The Centers for Disease Control and Prevention (CDC) issued a health alert on Monday, June 26, warning about five locally spread malaria cases. The alert marks the first such cases in the United States since 2003.

“In Florida, four cases within close geographic proximity have been identified, and active surveillance for additional cases is ongoing. Mosquito surveillance and control measures have been implemented in the affected area. In Texas, one case has been identified, and surveillance for additional cases, as well as mosquito surveillance and control, are ongoing. All patients have received treatment and are improving,” the CDC said in the health alert. “There is no evidence to suggest the cases in the two states (Florida and Texas) are related.”

About 2,000 U.S. cases of malaria are diagnosed each year. The vast majority of cases are in travelers coming from countries where malaria commonly spreads.

According to the CDC, there has been 11 outbreaks involving locally spread malaria cases since 1992. The last one occurred in 2003 in Palm Beach County, Florida, where eight cases were reported.

Malaria is caused by a parasite that spreads through mosquito bites. Infected people can suffer fever, chills and flu-like illness.

If it goes untreated, infected people can develop severe complications and die. The largest death toll in recent years has been seen in children in sub-Saharan Africa.

“Malaria is a medical emergency and should be treated accordingly,” the CDC said. “Patients suspected of having malaria should be urgently evaluated in a facility that is able to provide rapid diagnosis and treatment, within 24 hours of presentation.”

The CDC is warning doctors, especially those in southern states where the weather is more friendly to the type of mosquito that spreads malaria, to be aware of the possibility of infection. The centers said doctors should also think about how to access the IV drug that is the first-line treatment for severe malaria in the U.S.

“Despite these cases, the risk of locally acquired malaria remains extremely low in the United States,” the CDC said. “However, Anopheles mosquito vectors, found throughout many regions of the country, are capable of transmitting malaria if they feed on a malaria-infected person. The risk is higher in areas where local climatic conditions allow the Anopheles mosquito to survive during most of or the entire year and where travelers from malaria-endemic areas are found.”