of personnel with the killed formol vaccine or passive immunization with blood serum of convalescents is necessary.