Tick-Borne Encephalitis in Travelers
Abstract & Commentary
By Stan Deresinski, MD, FACP, Clinical Professor of Medicine, Stanford, Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center, is Editor for Infectious Disease Alert.
Synopsis: Tick-borne encephalitis virus infection may occur in travelers to endemic areas, but fortunately remains rare.
Source: CDC. Tick-borne encephalitis among U.S. travelers to Europe and Asia 2000-2009. MMWR. 2010;59:335-338.
A retrospective review of CDC laboratory records identified five cases of tick-borne encephalitis (TBE) among U.S. travelers between 2000 and 2009. All were males age 14 to 57 years. Each had traveled in the summer months, two to Russia ("eastern Russia" and Siberia), one to the Czech Republic and Russia (Siberia), one to Sweden, and one to China (Tianjin Province). A tick bite had been noted in four; four had a biphasic illness, while that of the traveler to China was monophasic. Two cases were classified as meningitis and three as encephalitis. The total cerebrospinal fluid (CSF) WBC ranged from 11 cells/mm3 to 112 cells/mm3; the CSF protein concentration was only modestly elevated from 55 mg/dL to 84 mg/dL in the four patients in whom this was measured. MRI was normal in three, while one demonstrated lesions of the basal ganglia and thalamus and the other had evidence of cerebral edema with ischemic changes in the thalamus and striatum. The three patients with normal MRI findings, two with meningitis, and one with encephalitis had complete neurological recovery. Of the two with abnormal brain MRI, one was left with residual mild cognitive impairment, while the other, the only patient with a monophasic illness and the one who had traveled to China, had more significant neurological residual impairment.
Commentary
TBE virus is a member of the tick-borne flavivirus group. Other members that are known to cause human disease include louping ill virus and Powassan virus, both of which also cause encephalitis, and three viruses that cause hemorrhagic fever Omsk hemorrhagic fever virus, Kayasnur Forest disease virus, and Alkhurma hemorrhagic fever virus.1 There are three main, closely related subtypes of TBE virus: European, Siberian, and far-eastern. The known geographic range of TBE virus is increasing, and the virus is currently endemic in areas ranging from northern China and Japan through far-eastern Russia to Europe. Approximately 10,000 cases of tick-borne encephalitis (TBE) are reported each year in Europe and Russia. The highest incidences of human cases are reported in Austria, Czech Republic, Estonia, Germany, Hungary, Latvia, Lithuania, Poland, Russia, Slovenia, Sweden, and Switzerland, while no cases have been reported from Belgium, Ireland, Luxembourg, the Netherlands, Portugal, Spain, and the United Kingdom.2
In western Europe, TBE virus is transmitted primarily by the Ixodes ricinus tick, while the vector for the Siberian and far-Eastern subtypes is Ixodes persulcatus both of which also may transmit, among other things, Lyme disease borrelia. The virus is maintained within discrete areas where both the tick vectors and animal hosts, predominantly rodents, are present, and cases peak in late summer when the ticks are most active.2 TBE virus also can be transmitted by ingestion of unpasteurized milk from infected animals.
As in the cases described here, infection with a European subtype generally causes, after a mean incubation period of approximately eight days, a biphasic illness, while that associated with the far-eastern subtype is more often monophasic. Approximately one-half of infected patients are believed to develop encephalitis. Health-care providers should contact their state or local health department or CDC's Division of Vector Borne Infectious Diseases (970-221-6400) for assistance with diagnostic testing. No specific therapy is available, so prevention is critical. In addition to taking measures to avoid tick bites in endemic areas, vaccination also is important. Two safe, effective inactivated TBE vaccines are available in Europe, in adult and pediatric formulations: FSME-IMMUN (Baxter, Austria) and Encepur (Novartis, Germany).2 The adult formulation of FSME-IMMUN also is licensed in Canada. Two other TBE vaccines are produced in Russia, but little information has been published about their safety and efficacy. The primary vaccination series requires at least nine months for completion. The overall risk of acquiring TBE for an unvaccinated visitor to an endemic area during the TBEV transmission season has been estimated at 1 case per 10,000 person-months of exposure.
References
- Mansfield KL, et al. Tick-borne encephalitis virus a review of an emerging zoonosis. J Gen Virol. 2009;90:1781-1794.
- http://wwwnc.cdc.gov/travel/yellowbook/2010/chapter-5/tick-borne-encephalitis.aspx
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