Last summer Arizona had an outbreak of St. Louis encephalitis virus, which is similar to West Nile virus. Both are spread most commonly by the bite of an infected mosquito, but other possible routes of infection include organ transplants, blood transfusions, or needle sticks in laboratories. Public health was notified that one person in Arizona diagnosed with St. Louis encephalitis had recently received a blood transfusion. Disease detectives at federal, state, and local agencies investigated, performed testing on samples, and coordinated with environmental services to collect mosquito testing data.
These efforts led officials to confirm that this is likely the first documented case of transmission of St. Louis encephalitis from a blood transfusion and not a mosquito. Public health tracked down the samples to ensure no one else was at risk from the donated blood. Currently, donated blood products are screened for West Nile virus (because of previous proof of transmission and availability of an FDA-approved screening test), but not St. Louis Encephalitis virus.