(CNN) Florida health officials are warning of an uptick in a mosquito-borne virus known as Eastern equine encephalitis (EEE).

Several sentinel chickens tested positive for EEE, which can spread to humans via infected mosquitoes and cause brain infection and swelling, the Florida Department of Health in Orange County said in a Thursday statement. Sentinel chickens are fowl that are tested regularly for the West Nile virus and EEE. Their blood can show the presence of the diseases, but they don't suffer from the effects of the viruses.

Following the positive tests for the sentinel chickens in Orange County, the health department said "the risk of transmission to humans has increased."

Only about seven cases of the EEE virus in humans are reported in the US each year, the Centers for Disease Control and Prevention says

However, the disease can be fatal: about 30% of people who contract it die, according to the CDC. Many survivors have ongoing neurologic problems.

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