In late May, in Sarasota County, Florida, health officials confirmed they had identified a locally transmitted case of malaria. In mid-June, they confirmed the second. On June 23, Texas joined in: its state health department announced that it had confirmed a case of local malaria transmission in Cameron County.
This is all very unusual. The United States has not documented a case of locally acquired malaria in 20 years.
Although approximately 2,000 people infected with malaria present themselves to the US healthcare system each year, these cases are all linked to travel outside the United States. Neither the people involved in the Florida cases nor the Texas case had travelled. This means that in both states the infection was acquired within US borders.
Experts say the three cases should not warrant panic over widespread malaria transmission in the United States. But this justifies asking some questions and being wary of the threat of more local transmission. Mosquitoes can infect multiple people even before a full outbreak is identified – so there could be more cases.
Even if it turns out not to be widespread, it’s a good reminder: Malaria could be making a comeback in the United States, and we — and our public health infrastructure — need to be prepared. This is especially true as a changing climate and changing weather patterns are increasingly causing mosquitoes to migrate to new places around the world, allowing malaria to take hold where it has not. previously.
These cases are not the worst form of the disease, but they are not mild
Malaria comes in a variety of flavors, all in the genus Plasmodium, and all parasites that infect and kill red blood cells. The resulting anemia is the primary cause of much of the bodily havoc that ensues.
The species that has been identified in Florida and Texas is P. vivax. Not the Worst Malaria Species: P. falciparum, the most serious form of malaria, is 10 times more deadly than vivax, according to an American study diagnosed between 1985 and 2011. But vivax is not an easy task. People with this infection can develop life-threatening brain swelling, lung congestion, and kidney failure.
The infection causes fevers that come and go, as well as a wide range of symptoms that can be mistaken for the flu, a stomach bug, or liver disease.
Vivax malaria is also sneakier than some of its counterparts: the parasite can lie dormant in the liver for years after initial infection, rearing its head long after exposure. (Globally, most vivax malaria infections occur in eastern Africa, southern Asia, and northern South America.)
Malaria was a huge problem in the United States until it was eradicated in the 1950s, largely by spraying homes and environmental areas with the insecticide DDT to kill the mosquitoes that spread it. (All forms of malaria are spread by Anopheles mosquitoes, and the United States still has plenty of them.)
Also keeping malaria at bay: the mosquitoes that transmit the disease like to bite at night – and as American homes have increasingly incorporated mosquito nets and air conditioning, those inside have been better protected from food for this particular kind.
So malaria is no longer something you would expect to get in the United States. But when things line up perfectly, all the ingredients are in place for malaria transmission to occur in the United States.
What does it take for malaria transmission to occur in the United States?
A big unanswered question right now is: Why are two geographically distant areas of the United States currently seeing local malaria transmission, especially after so many years without it?
Although both states see many travel-related cases of malaria each year, these cases do not usually lead to local spread. Will anything be different this year? Or is it just a coincidence?
To think about the possibilities, it helps to understand how malaria spreads.
Malaria is transmitted when a female Anopheles mosquito bites an infected person, then a week later bites an uninfected person. Between bites, the mosquito does mosquito business in warm, still water – but it’s picky about which water it prefers. These malaria-spreading mosquitoes typically like to breed in bodies of water with vegetation growing along the banks, Wade Brennan, a Sarasota County mosquito officer, wrote in an email. In other words, it prefers the forest to puddles near human habitation, such as those you might find in an empty bucket or garbage can outside your home.
To trigger local transmission, a person who has contracted P. vivax malaria abroad must come close enough to the habitat of an Anopheles mosquito to be bitten.. During the following days, the vivax infection is brewing in the mosquito’s intestine. About a week later, he is ready to infect another person. When the mosquito bites its next victim, particles of its saliva mix with the person’s blood – and before the mosquito lets go, some of these particles are injected into the person.
