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Florida and Texas are seeing some locally acquired cases of malaria - the first spread of the mosquito-transmitted disease inside the US in 20 years, officials warn in a health alert.

Active surveillance for more cases is continuing, the says.

The risk of catching malaria in the US remains extremely low, it says.

All five patients - four in , one in - have now had treatment.

Malaria is caused by being bitten by an infected mosquito. People cannot catch it from each other. But the insects catch it from infected people - and the cycle continues.

It is common in large areas of Africa, Asia and Central and South America but not the US.

However, Anopheles mosquitoes, found throughout many parts of the US, can transmit malaria, if they have fed on an infected person.

The risk is higher in areas where:

Infected people can suffer fever, sweats and chills. Malaria is an emergency and must be treated quickly with drugs to kill the parasite that causes the infection.

Using insect repellent and covering up can help protect against mosquito bites.

The CDC says it is working with the Florida and Texas health departments and those recently diagnosed and treated "are improving".

US doctors are being advised to consider malaria in any person with an unexplained fever, regardless of international travel history, particularly if they have visited or live in the affected areas of Florida or Texas.

Florida has issued a mosquito-borne illness alert after cases were discovered in Sarasota County and Manatee County, warning residents to drain standing water where mosquitoes can breed and wear long-sleeved shirts and pants.