Eastern equine encephalitis virus in mice II: pathogenesis is dependent on route of exposure

Abstract

BackgroundEastern equine encephalitis virus (EEEV) is an alphavirus with a case fatality rate estimated to be as high as 75 % in humans and 90 % in horses. Surviving patients often have long-lasting and severe neurological sequelae. At present, there is no licensed vaccine or therapeutic for EEEV infection. This study completes the clinical and pathological analysis of mice infected with a North American strain of EEEV by three different routes: aerosol, intranasal, and subcutaneous. Such an understanding is imperative for use of the mouse model in vaccine and antiviral drug development.MethodsTwelve-week-old female BALB/c mice were infected with EEEV strain FL93-939 by the intranasal, aerosol, or subcutaneous route. Mice were euthanized 6 hpi through 8 dpi and tissues were harvested for histopathological and immunohistochemical analysis.ResultsViral antigen was detected in the olfactory bulb as early as 1–2 dpi in aerosol and intranasal infected mice. However, histologic lesions in the brain were evident about 24 hours earlier (3 dpi vs 4 dpi), and were more pronounced following aerosol infection relative to intranasal infection. Following subcutaneous infection, viral antigen was also detected in the olfactory bulb, though not as routinely or as early. Significant histologic lesions were not observed until 6 dpi.ConclusionThese pathologic studies suggest EEEV enters the brain through the olfactory system when mice are exposed via the intranasal and aerosol routes. In contrast, the histopathologic lesions were delayed in the subcutaneous group and it appears the virus may utilize both the vascular and olfactory routes to enter the brain when mice are exposed to EEEV subcutaneously.

Encephalitic Arboviruses: Emergence, Clinical Presentation, and Neuropathogenesis

“…Within the CNS, neurons are the main target cells for encephalitic flaviviruses [154,170], alphaviruses [107,145], and members of the Bunyaviridae and Reoviridae [146,171,172]. Arboviral infection of other cell types of the CNS has also been reported (Table 2), and recent in vitro studies with ZIKV suggest it may target neural progenitor cells [173][174][175][176].…”

“…CNS invasion via olfactory bulb has been demonstrated for several encephalitic arboviruses, including SLEV [142], MVEV [143], VEEV [144], EEEV [145], and LACV [146]. Soon after onset of viremia, viral particles exit the fenestrated capillaries beneath the mucosa of the nasal cavity.…”

“…Neuronal damage and loss can occur by either direct arboviral infection or indirectly by uncontrolled immune responses to the replicating virus [181,182]. In vitro and in vivo studies have shown that neurons undergo morphologic changes that are characteristic of apoptosis, that is, nuclear and membrane condensation, after infection with several encephalitic flaviviruses, including WNV [183], JEV [154], TBEV [184], and alphaviruses [145,185,186]. Neuronal degeneration, necrosis, and apoptosis were also demonstrated in various CNS regions in mice infected with LACV [146].…”

Entry Sites of Venezuelan and Western Equine Encephalitis Viruses in the Mouse Central Nervous System following Peripheral Infection

“…exposure of outbred CD-1 mice to WEEV.McM leads to virus entry into the CNS through the long axonal projections of olfactory sensory neurons (OSNs) (6,7) and supports other studies showing that VEEV and EEEV utilize OSNs as an important route of entry into the brain following i.n. or aerosol inoculation (8)(9)(10). Alphaviruses then disseminate in the CNS along the neuronal axis, with infection of the olfactory bulb glomerular layer and lateral olfactory tract being prominent features following i.n.…”

“…Others have reported that s.c. injection of mice with VEEV leads to infection of the CNS through olfactory or peripheral nerves (12). A more recent study also supports the hypothesis that EEEV enters the CNS directly from blood following s.c. inoculation (9). However, the precise entry points of WEEV or EEEV in the brain remain to be determined.…”