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Tick-borne encephalitis (TBE)
Introduction
- Cause: tick-borne encephalitis virus (TBEV) from the Flavivrus genus of the Flaviviridae family.
- Signs: ataxia, proprioceptive deficits, seizures, tremor, paresis, paralysis and cranial nerve deficits. Neurological signs are often progressive and fatal.
- Diagnosis: serology, immunohistology.
- Treatment: supportive.
- Prognosis: variable in human cases, very guarded to grave in dogs.
Presenting signs
- Pyrexia.
- Lethargy.
- Ataxia.
- Paresis.
- Behavioral changes (aggression, nervous signs).
- Proprioceptive deficits.
- Facial nerve paralysis.
- Strabismus.
- Nystagmus.
- Seizures.
- Tremor.
- Progressive paralysis.
Acute presentation
- Pyrexia and lethargy accompanied by severe neurological signs.
Geographic incidence
- Present in Eurasia and Japan.
- Countries with incidence areas (incidence of human clinical TBE cases is > 10 per 105 inhabitants) are Russia, Latvia, Lithuania, Slovenia, and Estonia.
- TBE is also a significant issue in Germany, the Czech Republic, Poland, Switzerland, Sweden, Finland, Slovakia, and Hungary.
- Cases have also been reported in Austria, Denmark, France, Greece, Italy, Norway, the UK, Turkey, South Korea, China and Japan.
- Austria is the only country with progressively decreasing human incidence due to human vaccination policy but is unlikely reflective of underlying virus prevalence or disease rates in dogs.
- Canine disease incidence in endemic countries is lacking but experimental and field data suggest that canine disease as a result of infection is lower than in humans. In endemic areas in Switzerland there is a higher seroprevalence in dogs than humans and yet the incidence of clinical cases in dogs in Switzerland, Austria, Germany and Sweden are significantly lower than human incidence of disease.
Age predisposition
- None.
Breed/Species predisposition
- No cases of TBEV have been published in cats, the pathogenicity of TBEV in cats is likely to be low.
- Rottweiler Rottweiler dogs appear to be over represented in cases of TBEV.
Public health considerations
- Zoonotic. Dogs pose no direct to humans but infected unfed ticks falling off in the home may pose a small risk to people. Positive dogs are sentinels of infection, suggesting that humans walking the dog may have been exposed to infected ticks in the same environment.
Cost considerations
- Dogs developing clinical signs are likely to require prolonged hospitalization to have any realistic chance of recovery.
Special risks
- Dogs presenting with behavioral changes may represent an increased risk of injury, either to themselves or to those handling them if increased aggression is displayed.
Pathogenesis
Etiology
- Caused by TBEV. There are three subtypes of TBEV currently recognized (European, Siberian, and Far Eastern), although Baikalian and Himalayan subtypes have also recently been proposed.
- The far Eastern subtype is more pathogenic in people with a higher mortality rate (25-30% compared to 1-5% mortality in the European strain). Although disease is rare in dogs, when it develops it is often fatal, whichever strain is involved.
Predisposing factors
General
- Immune status of the dog.
- The amount of time spent exposed to ticks in endemic countries.
- Stress.
- Immunosuppression.
- Concurrent disease.
Specific
- Exposure to infected ticks (primarily Ixodes ricinus Ixodes ricinus in Western and Central Europe, Ixodes persulcatus, in Central and Eastern Europe and Ixodes ovatus in Japan but also less commonly Dermacentor reticulatus Dermacentor reticulatus).
- Alimentary transmission has been reported from the consumption of unpasteurized milk and milk products from livestock.
Pathophysiology
- Viral infection leads to severe encephalitis Encephalitis/ meningoencephalitis in the absence of gross visceral changes.
- Large areas of brain affected by inflammation with infiltrates of lymphocytes, histiocytes and plasma cells.
- Pyrexia Pyrexia: overview and altered behavior caused by inflammation in the thalamus and cerebral cortex.
- Proprioceptive deficits caused by inflammation in the mesencephalon.
- Motor neuron deficits caused by spinal cord inflammation.
- Severe neck pain caused by meningitis Meningitis.
- Head tilt, nystagmus and strabismus caused by inflammation of the brainstem.
- The pathological changes are similar in humans and dogs.
Timecourse
- Peracute lethal 3-7 days.
- Acute 1-3 weeks.
- Chronic subclinical - months and milk products from livestock.
Epidemiology
- TBEV is a seasonal disease which depends on the climate related activity of the ticks.
- Rodents act as reservoir hosts capable of maintaining the pathogen once it has moved to a new area.
- Larger wild animals such as deer are not considered to be competent hosts for virus transmission but serve as reproductive and transport hosts for infected ticks, allowing maintenance of tick populations and geographical spread of TBEV.
- Migratory birds are also likely to play a significant role in carrying infected ticks over large distances.
- Within the tick population, the virus is maintained via the transstadial route and possibly to a lesser extent, via transovarial transmission to the next developmental stage of the tick's life cycle.
Diagnosis
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Treatment
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Prevention
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Outcomes
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Further Reading
Publications
Refereed Papers
- Recent references from PubMed and VetMedResource.
- Holding M, Dowall S D, Medlock J M, Carter D P, Pullan S T et al (2020) Tick-Borne Encephalitis Virus, United Kingdom. Emerg Infect Dis 26, 90-96 PubMed.
- Dai X, Shang G, Lu S, Yang J & Xu J (2018) A new subtype of eastern tick-borne encephalitis virus discovered in Qinghai-Tibet Plateau, China. Emerging Microbes and Infections 7, 74 PubMed Full Article.
- Jahfari S, de Vries A, Rijks J M, Van Gucht S, Vennema H, Sprong H et al (2017) Tick-borne encephalitis virus in ticks and roe deer, the Netherlands. Emerg Infect Dis 23, 1028-1030 PubMed.
- Imhoff M, Hagedorn P, Schulze Y, Hellenbrand W, Pfeffer M et al (2015) Review: Sentinels of tick-borne encephalitis risk. Ticks and Tick Borne Diseases 6, 592-600 PubMed.
- Pfeffer M & Dobler G (2011) Tick-borne encephalitis virus in dogs – is this an issue? Parasites & Vectors 4, 59 PubMed Full Article.
- Lindquist L & Vapalahti O (2008) Tick-borne encephalitis. Lancet 371, 1861-1871 PubMed.
- Gritsun T S, Nuttall P A & Gould E A (2003) Tick-borne flaviviruses. Adv Virus Res 61, 317-317 PubMed.
Organisation(s)
- The European Scientific Counsel for Companion Animal Parasites (ESCCAP): www.esccap.org/
- World Health Organisation (WHO): www.who.int/ith/diseases/tbe/en/