Balamuthia mandrillaris, an opportunistic agent of granulomatous amebic encephalitis, infects the brain via the olfactory nerve pathway.

Balamuthia mandrillaris is a free-living ameba and an opportunistic agent of lethal granulomatous amebic encephalitis (GAE) in humans and other mammals. Its supposed routes of infection have been largely assumed from what is known about Acanthamoeba spp. and Naegleria fowleri, other free-living amebae and opportunistic encephalitis agents. However, formal proof for any migratory pathway, from GAE patients or from animal models, has been lacking. Here, immunodeficient mice were infected with B. mandrillaris amebae by intranasal instillation, the most likely natural portal of entry. By means of classical and immunohistology, the amebae are shown to adhere to the nasal epithelium, progress along the olfactory nerves, traverse the cribriform plate of the ethmoid bone, and finally infect the brain. A similar invasion pathway has been described for N. fowleri. The data suggest that the olfactory nerve pathway is a likely route for natural infection of the brain by B. mandrillaris amebae.