Eastern equine encephalitis virus in mice I: clinical course and outcome are dependent on route of exposure.

Background: Eastern equine encephalitis virus (EEEV), an arbovirus, is an important human and veterinary pathogen belonging to one of seven antigenic complexes in the genus Alphavirus, family Togaviridae. EEEV is considered the most deadly of the mosquito-borne alphaviruses due to the high case fatality rate associated with clinical infections, reaching up to 75 % in humans and 90 % in horses. In patients that survive acute infection, neurologic sequelae are often devastating. Although natural infections are acquired by mosquito bite, EEEV is also highly infectious by aerosol. This fact, along with the relative ease of production and stability of this virus, has led it to being identified as a potential agent of bioterrorism.

Methods: To characterize the clinical course and outcome of EEEV strain FL93-939 infection, we compared clinical parameters, cytokine expression, viremia, and viral titers in numerous tissues of mice exposed by various routes. Twelve-week-old female BALB/c mice were infected by the intranasal, aerosol, or subcutaneous route. Mice were monitored for clinical signs of disease and euthanized at specified time points (6 hpi through 8 dpi). Blood and tissues were harvested for cytokine analysis and/or viral titer determination.

Results: Although all groups of animals exhibited similar clinical signs after inoculation, the onset and severity differed. The majority of those animals exposed by the aerosol route developed severe clinical signs by 4 dpi. Significant differences were also observed in the viral titers of target tissues, with virus being detected in the brain at 6 hpi in the aerosol study.

Conclusion: The clinical course and outcome of EEEV infection in mice is dependent on route of exposure. Aerosol exposure to EEEV results in acute onset of clinical signs, rapid neuroinvasion, and 100 % mortality.