A Locally Acquired Case of Malaria in Arkansas, 2023

For 20 years no one in the United States was recorded catching malaria from a local mosquito. Then, from May to August 2023, nine such cases were reported to CDC. In September 2023 a tenth was diagnosed, in Saline County, Arkansas.

The case

On September 28, 2023, a previously healthy Saline County resident went to a local hospital after 10 days of headache, fever, chills, night sweats and fatigue, and a day of nausea and vomiting. The patient had never traveled abroad and had no history of transfusion, transplant or other bloodborne exposure. Tests showed anemia, thrombocytopenia and hyperbilirubinemia, and the patient was admitted for a possible hematologic malignancy. As the anemia and thrombocytopenia worsened (hemoglobin 7.3 g/dL; platelets 14 K/µL), the patient received a unit of red cells and a unit of platelets.

On September 30 a repeat blood smear showed ring forms suggesting malaria, and a rapid diagnostic test confirmed a non-falciparum Plasmodium seven hours later. With the species and parasite level not yet known, the patient was started on intravenous artesunate. A pathologist then found P. vivax/P. ovale ring forms and gametocytes, with 0.26% of red cells infected, and the patient was switched to artemether-lumefantrine, plus primaquine to prevent relapse. CDC confirmed the parasite as P. vivax by microscopy and PCR. A smear taken before the transfusion already showed the parasites, ruling out transfusion as the source. The patient completed treatment and fully recovered.

The response

The Arkansas Department of Health (ADH) led the response with CDC's support.

• Investigation. The patient had never been abroad and had no other risk factors. The places they had spent time outdoors in the four weeks before falling ill did not overlap in place or time with any of Arkansas's 2023 imported malaria cases, all of which were P. falciparum.
• Case finding. Everyone in the household was well. From October 9, ADH searched hospital records through the ESSENCE syndromic surveillance system, adapting a query from the Florida Department of Health; only three of eight local hospitals in Saline County and Little Rock were fully connected. Over 9 weeks from the patient's first symptoms, eight possible cases turned up, and all had other diagnoses.
• Mosquito surveillance. From October 5 to 24, ADH trapped mosquitoes at two sites near the patient's home and where they had spent time outdoors, running five light traps (with and without carbon dioxide, which attracts mosquitoes) at each site on seven nights. Of 244 mosquitoes caught, 25 were female Anopheles; CDC tested them all, and none carried Plasmodium.
• Mosquito control. The town had no routine mosquito control, so a neighboring municipality lent insecticide, an ultra-low-volume sprayer and a truck. From October 5 to 11 crews sprayed on five nights — first within a mile of the patient's home, then in widening one-mile rings, then at likely breeding sites — using permethrin with piperonyl butoxide. Spraying stopped because resources ran short and cooling weather reduces mosquito activity.
• Hospital readiness. Seven of eight local hospitals could read thick and thin blood smears and two used rapid tests; the state public health laboratory offers smear consultation and ordered 15 boxes of rapid test kits. Three hospitals stocked artemether-lumefantrine, the first-line drug for uncomplicated malaria, but none stocked IV artesunate, the first-line drug for severe malaria.
• Outreach. ADH issued a press release on October 4, a Health Advisory to clinicians, hospitals and labs on October 5, and held a clinician webinar on October 11.

As of September 2024, no further locally acquired cases had been found in Arkansas.

Why it can happen

U.S. malaria cases rose from 1972 to 2019, reaching 2,048 in 2019, most tied to travel to 85 countries where malaria is still endemic. Anopheles mosquitoes live across the United States and can pick up the parasite from a traveler with malaria, then pass it to someone who has never traveled. The source of the Arkansas case is unknown; local mosquitoes may have fed on a person with undiagnosed travel-related malaria nearby.

What clinicians and travelers should do

• Think of malaria in anyone who has traveled to an endemic country — or who has unexplained fever, whatever their travel history.
• Diagnose carefully. Thick and thin blood smears are the gold standard, but few U.S. labs follow every Clinical and Laboratory Standards Institute guideline, which can delay diagnosis. Rapid tests help but are less sensitive and cannot reliably identify the species or measure parasite levels, so they should be used with microscopy.
• Be ready to treat. Stocking IV artesunate could speed treatment of severe malaria and reduce deaths; emergency supplies are also available. Before primaquine or tafenoquine, test G6PD levels, since deficiency risks hemolytic anemia. CDC's Malaria Hotline gives clinical advice.
• Report cases. Malaria is nationally notifiable, and hospitals that join state syndromic surveillance help find cases.
• Travelers to malaria areas should ask their provider about preventive medicine and avoiding mosquito bites.

Sources

• Courtney AP, Boyanton BL Jr, Strebeck PV, et al. "Locally Acquired (Autochthonous) Mosquito-Transmitted Plasmodium vivax Malaria — Saline County, Arkansas, September 2023." MMWR 73(42). CDC. The report's blood smear photographs, credited to Arkansas Children's Hospital, are not reproduced.