JOURNAL OF THE LOUISIANA STATE MEDICAL SOCIETY
Figure 1: Ixodes scapularis , the Eastern black-legged tick or deer tick, adult female and nymphs. These are the most common arthropod vectors of deer tick and Powassan virus diseases as well as Lyme disease and babesiosis, and have transmitted both infections concurrently in regions of high prevalence, such as the New England states. The diminutive nymphs are often unobserved and their bites are often painless and unnoticed. (From Centers for Disease Control and Prevention [CDC], Atlanta, GA. Public Health Image Library, image 1205.
The tickborne viral infections share several common clinical and epidemiologiccharacteristics, includingthefollowing: incubation periods of approximately one week; biphasic illnesses separated by symptom-free periods, beginning with flulike viremic stages and ending with CNS or hemorrhagic manifestations with increased case fatality rates (CFRs); nonspecific serodiagnosis by comparing acute and convalescent sera for increased antibody titers or by hemagglutination inhibition; specific serodiagnosis by enzyme-linked immunosorbent assay (ELISA) and antigen nucleic acid detection in blood or cerebrospinal fluid (CSF) by reverse-transcriptase (RT)-PCR; no specific treatments other than supportive therapy; and significantly increased post- infection morbidity. 2 From a global distribution perspective, the tickborne encephalitis viruses (TBEVs) are also stratified geographically into the Old World (Eastern Hemisphere) and New World (Western Hemisphere) strains, with the OldWorld strains having significantly higher CFRs (20- 40%) and permanent neurologic morbidity rates (28-30%) than the New World strains (CFR, 10- 15%; morbidity rate <10%). 2 New World strains are, however,
evolving with new and more virulent strains, such as the deer tick virus and the Powassan virus diseases in the U.S.
Although additional Old World flaviviral strains have now been discovered in sheep reservoirs, the most common Old World TBEVs have been even further stratified regionally into three major subtypes: (1) European or Central European (TBEV-Eu) subtypes, (2) Siberian or Russian spring-summer (TBEV-Sib) subtypes, and (3) Far Eastern (TBEV-FE) subtypes (Table 1). Except for some of the OldWorld TBEVs with sheep reservoirs, all of the TBEVs are transmitted by the injection of infected saliva from viremic ixodid (Family Ixodidae) ticks. During painless blood-feeding, viruses in tick saliva increase up to 10-fold and render early removal of the feeding tick ineffective in preventing disease. 2 This has become a very serious problem in the transmission of deer tick and Powassan virus disease- associated encephalitis in the US. The preferred wild animal reservoirs for TBEVs worldwide include rodents, insectivores, medium-sized mammals, deer and other ungulates, birds, and, less often, domestic animals (Table 1). 2
48 J La State Med Soc VOL 170 MARCH/APRIL 2018
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