Fever of the Western Nile

Fever of the western Nile (encephalitis of the Western Nile) is an acute viral zoonotic natural focal disease with a transmissible mechanism of transmission of the pathogen. Characterized by an acute onset, expressed feverishly intoxication syndrome and CNS damage.

ICD-10 code

A92.3. Fever of the Western Nile

Epidemiology of West Nile fever

The reservoir of the West Nile fever virus in nature is a bird of the water-and-water complex, the carrier is mosquitoes, primarily the ornithophilic mosquitoes of the Sylech family. Between them, the virus circulates in nature, they determine the possible area of distribution of the fever of the western Nile - from the equatorial zone to regions with a temperate climate. Currently, the West Nile fever virus is isolated from more than 40 species of mosquitoes that enter not only into the genus Sylech, but also into the genera Aedes, Anopheles, and others. The significance of specific mosquito species in the epidemic process occurring in a certain territory has not been clarified. The work of Russian scientists has established the contamination of argas and ixodid ticks in the natural foci of the western Nile fever.

An additional role in the conservation and spread of the virus can play the synanthropic birds. The outbreak of the Western Nile fever in New York in 1999 was accompanied by a massive death of crows and the death of exotic birds in the zoo; in 2000-2005. Epizootics spread throughout the United States. Epidemics in Israel in 2000 were preceded by an epizootic in 1998-2000. Among geese on farms. About 40% of poultry in the Bucharest area in autumn 1996 had antibodies to the West Nile fever virus. Together with the "urban" ornithophilic and anthropophilic mosquitoes, domestic and urban birds can form the so-called urban, or anthropurgic focus of the western Nile fever.

Described diseases of mammals, in particular, epizootics of horses (from tens to hundreds of cases).

Due to the high incidence of fever of the western Nile in the USA in 2002-2005, noted cases of infection with Western Nile fever recipients of blood and organs.

In countries with a temperate climate, the disease has a pronounced seasonality, due to the activity of mosquito vectors. In the northern hemisphere, the incidence is observed from the end of July, reaches a maximum in late August - early September and ends with the onset of cold weather by October-November.

The susceptibility of man to the western Nile fever appears to be high, the subclinical course of the infection predominates. Postponed fever of the western Nile leaves behind a pronounced immunity. This is evidenced by the fact that in hyperendemic regions (Egypt) children of younger age groups are ill, and antibodies are detected in more than 50% of the population, while in countries from hypoendemic regions the level of population immunity is below 10% and is mainly sick in adults, in In particular, in the southern regions of Russia (Volgograd and Astrakhan regions, Krasnodar and Stavropol Territory). 

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What causes the fever of the western Nile?

The western Nile fever is caused by the West Nile fever virus belonging to the Flavivirus family of the Flaviviridae family . The genome is represented by single-stranded RNA.

Virus replication occurs in the cytoplasm of affected cells. The virus of the West Nile fever has a significant ability for variability, which is due to the imperfection of the mechanism of copying genetic information. The greatest variability is characteristic for genes encoding the envelope proteins responsible for the antigenic properties of the virus and its interaction with the membranes of tissue cells. Strains of the West Nile fever virus, isolated in different countries and in different years, do not have a genetic similarity and have different virulence. A group of "old" strains of the western Nile fever, which were mainly allocated before 1990, is not associated with severe CNS lesions. A group of "new" strains (Israel-1998 / New York-1999, strains Senegal-1993 / Romania-1996 / Kenya-1998 / Volgograd-1999, Israel-2000) is associated with massive and severe human diseases.

What pathogenesis does the western Nile fever have?

The fever of the western Nile is little studied. It is assumed that the virus spreads hematogenously, causing damage to the vascular endothelium and microcirculatory disorders, in some cases - the development of thrombohemorrhagic syndrome. It was established that the virusemia is short-term and non-intensive. Leading in the pathogenesis of the fever of the western Nile - the defeat of the membranes and brain material, leading to the development of meningeal and cerebral syndromes, focal symptomatology. Death occurs, as a rule, on the 7-28th day of the disease due to disruption of vital functions due to swelling-swelling of the brain substance with the dislocation of stem structures, necrosis of neurocytes, and hemorrhages into the brain stem.

