Abstracts

Rationale: CNS infections are recognized as major risk factors for the development of epilepsy. However, the mechanisms behind post-infection epileptogenesis are not well understood, partly due to the lack of an animal model that accurately reproduces the pathologic and phenotypic features of human encephalitis-induced epilepsy. Recently we reported that Theiler’s virus infection of C57BL/6 (B6) mice leads to the development of seizures during the acute encephalitic phase of the infection. These seizures are accompanied by other pathological features such as neuronal cell death and viral antigens in the brain, particularly the hippocampus and cortical regions. The purpose of this study was to examine seizure progression and chronic CNS histopathology in TMEV-infected B6 mice to test the hypothesis that animals exhibiting seizures during the acute phase of infection subsequently develop epilepsy. Methods: Acute: 4-week-old B6 mice were surgically implanted with bilateral surface electrodes. After a 1-week recovery period, mice were intracerebrally injected with either PBS (n = 8) or TMEV (n = 16; 2 x 10 PFU) and continuously video-EEG monitored for 2 weeks to assess acute seizures. Chronic: 5-week-old B6 mice were injected with Theiler’s virus and visually observed 2 hours per day for 2 weeks to determine the presence of seizures. At 7 weeks post-inoculation, mice that exhibited behavioral seizures during the acute phase of infection (n = 10) were implanted with electrodes, allowed 1 week recovery and then continuously video-EEG monitored for 1 week. Control PBS-injected mice (n = 8) were monitored simultaneously. Coronal brain sections (40 μm) taken from infected and control mice (n = 5 each) at 8 weeks post-infection (p.i.) were stained with cresyl-violet to assess chronic histopathology. Results: A significant fraction (75%) of infected mice displayed behavioral seizures during the first week of infection, with most of the seizures fully generalizing to Stage 4/5. Interestingly, seizures completely abate by the second week post inoculation, suggesting a latent period. At 8-9 weeks p.i. 50% of infected mice that had acute seizures went on to develop spontaneous seizures. Morphological assessment revealed considerable hippocampal and cortical neuronal cell loss in infected mice, particularly in the pyramidal cell layers and parietal cortex. PBS-injected mice did not display either seizures or cell death. Conclusions: These findings establish Theiler’s virus infection of B6 mice as a novel model of infection-induced epilepsy. We anticipate that this model will be instrumental in future experiments designed to understand the mechanisms through which CNS infection ultimately result in epilepsy and provide the framework for future studies to identify therapies for the prevention of epilepsy following CNS infections.