Local Malaria Transmission in the United States

A recent health alert from the Centers for Disease Control and Prevention (CDC) has raised concern regarding local malaria transmission in the United States. Malaria is a potentially lethal disease and is a true medical emergency requiring prompt diagnosis and treatment. Unfortunately, the cases identified by the CDC are from locally transmitted clusters, which has serious implications.

Historic Impacts of Malaria in the United States

Through the 19^th century, malaria was one of the most significant endemic diseases impacting the United States. Most individuals in the Anopheline mosquito range were chronically infected with the malaria-causing plasmodium. Malaria was also one of the leading causes of death and resulted in almost incalculable health and economic impacts. However, malaria was eradicated as an endemic disease in the United States in the 1950s.

Current State of Malaria Globally

Malaria now accounts for more than one million annual deaths globally. New global malaria cases range from 300 to 500 million each year – even with prevention and treatment efforts. About 95% of these cases are in Africa. Although there are other diseases with greater short-term impacts, estimations are that malaria accounts for more long-term health and economic impacts than almost any other infectious disease. Infected people serve as the reservoir for human malaria.

Current State of Malaria in the United States

Before the pandemic, all of the 2,000 or so malaria cases seen in the United States annually were imported from endemic regions of the globe. Around 15% of these people developed severe disease and 1.6% to 3.3% died. The pandemic disrupted the seasonal characteristics of many infectious diseases, including malaria. However, many diseases are starting to resume similar pre-pandemic characteristics. As such, malaria is typically diagnosed during the summer and early fall. In addition, the current travel surge in the United States indicates a potentially congruent surge of imported malaria.

The Anopheles Mosquito and Malaria

The Anopheles mosquito serves as the only natural insect vector for malaria. These mosquitoes are endemic in the United States. While experts have been able to control the mosquito population density, they can never be completely eradicated, and unfortunately, the range is expanding.

Why is the Range of Anopheles Expanding?

Persistent warming of the climate is the primary factor affecting expansion. The southern, southeastern and northeast regions of the United States are increasingly experiencing such a climate shift, with the detection of the Anopheles mosquito progressing north of their previously known range. This is increasing the risk in the highly populated northeast and northern seaboard regions. In addition, most areas of the country no longer have active mosquito control protocols or programs, particularly in regions previously without the Anopheles mosquito as a resident species. Finally, human populations do not have a good knowledge base or an adequate awareness of mosquito control and bite prevention.

The Current Outbreak of Local Malaria Transmission

Over the last few weeks, five new locally transmitted malaria cases have been reported (four in Florida and one in Texas). In addition, the Florida and Texas cases are not related – but come from their local reservoirs of infected people. This raises the additional concern that the conditions for local transmission of malaria are not limited to a clustered location, but rather extend over a much wider geographic region, which makes epidemiologic tracking, case identification and locating any patients serving as reservoirs more difficult.

What are the Important Implications of Local Malaria Transmission?

The primary concerns include increased numbers of individual malaria cases and the risk of reestablishing endemic malaria in the United States. Because the malaria-transmitting mosquito is already a resident species, any infected individual or group represents a potential reservoir allowing local transmission. Reestablishment of endemic malaria would bring along the associated serious health and healthcare system impacts of this disease.

Malaria Clinical Presentation

The clinical manifestations of malaria are usually nonspecific and include fever, chills, headache, myalgia, arthralgia and fatigue. In addition, nausea, vomiting and diarrhea may occur. Most people develop symptoms within 10 days to four weeks of mosquito bite, but initial symptoms may start seven days to one year after exposure. Altered mental status, seizures, renal failure, acute respiratory distress syndrome, hepatic insufficiency and coma indicate progression to severe disease. A diagnosis of severe malaria carries a 90% risk of death for patients who remain at home and 20% for hospitalized persons. The initial presentation may be mild enough to be obscured. Many cases of malaria may also be asymptomatic.

Malaria Diagnosis

Diagnosis should be made within 24 hours of presentation to reduce morbidity and mortality. While clinicians should take a travel history, it should not be a deciding factor before including malaria in the differential. Rather, the clinician should consider malaria in the differential of all patients presenting with an acute febrile illness. Laboratory findings are critical. General laboratory abnormalities can include anemia, thrombocytopenia, hyperbilirubinemia and elevated transaminases. Critical diagnostic tests include microscopic examination of thin and thick blood smears, and a rapid diagnostic test (RDT) to diagnose malaria as soon as possible. More specific diagnostic criteria can be found here.

Initial Management of Malaria Patients

Malaria is a true medical emergency. Treatment should be initiated as soon as practical. Patients should be admitted for diagnostic evaluation, infectious disease consultation and initiation of treatment. Malaria treatment depends on the species of plasmodium, the severity of the disease, the potential transmission location and any known drug resistance likely to be present. More treatment recommendations can be found here.