What are the symptoms of West Nile fever?

The incubation period of the western Nile fever lasts from 2 days to 3 weeks, usually 3-8 days. Fever of the western Nile begins sharply with an increase in body temperature to 38-40 ° C, and sometimes even higher for several hours. The fever is accompanied by severe chills, intense headache, pain in the eyeballs, sometimes with vomiting, pain in the muscles, lower back, joints, a sharp general weakness. Intoxication syndrome is expressed even in cases that occur with a short-term fever, and after the temperature is normal, asthenia persists for a long time. The most characteristic symptoms of the western Nile fever caused by the "old" strains of the virus, in addition to those listed, are scleritis, conjunctivitis, pharyngitis, polyadenopathy, rash, hepatolyenal syndrome. Dyspeptic disorders are frequent (enteritis without pain syndrome). The defeat of the central nervous system in the form of meningitis and encephalitis is rare. In general, the fever of the western Nile is benign.

How is the West Nile fever diagnosed?

Clinical diagnosis of West Nile fever is problematic. In the endemic region of the Western Nile region, any case of influenza-like illness or neuro-infection in June-October is suspected of West Nile fever, but can only be diagnosed using laboratory tests. During outbreaks, the diagnosis can be made with a significant degree of reliability on the basis of clinical and epidemiological data: the connection of the disease with mosquito bites, travel outside the city, living near open water bodies; absence of repeated cases of diseases in the outbreak and the connection of the disease with the use of food products, water from open reservoirs; Increase in the incidence of neuroinfections in the region in the warm season.

How is the West Nile fever treated?

Fever of the western Nile is treated with syndromic therapy, since the effectiveness of antiviral drugs has not been proven. To combat cerebral hypertension, furosemide is administered to adults 20-60 mg per day, maintaining a normal volume of circulating blood. When the symptoms of swelling-swelling of the brain increase, mannitol is prescribed in a dose of 0.5 g / kg of body weight in a 10% solution, administered rapidly for 10 minutes, followed by 20-40 mg of furosemide intravenously. In severe cases (coma, respiratory disorders, generalized convulsions), additionally, dexamethasone (dexazone) is administered at a dose of 0.25-0.5 mg / kg per day for 2-4 days. Detoxification and compensation of fluid loss are carried out by intravenous infusions of polyionic solutions (trisol solution). Polarizing mixture and colloidal solutions (10% albumin solution, cryoplasma, rheopolyglucin, reoglumane) in a ratio of 2: 1. The optimal daily volume of injected fluid, including oral and probe injection, is 3-4 liters for adults and 100 ml / kg of body weight for children.

How is the West Nile fever prevented?

Fever of the western Nile is prevented with the help of measures that are aimed at reducing the number of mosquitoes, which is achieved by anti-mosquito treatments of mosquito breeding sites in the city and in the surrounding areas, as well as in areas near suburban recreation centers, dispensaries, children's camps. The basements of residential and public buildings in urban and rural areas are subject to disinfestation. Treatment can be carried out outside the epidemic season for the destruction of mosquitoes that hibernate in the adult stage. It is recommended to reduce the density of populations of synanthropic birds (crows, jackdaws, sparrows, pigeons, gulls, etc.). Measures to publicly prevent fever of the western Nile are conducted according to epidemiological indications on the basis of regular epidemiological surveillance and survey of the territory.

The measures of nonspecific individual prophylaxis are reduced to the application in the epidemic period (June-October) of repellents and clothing, which protects against mosquito bites, minimization of time spent outdoors during maximum activity of mosquitoes (in the evening and in the morning), window sifting, the smallest number of mosquitoes. In the endemic regions, sanitary and educational work among the local population and visitors is very important.