Malaria Resurgence in the US: Understanding the Past and Preparing for the Future

Apr 25, 2024

Malaria has a long history in the United States. Back in the mid-20th century, malaria posed a major threat to American lives, particularly in southern regions of the country. Through concerted efforts by the Centers for Disease Control and Prevention, including the elimination of mosquito breeding grounds, the use of insecticides and improved public health measures, malaria was considered successfully eliminated from the U.S. by 1951.

Fast forward to today: malaria is once again making headlines in the United States. On average, 2,000 to 2,500 cases of travel-associated malaria are reported annually in the U.S. among individuals who have returned from travel to malaria-endemic regions like sub-Saharan Africa as well as parts of South America and Southeast Asia. However, for the first time since 2003, nine locally-transmitted cases of malaria have been reported in three states: Florida, Maryland and Texas. This has prompted public concern and is compelling experts to closely examine the situation. 

The Culprits: Mosquitoes and Parasites

Malaria transmission is a complex process involving Anopheles mosquitoes and the malaria parasite, influenced by various factors. Female Anopheles mosquitoes must feed on blood to mature their eggs, potentially allowing them to transmit the parasite to multiple individuals. Different Anopheles species have varying preferences for feeding habits, some primarily biting humans and some targeting other mammals. Moreover, several malaria-causing parasites exist; Plasmodium falciparum, the deadliest, is primarily prevalent in sub-Saharan Africa. All this complexity makes malaria transmission a puzzling process.

How Malaria Found Its Way Back to the U.S.

Anopheles mosquitoes capable of carrying malaria are still present in the U.S.—they have just had very few opportunities to transmit the parasite due to the scarcity of infected people to feed on. Experts believe that the recent surge in locally-transmitted cases resulted from a specific set of circumstances: an infected individual traveled from a malaria-endemic region and was then bitten by a local Anopheles mosquito, which picked up the parasite and passed it on to someone else. While one case in Maryland has been confirmed to be linked to P. falciparum, those in Florida and Texas are believed to be linked to P. vivax, likely originating from travelers returning from South America.

Symptoms and Treatment

Malaria manifests with flu-like symptoms, including fever, body aches, vomiting and chills. Clinicians should consider malaria as a potential cause of unexplained fevers, especially in patients with a recent international travel history. A key presentation of infection is the occurrence of recurring fever cycles, corresponding to the parasite’s life cycle in the bloodstream. When a patient is suspected of having malaria, it is critical to ensure they receive immediate evaluation at a healthcare facility capable of providing rapid diagnosis and treatment, ideally within 24 hours of their initial presentation. Hospitals must have a well-prepared plan in place to facilitate rapid malaria diagnosis and treatment. Swift treatment is imperative, as untreated malaria can progress to severe or cerebral malaria, with high mortality rates.

Guidelines for Laboratories

The clinical features of malaria can overlap with other febrile illnesses, underscoring the importance of confirming the diagnosis through parasite-specific laboratory tests. Laboratories, especially those affiliated with healthcare facilities, should have malaria diagnostic tests readily available, along with qualified staff to perform and interpret these tests. In cases where these tests are not readily accessible, patient samples should be sent to a reference laboratory for prompt evaluation.

There are various methods for laboratory diagnosis of malaria, including microscopic examination of stained blood films, rapid detection tests (RDTs) for parasite antigen and nucleic acid amplification tests (NAATs). Regardless of the method used, testing for malaria should be available 24/7 due to the potentially life-threatening nature of the infection. 

Microscopic examination, involving both thick and thin blood films, remains the gold standard for malaria diagnosis. Accurate diagnosis relies on proper blood collection, smear preparation and staining. Giemsa stain is recommended for species identification. In cases of low parasitemia, repeated blood smears may be necessary to confirm the diagnosis. To monitor therapeutic responses effectively, laboratory professionals can examine consecutive blood smears, quantifying parasites and observing a progressive decrease in percent parasitemia until complete parasite clearance is achieved. 

RDTs are lateral flow assays detecting specific parasite antigens. These can be useful in settings where microscopy is not readily available, but should be used alongside other diagnostic methods, especially for non-falciparum infections. NAATs provide high sensitivity for detecting low levels of parasitemia and confirming the infecting species, including mixed infections. However, their cost and complexity limit their widespread use. Additionally, unless NAAT can be performed within several hours of specimen collection, the laboratory must have an alternative method for rapid laboratory detection of malaria. Both RDTs and NAATs should not be used to evaluate therapeutic responses because malaria antigens and nucleic acids may remain in the blood stream after successful treatment.

While serologic testing for antibodies is generally not recommended for routine diagnosis, it may have specific uses, such as in cases of repeatedly smear-negative febrile patients with travel history or for screening blood donors.

Regardless of the method used, testing should be performed by experienced and qualified laboratory professionals. Quality assurance measures, including staff training, competency assessment and proficiency testing, are essential for reliable results.

Malaria is nationally reportable in the US, and all laboratory-confirmed cases should be reported to local health departments. Epidemiological and clinical data are transmitted to the CDC through the National Malaria Surveillance System. Proper diagnosis and reporting are crucial for effective malaria control and management.

Parting Thoughts

Currently, the U.S. is not conducive to a major malaria outbreak; however, worrying trends are emerging. Warmer winters have allowed Anopheles mosquitoes to reproduce earlier, increasing the likelihood of parasite transmission. Furthermore, heightened travel and globalization, including post-COVID surges, have expanded the pool of potential infection sources. Mosquitoes can hitch rides on ships and aircrafts, potentially introducing the disease to new locations. Whether this resurgence is an isolated occurrence or a precursor of future challenges remains uncertain. Only time will tell. Nevertheless, the next five years will be pivotal in assessing the threat of malaria in the U.S. 

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