An unexpected human outbreak of the mosquitoborne Sindbis virus occurred in a previously nonendemic area of Sweden. At follow-up, 6-8 months after infection, 39% of patients had chronic arthralgia that affected their daily activities. Vectorborne infections may disseminate rapidly into new areas and cause acute and chronic disease.
Keywords: Ockelbo disease; Sindbis virus; Sweden; alphavirus; chronic arthritis; outbreak; vector-borne infections; viruses.