A seventh person has been diagnosed with a locally acquired case of malaria in Florida's Sarasota County, state health officials reported this week.

The rare outbreak is now in its third month after authorities in the Sunshine State reported the first case in May. When Florida had identified four cases by late June, the Centers for Disease Control and Prevention released a health alert to clinicians calling locally acquired malaria cases in the US a "public health emergency."

Florida's outbreak and a single unrelated case in Texas from June collectively mark the first time in two decades that the US has seen locally acquired malaria cases, which, if left untreated, can be deadly. In the last instance, in 2003, Florida officials reported a small outbreak of at least seven people in Palm Beach.

The US usually sees around 2,000 malaria cases each year, usually all in people who had recently traveled from areas where the mosquito-borne pathogen is endemic. Outbreaks are uncommon but usually limited in scope. The mosquitoes that transmit malaria parasites, in the Anopheles genus, are typically evening and nighttime biters. Transmission is significantly hindered by people having screened windows and air conditioning. Thus, outbreaks in the US tend to fizzle out.

But, the Florida outbreak appears to be stubbornly hard to smack down. The Sarasota area has been under a mosquito-borne illness alert since June 19. In the last week of June, the area reported two new cases, bringing the total at the time to six. In the first week of July, there were no new cases, offering hope that the applications of insecticide by aircraft, vehicles, and crews had successfully knocked out the parasite's spread. But, this week, authorities identified a new case.

According to the Sarasota Herald-Tribune, authorities began investigating the new case last Thursday, and testing by the CDC has since confirmed the infection. So far, all of the cases have been in the Desoto Acres and Kensington Park areas in north Sarasota County, according to Wade Brennan, the manager of Sarasota County Mosquito Management Services. Brennan told reporters Tuesday night, "We still need everybody to be diligent about avoiding mosquito bites."

The mosquito management officials have been trapping mosquitoes and sending them off for testing at the CDC. Of the 130 or so tested so far, three mosquitoes have been positive for the malaria parasite Plasmodium vivax, but those were all from early in the outbreak. No mosquitoes have tested positive since June 5, officials said.

P. vivax causes a relatively milder type of malaria than some other Plasmodium species, namely P. falciparum, the deadliest cause of the disease that is concentrated in some African countries. However, P. vivax is known for going dormant in the liver, leading to chronic and relapsing disease if left untreated. People infected are typically given an intravenous antimalarial treatment for several days to kill the parasite in the blood and then a longer oral course to kill parasites in the liver.

Dr. Manuel Gordillo, an infectious disease specialist at Sarasota Memorial Hospital, told NBC News that some of the local cases have required more extensive treatment.

“They are dehydrated, some of them. They have low blood counts, especially platelets, which puts them at risk for bleeding. Some of them have had renal kidney failure, which is one of the complications of malaria,” Gordillo said.

State health officials did not immediately respond to Ars' request for more information about the cases and the state of the outbreak.