Tick-Borne Viral Encephalitides

Clinical Description

Identification

A group of viral diseases clinically resembling the mosquito-borne encephalitides, caused by tick-borne encephalitis virus (TBEV), louping ill virus (LIV), and Powassan encephalitis virus (POWV). There are three subtypes of TBEV: Far Eastern, Central European and Siberian. The Far Eastern subtype causes the most severe disease, with up to 50% of cases developing neurological disease, and 20% mortality; the others cause an uncommon and milder neurological illness, with 1–3% mortality for the Siberian subtype and 1–2% for the Central European subtype. Central European TBEV typically causes a diphasic illness with a week of influenza-like illness, followed by an asymptomatic period of a few days, then neurological illness in one-third of cases. Powassan encephalitis (PE) has a similar clinical course, with a 10% case-fatality rate and neurological sequelae among 50% of survivors. LIV is closely related to Central European TBEV and causes a similar illness. All may cause a polio-like flaccid paralysis.

Infectious Agent

A complex within the flaviviruses; minor antigenic differences exist, more with Powassan than others, but viruses causing these diseases are closely related.


Diagnosis

Specific identification is made through demonstration of specific IgM or nucleic acid in acute phase serum or CSF, serological tests of paired sera, virus isolation from blood during acute illness, or from brain postmortem (inoculation of suckling mice or cell culture). Common serological tests distinguish the group from most other similar diseases, but differentiating within this group requires characterization of the viral RNA.


Epidemiology

Mode of Transmission

Bites of infective ticks or consumption of milk from certain infected animals. Larval ticks ingest virus by feeding on infected vertebrates, including rodents, other mammals or birds. Central European TBEV may be acquired through consumption of infected raw milk.

Incubation Period

Usually 7–14 days.

Period of Communicability

No direct person-to-person transmission. A tick infected at any stage remains infective for life. Viremia may last for days in vertebrates; in humans, up to 7–10 days.

Reservoir

The tick, or ticks and mammals in combination, appear to be the true reservoir; trans-ovarian tick passage of some tick-borne encephalitis viruses has been demonstrated. Sheep and deer are the primary vertebrate hosts for LIV, while rodents and other small mammals and birds serve as sources of tick infections with TBEV and POWV.

Susceptibility

Men and women of all ages are susceptible. Infection, whether unapparent or overt, leads to immunity.

Occurrence

Disease of the CNS caused by this complex is distributed spottily over much of Russia, other parts of eastern and central Europe, Scandinavia, and the UK. Far Eastern TBEV is in northeast Russia, China, and northern Japan; Central European TBEV is found in an area extending from Scandinavia down to the Adriatic region, and east to the Urals; and the Siberian TBEV is found in Siberia and the Baltic region. Ixodes persulcatus is the major vector of the Far Eastern and Siberian subtypes, and Ix. ricinus for the central European subtype. LIV is found in the UK and Ireland and southwestern Europe, and is transmitted by Ix. ricinus. Powassan virus is present in Canada, the USA, and Russia, and is transmitted by Ix. cookei in North America and by Ix. persulcatus and Haemaphysalis longicornis in Russia. Seasonal incidence depends on density of the tick vectors: activity peaks in spring and early summer in eastern Asia; early summer and early autumn in Europe; and June to September in Canada and the USA.

Areas of highest incidence are those where humans have intimate association with large numbers of infected ticks, generally in rural or forested areas but also in urban populations. Local epidemics of Central European TBEV disease have occurred among people consuming unpasteurized milk and dairy products from goats and sheep, hence the name “diphasic milk fever.” The age pattern varies in different regions and is influenced by opportunity for exposure to ticks, consumption of milk from infected animals, and previously acquired immunity. Laboratory infections are common, some with serious sequelae, including death.


Prevention and Control

1)    Preventive measures:

a)    See section on Lyme Disease, for measures against ticks.

b)    Inactivated virus vaccines have been used extensively in Europe and the Russian Federation, with reported safety and effectiveness.

c)    Boil or pasteurize milk of susceptible animals in areas where Central European TBEV occurs

2)    Control of patient, contacts and the immediate environment:

a)    Report to local health authority: In selected endemic areas; in most countries not a reportable disease, Class 3.

b)    Isolation: None, after tick removal.

c)    Concurrent disinfection: Not applicable.

d)    Quarantine: Not applicable.

e)    Immunization of contacts: Not applicable.

f)    Investigation of contacts and source of infection: Search for missed cases, presence of tick vectors, and animals excreting virus in milk.

g)    Specific treatment: None.

3)    Epidemic measures:

See page on Lyme Disease.

4)    Disaster implications:

None.

5)    International measures:

WHO Collaborating Centres provide support as required. More information can be found at: http://www.who.int/collaboratingcentres/database/en/
 

Source: Heymann (Ed.). (2008). Control of Communicable Diseases Manual, 19th edition. Washington, DC: American Public Health Association.

Common Disease Taxonomy: