Why are we seeing new malaria cases in the U.S.?

Manoj Duraisingh
The Big 3: Three questions, three answers

July 12, 2023 – On June 26, the Centers for Disease Control and Prevention reported several cases of locally acquired malaria in Florida and Texas, meaning that patients were infected within the U.S.—the first such cases in over two decades. Manoj Duraisingh, John LaPorte Given Professor of Immunology and Infectious Diseases at Harvard T.H. Chan School of Public Health, explains the significance of the cases and how individuals and public health officials should respond to the situation.

Q: Why are the recent malaria cases notable, and what do experts know so far about them?

A: Malaria is caused by Plasmodium parasites, which are transmitted from one person to another by the bite of a certain group of mosquitoes, the Anophelines. While endemic, or continuous transmission of malaria was eliminated from the U.S. in the 1950s, mosquitoes that can transmit malaria are still present.

The six cases in Florida and the single case in Texas are attracting attention because these are rare cases of local transmission within the U.S. In contrast, the approximately 2,000 annual cases of malaria that are usually reported in the U.S. are in individuals traveling back from malaria-endemic areas, where they were infected. There have been 24 such cases so far this year in Florida, for example. But imported malaria cases are seen in almost every state, suggesting that health professionals need to be vigilant, asking patients about their travel histories to determine whether they have traveled to malaria-endemic areas.

In terms of the locally transmitted cases, by far the most likely scenario is that local mosquitoes bit individuals who were infected in a malaria-endemic country and then traveled back to the U.S., and then the locally infected mosquitoes subsequently bit the unsuspecting patients. The individuals with the initial imported infections could have been symptomatic or asymptomatic.

The cases within Florida appear to cluster in one county—six cases were reported there over the course of a month, and infected mosquitoes have now been found in the area. However, there appears to be no evidence that the cases between Florida and Texas, two geographically distant states, are linked. Genomic studies of the parasites will help to track their origins.

All of the patients were infected with the parasite species Plasmodium vivax. This is the most widespread of the different malaria parasites that infect humans and is very resilient to public health elimination efforts. While it can sometimes cause severe disease, it is rarely fatal. However, Plasmodium vivax presents a challenge in that it can remain dormant in an infected individual’s liver. If the parasites are not initially killed by intensive treatment, they can be reactivated several years later, resulting in new infections.

Q: Why do you think these locally acquired cases are popping up now in the U.S.?

A: For successful local transmission, you need several ingredients: an infected person, mosquitoes that can transmit the parasite, temperatures warm enough for the parasite to develop in the mosquito, and for the mosquito to bite an exposed and susceptible human. All these conditions are met very rarely in the U.S.

The current cases are most likely due to the fact that international travel has picked up again this season, meaning that there are infected individuals to seed local transmission. Another factor is a likely increase in mosquito populations in the hot and humid areas where the cases occurred. Importantly, the cases highlight the potential for a return of malaria in the future. For instance, higher temperatures driven by climate change could significantly increase mosquito populations, and we should be prepared at a public health level.

Q: What should individuals and public health officials do to reduce the risk of malaria?

A: First of all, we should remember that the chance of becoming infected with the malaria parasite in the U.S., including Florida and Texas, remains extremely low. Nevertheless, individuals who work outdoors and are exposed to mosquitos are at the highest risk, particularly at dawn and dusk when Anophelines tend to bite. Those living without adequate insect screens or air-conditioning in their homes would also be more exposed.

People can adopt well-known methods to avoid being bitten, for example using mosquito repellents and wearing long sleeves—all are tried and tested. They can also eliminate small bodies of water where mosquitos can breed, such as ponds and birdbaths in their backyards, as well as fix any holes in insect screens.

On a broader level, malaria is very unlikely to become endemic in the U.S. Public health efforts will be focused on stamping out these flare-ups, as they have successfully done in the past. Some cases of malaria can be severe and the disease can progress quickly, and hence prompt diagnosis and treatment is critical. Active surveillance for additional cases is important, and immediate access to intravenous artesunate—the first-line drug for treating severe malaria—should be ensured. Most important will be the reduction of mosquito populations in the hotspot areas by spraying insecticides, both by air and on the ground. This action will greatly reduce the chance of transmission and will have the added benefit of reducing the general nuisance of Anopheline mosquito bites.

Jay Lau