Three mosquitoes collected near Sarasota, Florida, have tested positive for malaria amid an unusual cluster of locally acquired cases. It is the first time in two decades that US mosquitoes have tested positive for malaria in connection to US-based cases.

Four cases have so far been confirmed in Florida, all in close geographic proximity, health officials reported on Monday. The Sarasota Herald-Tribune reported Wednesday that officials are investigating a possible fifth case. //

The mosquitoes that transmit the malaria parasite are of the Anopheles genus. There are multiple species of Anopheles mosquitoes in the US, which have been found in at least 32 states. //

The cluster of locally acquired malaria cases around Sarasota—as well as an unrelated case in Texas—marks the first time the parasite is known to have spread in the US since 2003, when there was a cluster of cases in Palm Beach, Florida. In that outbreak, no captured mosquitoes were found to be positive for malaria.

However, an investigation around a cluster of locally acquired cases in Loudon County, Virginia, in 2002 turned up malaria-positive mosquitoes. It was the first time since 1957 that US mosquitoes linked to locally acquired cases had tested positive for a malaria parasite.