An electron microscopical study of the replication of avirulent Semliki Forest virus in the retina of mice

Abstract Electron microscopical (EM) studies were carried out on the retinas of 2–3-(baby), 12-, 14- and 21–28-day-old (adult) mice infected with avirulent (A774) Semliki Forest virus (SFV). Virions (mature virus), spherules and advanced stages of virus replication, cytopathic vacuoles type II (CPV II), were seen in the retinal neurons of baby mice after intracerebral (i.c.) or intraperitoneal (i.p.) infection. Some virions and spherules were also seen in the retinas of 12- and 14-day-old mice. Virions and advanced stages of virus replication were not seen in adult mice despite high virus titres. Some neurons of the inner nuclear layer and some ganglion cells showed reduced basophilia and appeared pale and occasionally some dense clumps of fine granules (DC) were seen in the neurones of the inner nuclear layer in these mice. A few small spherules were seen in the extracellular spaces. Some infiltrating cells were seen in the retinas in all ages of mice. We suggest that SFV causes retinopathy in baby mice and the neurophysiological changes reported in adult mice may be contributed to by virus replication in the retinal neurones and the presence of infiltrating cells in the retina.