Electron microscopic diagnosis of human flavivirus encephalitis - Use of confocal microscopy as an aid

The distinction between intracranial viral infections and inflammatory cond itions requiring immunosuppression is important. Although specific laborato ry reagents are readily available for some viruses, diagnosis of arbovirus infection is more difficult. Transmission electron microscopy (TEM) theoret ically allows identification of viral particles independent of reagent avai lability, but it has limited sensitivity. We report two cases of human flav ivirus encephalitis diagnosed by TEM. Laser scanning confocal microscopy (L SCM) was used in one case to survey unembedded tissue slices for focal abno rmalities, from which fragments smaller than 1 mm(2) were excised for epoxy embedding. This facilitated TEM identification of intracytoplasmic, buddin g, 35-40 nm spherical virus particles, confirmed by serology as St. Louis e ncephalitis. In contrast to mosquitoes and newborn mice, in which high vira l loads are associated with minimal tissue responses, these biopsies showed florid angiodestructive inflammation and microgliosis, with rare virions i n necrotic perivascular cells and astrocytes. To our knowledge, this repres ents the first ultrastructural study of St. Louis encephalitis in humans, i ndicating the potential value of LSCM-aided TEM.