Malaria in the US

Malaria should stay on the differential for fever after travel to endemic regions, and recent locally acquired U.S. cases show it is not purely an imported disease. Severe disease is most associated with Plasmodium falciparum, while Plasmodium vivax can relapse years later because of dormant hepatic hypnozoites.

Recognizing Malaria in U.S. Practice

• Travel and exposure clue: Fever with chills, headache, myalgias, and nausea or vomiting after travel to a malaria-endemic region is malaria until proven otherwise, and the recent U.S. cluster involved locally acquired Plasmodium vivax.
• Severe disease patterns: Altered mental status, shock, pulmonary edema, renal failure, or metabolic acidosis should raise concern for severe malaria, especially with Plasmodium falciparum and in patients without prior malaria exposure.
• Vector and incubation window: Female Anopheles mosquitoes transmit malaria, typically feeding from dusk to dawn, with symptoms emerging about 7 to 30 days after the infective bite.
• Relapsing species behavior: Plasmodium vivax and Plasmodium ovale can reactivate from latent liver hypnozoites years after the initial infection, a distinction worth hearing in the episode.
• U.S. epidemiology shift: Most U.S. malaria cases are still travel-associated at roughly 2,000 per year, but Florida and Texas both reported unrelated local transmission in 2023.

Diagnosis, Treatment, and Prevention

• Smear-based diagnosis first: Thick and thin blood smears remain the core diagnostic test for malaria because they detect parasitemia and help identify species, with repeat sampling improving yield.
• Rapid test role: Rapid diagnostic antigen assays can identify malaria from a small blood sample faster than PCR, making them useful when immediate bedside suspicion is high.
• PCR confirmation limits: Malaria PCR is species-specific and widely available in many labs, but turnaround is slower and mixed infections can still create interpretation challenges.
• Species-specific therapy planning: Treatment depends on the Plasmodium species and travel context, and infectious disease consultation plus CDC guidance are the safest starting points. We get into the practical workflow in the chapter.
• Prevention at the bedside: Mosquito avoidance is foundational: insecticide-treated bed netting, protective clothing, repellents, and travel chemoprophylaxis remain the most reliable prevention tools.
• Vaccine reality check: Malaria vaccines now reduce severe disease burden in children, with RTS,S lowering severe-malaria hospitalizations by about 30% and R21 reaching the WHO 75% efficacy target.

References:  

1. Kimberlin, D. W., Barnett, E. D., Lynfield, R., & Sawyer, M. H. (2021). Red Book: 2021-2024 report of the Committee on Infectious Diseases. American Academy of Pediatrics. 
2. Centers for Disease Control and Prevention. (2023, June 28). CDC - Malaria - diagnosis & treatment (United States) - treatment (U.S.) - guidelines for clinicians. Centers for Disease Control and Prevention. https://www.cdc.gov/malaria/diagnosis_treatment/clinicians1.html 
3. World Health Organization. (2023, October2). WHO recommends R21/matrix-M vaccine for malaria prevention in updated advice on immunization Centers for Disease Control and Prevention. World Health Organization. https://www.who.int/news/item/02-10-2023-who-recommends-r21-matrix-m-vaccine-for-malaria-prevention-in-updated-advice-on-immunization#:~:text=The%20R21%20vaccine%20is%20the,have%20high%20public%20health%20impact