A few weeks later, this second person falls ill. During this time, the mosquito still has a few weeks to live – and during this time it is still feeding on other people, potentially infecting them as well. But even if he dies, his friends could bite this newly infected person and pass on the infection.
So: The basic elements of malaria transmission are a source (the infected returning traveller), a vector (the mosquito), an uninfected target (the newly identified cases) and an environment that allows them to come into contact . An increase in any of these could make local transmission more likely.
A robust public health system helps understand the causes in cases like this and is essential to the response
It is not yet clear whether changes in mosquito populations or in human behavior are the reason for this current wave of locally acquired cases. But public health investigations in both states have already identified risk factors for those infected — and environmental reservoirs of infection.
Michael Drennon, an epidemiologist with the Sarasota County Health Department, could not share demographic or location information on the two locally infected people. However, he noted that the two were adults who spent a lot of time outdoors at night and hadn’t traveled anywhere outside the United States. A representative from the Texas Department of State Health Services said the case identified in that state was an adult who was working outdoors.
Brennan said her Sarasota County Mosquito Management Team found malaria-infected mosquitoes in an area swamp and focused prevention efforts there, applying insecticides that kill both forms adults and juveniles of the mosquito. “We were able to ascertain that the mosquito population in this area is extremely low,” he wrote.
It is not known whether more Anopheles mosquitoes than usual are circulating in either state. More mosquitoes would increase the chances of an infected person’s parasites spreading to an uninfected person – and make the disease harder to eradicate.
Overall, the environment in the United States is increasingly conducive to growing mosquito populations, which can increase the risk of malaria transmission. Climate change “certainly plays a role in vector-borne diseases” across the United States, said Estelle Martin, an entomologist at the University of Florida in Gainesville who studies mosquito-borne diseases. Over the past two decades, rising temperatures and extreme weather conditions have favored the replication of mosquitoes – and they also favor the replication of malaria parasites. But it is unclear exactly what role this dynamic plays in these three cases.
A spokesperson for the Centers for Disease Control and Prevention wrote in an email to Vox on Friday: “While we generally know that climate can be one of many factors that can impact vector-borne diseases , in this situation, there is no compelling reason to think so.” The forces of migration were more likely to be at play, the agency said: “Today, global travel and trade allow vector-borne diseases to move around the world and be transmitted by mosquitoes. or local ticks, especially in places where these diseases may have once been common.
Now is not the time to panic, but it’s time to protect yourself
Even though several people have been infected and there are mosquitoes with the parasite in their bellies, that does not mean that the United States is heading for an explosive epidemic of malaria. “It’s always worrying to have local transmission in an area,” Martin said, but added there was no need to panic. And there are many things people can do to protect themselves.
It’s important that people do their best to avoid mosquito bites, Drennon said. He advises people to wear long sleeves and cover their legs – which he says is difficult in the Florida heat – and to use mosquito repellent.
Eliminating mosquito hangouts is also very important: although Anopheles mosquitoes do not usually congregate near houses, he advises people to drain any standing water and eliminate places where water can collect. accumulate because other mosquitoes that spread other infections can breed there. “It’s not uncommon to see dengue fever here,” he said; during the Zika outbreak in 2016-17, the state was also one of two places in the United States where this virus was transmitted locally.
It is also important that local health care providers are aware of the local malaria risk. Mosquitoes can infect several people even before the epidemic is identified. So, once a case is detected, public health officials need to spring into action, alerting health workers, educating the public about prevention, and coordinating search and kill missions for mosquitoes.
Sarasota County’s first case was initially diagnosed while the patient was being evaluated for a fever; blood test abnormalities prompted further testing, Drennon said. Area health care providers were alerted to the case and the second case was reported shortly thereafter.
Even if these epidemics prove to be a jolt, the threat of a stronger grip of malaria in the United States remains. “We know we have people who travel all over the world who may end up here with malaria,” Drennon said. “And we know we have the vector species here that can transmit it.” It is essential that the public “take precautions to prevent not only malaria, but also other mosquito-borne diseases that we have in Florida.”