Malaria is a parasitic infection of red blood cells by Plasmodium species. The route of infection is via the bite of an infected female Anopheles mosquito. Less commonly, malaria may be transmitted via transfusion of blood products or by vertical transmission. In the United States, physicians encounter malaria as one of most common causes of acute febrile illness in travelers returning from endemic areas, especially sub-Saharan Africa and South Asia. In addition, cases of malaria in the United States have increasingly been found in refugees relocating from sub-Saharan Africa to the United States.
Approximately 1500 cases of malaria are reported in the United States each year. Most are diagnosed in travelers returning from malaria-endemic regions (from high to low risk: West Africa, Oceania, other parts of Africa, South Asia, South America, Central American, and other parts of Asia). Five Plasmodium species are responsible for human disease: Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, Plasmodium malariae, and Plasmodium knowlesi. Geographic distribution of each species is different. Plasmodium falciparum is more likely to be identified as a cause of malaria in travelers returning from Africa, and P vivax is more likely to be identified as a cause of malaria in travelers returning from Asia, Central America and the Caribbean, or South America. Plasmodium knowlesi is a simian malaria species that has increasingly been recognized as the cause of human infection in Malaysian Borneo and surrounding countries. Coinfection with more than one species of malaria can occur in regions where multiple Plasmodium species are present.
The typical incubation period is 9-18 days, except for P malariae, for which it is longer (18-40 days); 80% of malaria cases with known onset of illness reported symptoms within 30 days after return from travel (90% of cases with P falciparum but only 54% of cases with P vivax).
Symptoms of uncomplicated malaria are nonspecific and include fever, headache, back pain, chills, sweating, myalgia, nausea, vomiting, diarrhea, and cough. Differentiating malaria from other infectious and non-infectious diseases is difficult if based on clinical symptoms alone.
The fever may become paroxysmal, preceded by rigors for 1-2 hours and followed by an intense diaphoresis. Regularly periodic fever may be seen with P vivax or P ovale, with a febrile episode every 48 hours (tertian fever), or with P malariae, with a febrile episode every 72 hours (quartan fever).
Relapsing malaria may occur with P vivax and P ovale, both of which have a dormant hypnozoite liver stage as part of the life cycle. Relapses can occur months or even years after infection.
Malaria should be included in the differential diagnosis of every patient with acute febrile illness and travel history to endemic areas.
Patients with severe or complicated malaria, which has very high mortality, may present with prostration, coma, respiratory distress (acidotic breathing), severe anemia, renal failure, acute respiratory distress syndrome (ARDS), pulmonary edema, hypoglycemia, shock, disseminated intravascular coagulation (DIC), bleeding, and convulsion.
Other complications include ST changes or prolonged QT on ECG, chest pain, and myocarditis / pericarditis.
Species identification is important because uncomplicated malaria with P falciparum can rapidly develop into severe infection, though any species can cause severe disease. Physicians cannot rely on symptoms and signs to diagnose malaria or to specify the infecting species.
Malaria mainly due to P falciparum has been associated with increased susceptibility and severity among pregnant individuals.