Can a roundworm infection cause encephalitis?

Unlikely. I believe this is unlikely. I don't believe ascaris lumbricoides can travel into the brain, as it is fecally transmitted, and the parasite and its eggs do not enter the bloodstream.
Yes! Angiostrongyloides cantonensis can cause an eosinophilic meningitis or meningoencephalitis. Gnathostoma larvae, another nematode, can cause an eosinophilic encephalomyelitis. Even in the continental usa, larvae from the raccoon round worm causes a severe encephalitis.