West Nile fever - Diagnosis

Clinical diagnosis of sporadic West Nile fever cases is problematic. In the region where West Nile fever is endemic, any case of influenza-like illness or neuroinfection in June-October is suspected of being West Nile fever, but can only be diagnosed using laboratory tests. During outbreaks, diagnosis can be made with a high degree of certainty based on clinical and epidemiological data: association of the disease with mosquito bites, trips out of town, residence near open water bodies; absence of repeated cases of the disease in the outbreak and association of the disease with consumption of food products, water from open water bodies; increase in the incidence of neuroinfections in the region during the warm season.

West Nile fever virus can be isolated from blood and, less commonly, cerebrospinal fluid samples taken from patients during the acute phase of the disease, usually up to the fifth day after the onset of the disease. Laboratory models for virus isolation include newborn and young mice and various types of cell cultures.

During the same time frame, it is possible to detect West Nile fever virus RNA using PCR. Material for PCR testing (plasma and/or blood serum, cerebrospinal fluid) must be collected using only disposable test tubes and medical instruments in compliance with aseptic rules and stored at -70 °C or in liquid nitrogen until the time of testing.

Serological diagnostics of West Nile fever is possible using the methods of RTGA, RSK, RN. At present, the most widely used in practice is ELISA, which allows detecting antibodies to the virus of class IgM and IgG. Early antibodies of class IgM are determined in the first days of the disease, and their titers reach a very high level 1-2 weeks after the onset of the disease.

For serological diagnostics, it is necessary to take two blood samples: the first sample - in the acute period of the disease up to the 7th day from the onset of the disease; the second sample - 2-3 weeks after taking the first.

The diagnosis of West Nile fever can be made by detecting IgM antibodies to the virus in a single blood sample taken during the acute phase of the disease, as well as by determining a decrease or increase in IgM levels in paired blood sera.

Indications for hospitalization

West Nile fever is a reason for hospitalization, in the presence of hyperthermia of 40 °C and above, and the presence of meningeal, general cerebral and focal neurological symptoms.

Differential diagnosis of West Nile fever

Differential diagnostics of West Nile fever is carried out depending on the clinical form of the disease. Unlike influenza, West Nile fever does not show signs of laryngotracheitis, the duration of fever often exceeds 4-5 days. West Nile fever differs from ARVI by the absence of catarrhal symptoms in the upper respiratory tract, high fever and severe intoxication.

The meningeal form of West Nile fever differs from meningitis of other etiologies, primarily enterovirus, by high and prolonged fever, severe intoxication, mixed pleocytosis, and slow sanation of the cerebrospinal fluid. In enterovirus meningitis, neutrophilic and mixed pleocytosis is possible during the first examination of the cerebrospinal fluid in the early stages, and after 1-2 days it becomes lymphocytic (more than 90%).

The most difficult differential diagnosis of West Nile fever is with herpes encephalitis. In its presence, often against the background of fever, a sudden attack of generalized convulsions followed by coma is observed, however, differential diagnosis is possible only on the basis of blood and cerebrospinal fluid tests using a full range of immunological methods and PCR, as well as CT or MRI of the brain.

Unlike bacterial meningitis, in meningeal and meningoencephalic variants of West Nile fever, the cerebrospinal fluid is transparent or opalescent; there is a clear discrepancy between the severe picture of the disease and the mild inflammatory reaction of the cerebrospinal fluid, with elevated or normal glucose levels in it. Even in the presence of blood leukocytosis, there is no neutrophilic shift to the left.

Symptoms of CNS damage in patients with West Nile fever differ from tuberculous meningitis in that they appear earlier and increase in the first 3-5 days of the disease (in tuberculous meningitis - on the 2nd week). Fever and intoxication are more pronounced in the first days of the disease, on the 2nd-3rd week the condition improves, the fever decreases, neurological symptoms are recorded, and against the background of decreasing cytosis of the cerebrospinal fluid, the glucose level does not change.

Unlike rickettsioses, West Nile fever has no primary affect, characteristic rash, hepatosplenic syndrome, inflammatory changes in the cerebrospinal fluid are observed with greater constancy, RSK and other serological tests with rickettsial antigens are negative. The area of distribution, seasonality of West Nile fever may coincide with the area of Crimean hemorrhagic fever, however, with Crimean hemorrhagic fever, hemorrhagic syndrome is detected, inflammatory changes in the cerebrospinal fluid are absent. When examining blood from the 3rd-5th day of the disease, leuko- and neutropenia, thrombocytopenia are detected.

Unlike malaria, the fever in patients with West Nile fever is remittent, there is no apyrexia between attacks, repeated chills and hyperhidrosis, no jaundice, hepatosplenic syndrome, or anemia.

Differential diagnosis of West Nile fever with other diseases without central nervous system involvement

Indicator	LZN	ARVI	Flu	Enterovirus infection
Seasonality	July-September	Autumn-winter-spring	Autumn-winter	Summer-autumn
Fever	Up to 5-7 days 37.5-38.5 °C	2-3 days 37.1-38.0 °C	Up to 5 days 38.0-40.0 °C	2-3 days up to 38.5 °C
Headache	Expressed	Weak, moderate	Sharply expressed	Expressed
Vomit	Possible	Not typical	Possible	Possible
Chills	Possible	Not observed	Possible	Not typical
Myalgia	Characteristic	Not typical	Characteristic	Possible
Cough	Not typical	Characteristic	Characteristic	Not typical
Runny nose	Not typical	Characteristic	Characteristic	Not typical
Hyperemia of the pharynx	Not typical	Characteristic	Characteristic	Possible
Facial hyperemia	Possible	Not typical	Characteristic	Characteristic
Injection of sclera and conjunctiva	Possible	Possible	Characteristic	Characteristic
Cervical lymphadenitis	Not typical	Possible	Not observed	Possible
Rash	Possible	Not observed	Not observed	Possible
Enlarged spleen	Not observed	Not typical	Not observed	Maybe
Diarrhea	Not typical	Not typical	Not observed	Possible
White blood cell count	Leukocytosis is possible	More often leukopenia	More often leukopenia	More often leukocytosis

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