Flaviviruses(Tick-borne Encephalitis virus): An overview
▶The History of Virus Discovery
Tick-borne encephalitis virus (TBEV) was discovered in 1937 by the Soviet virologists L. Zilber, M. Chumakov, and E. Levkovich during their Far East expedition.
Tick-borne encephalitis virus (TBEV) pertains to Flaviviridae family and genus Flavivirus. The genus encompasses 53 zoonotic viral species that pertain to arboviruses, contracted by arthropod vectors to animal and human hosts.
▶Structure of Virus
TBEV is a middle- or small size virus (20-40 nm) of spherical shapes with lipid envelope. TBE virion has icosahedral symmetry.
Viral genome consists of positive single-stranded non-segmented RNA.
Viral particle has a RNA-bound nucleocapsid C protein and envelope proteins M and E
External E glycoproteins make viral spikes with receptor functions, promoting viral binding to the cells. They possess hemagglutinating activity.
Genomic RNA encodes a number of non-structural proteins (NSP) with enzymatic activity (helicase, protease, RNA polymerase).
TBE virus has 3 distinct genetic subtypes: European, Siberian, and Far Eastern.
All TBE viruses form similar common antigenic complex.
TBE virus as all flaviviruses is moderately resistant to the external influences. Nevertheless, it remains long stable at low temperatures. Virus maintains viability at least for 10 days at 16-18oC.
Within contaminated dairy products (milk) it may stay viable for 2 months.
Heating at 60oC for 10 min and boiling for 2 min irreversibly inactivates virus.
Virus is sensitive to UV light and all conventional disinfectants (halides, aldehydes, phenol, H2O2, detergents, ethanol, etc.)
▶Viral Replication Cycle
TBEV infects various types of cells, including leukocytes, endotheliocytes, hepatocytes, and neurons.
The virus attaches to the host membrane receptors by spikes and enter the cells by endocytosis. Viral uncoating is facilitated with endosome acidification resulting in fusion of lipid envelope with endosomal membrane. Viral nuclecapsid passes into cytoplasm, where uncoating and further replication occurs.
Genomic (+) RNA translation results in primary polyprotein synthesis that further processed by viral and cellular proteases into mature viral proteins.
Newly formed viral RNA polymerase enzyme activates genomic RNA replication. Viral genome is transcribed through intermediate minus RNA strand that serves as the template for final positive sense RNA synthesis.
Virion assembly and maturation is followed by egress of progeny viruses that leave the cell by budding. Active viral propagation results in the destruction of infected cells.
Virus is easily cultured in embryonated chicken eggs, various continuous (Vero and BHK cells) or primary cell lines, and in experimental animals (mice) under intracerebral inoculation.
▶Pathogenesis, Clinical Findings and Immunity in Tick-Borne Encephalitis
Tick-borne encephalitis is an endemic zoonotic disease registered in the areas of habitations of tick vectors. The broad endemic regions of infection cover Far East, Siberia, Urals, Central Asia, Northern, Central and Eastern Europe. The disease cases were indicated from France to Japan. The largest number of cases is regularly reported from Russian Federation – 5,000 to 13,000 annually.
Each of genetic subtypes of viruses – European, Siberian, and Far Eastern – prevails in its corresponding geographical areas.
Belarus is also an endemic country for TBE caused by European subtype of virus. Above 110 cases of the disease were registered in 2014.
The sources of infection are the numerous species of wild and domestic animals (primarily, rodents, hares, ungulates, predators, birds and many others, totally about 130 species).
The infection has a predominant vector-borne route of transmission via the bites of infected ticks.
The main vector for Siberian and Far Eastern virus subtypes is tick species Ixodes persulcatus, for European subtype – Ixodes ricinus.
According to local data, about 15% of tick vectors are infected by TBEV. Tick vectors have the transovarial transmission of virus
Rare alimentary transmission was occasionally registered in past via contaminated non-pasteurized goat or cow milk.
The disease is characterized with spring-summer seasonality.
Incubation period of tick-borne encephalitis averages 1-2 weeks.
According to seroprevalence studies, it has been established that about 90% of cases may be subclinical or asymptomatic.
After tick bite the virus enters the blood and lymph. It may primarily propagate in the site of bite and infect leukocytes and endotheliocytes. Hematogenous and lymphogenous dissemination spreads the pathogen throughout the body; the virus appears in spleen and liver and finally reachs CNS.
Viral propagation causes degeneration of neurons in spinal cord and brain followed by meningeal damage. The most suffering are the motor neurons of brainstem, cervical and upper thoracic parts of spinal cord.
In case of manifested TBE disease the onset is sharp with fever, headache and vomit.
Meningeal symptoms, sensory and coordination disorders and muscular pareses are observed. In severe cases the patients develop acute flaccid paralysis that predominantly affects the muscles of neck, shoulders, and upper limbs. The muscular weakness and paralysis may stay long after the end of acute disease course. Progression of the disease may be fatal.
The most severe are the clinical cases caused by Far Eastern subtype of virus. They demonstrate lethality of 20-40% and the high rate of poorly resolving neurologic sequelae.
The disease, caused by European virus subtype, has a milder course usually followed by complete recovery with lethality less than 1-2%.
Post-infectious immunity is long-term and stable. Virus-neutralizing antibodies maintain their protecting levels for a long time.
▶Laboratory Diagnosis of TBE
Clinical specimens of patient’s blood, serum, cerebrospinal fluid or autopsy materials are taken for examination.
For rapid detection of virus immunofluorescence assay, ELISA and RT-PCR are elaborated.
Serological testing is actively used in routine laboratory practice. Anti-TBEV IgM antibodies are determined in patient’s sera by ELISA indicating acute disease. The elevation of specific antibodies in the course of infection is determined in paired sera tests by hemagglutination inhibition assay, ELISA, or neutralization tests in the cell cultures.
Virus isolation is made by inoculation of filtrates of virus-containing specimens into various cell lines. TBE virus is detected in cell cultures by immunofluorescence test and PCR.
Animal experimental infection is performed by intracerebral inoculation of mice followed by animal postmortem examination with identification of virus (immunofluorescence assay, ELISA, or PCR).
Infectivity of ticks is determined by immunofluorescence and ELISA.
▶Prophylaxis and Treatment of Tick-borne Encephalitis
Specific antiviral treatment of tick-borne encephalitis is not elaborated. As the result, the treatment is largely symptomatic (infusion and detoxication therapy, neurological support). Passive protection with antiviral immune globuline might be worthy at early course of the disease.
Specific prophylaxis is performed by vaccination with inactivated cell culture-derived TBE vaccines.
The persons from the professional groups of risk of tick exposure that work in the endemic areas are vaccinated (e.g., forest workers, hunters, laboratory personnel, operating with infectious materials, etc.). In hyperendemic areas mass vacination of population should be conducted.
Post-exposure prophylaxis of individuals subjected to infected tick attack is performed by specific antiviral immune globuline being efficient within 2-3 days after the exposure.
Non-specific prophylaxis is primarily based on tick control measures achieved by broad use of chemical acaricidal agents in endemic areas. This substantially reduces the total number of living vectors, and therefore, the risk of infection.
Personal protection against tick bites should be maintained as well.