Although Ebola has been all the furor in the recent weeks, fever in travelers is NOT Ebola most of the times. Depending on the region and season, the usual culprits are more likely than Ebola, these include malaria, dengue, yellow fever, chikungunia virus, hepatitis, salmonella and other intestinal bugs, multiple parasites among others. Here is the Q&A for malaria and babesia from this month's NEJM.
What is the annual incidence of malaria in the United States?
In the United States, the annual incidence of malaria is approximately 1500 cases. In 2010, a total of 1691 cases were reported to the Centers for Disease Control and Prevention (CDC), the largest number reported since 1980; P. falciparum, P. vivax, P. malariae, and P. ovale were identified in 58%, 19%, 2%, and 2% of cases, respectively.
How do malaria and babesiosis differ in appearance on a peripheral blood smear?
Intraerythrocytic parasites are seen in both malaria and babesiosis. Plasmodia metabolize heme to form an intracellular crystallized pigment, hemozoin. Although hemozoin is not invariably identified in cases of malaria, its presence reliably distinguishes malaria infection from babesia infection. Malaria parasites can be distinguished from B. [Babesia] microti by the presence of recognizable gametocytes (characteristically banana-shaped in Plasmodium falciparum and round, with a granular appearance, in nonfalciparum species). In addition, intracellular vacuoles and extracellular merozoites are unusual in malaria but common in babesiosis, and the classic “Maltese cross” (a tetrad of parasites budding at right angles) is unique to babesia species.
Which malaria species can remain dormant in the liver?
In the case of P. vivax and P. ovale, some sporozoites (immature malaria parasites) do not replicate immediately when they invade hepatocytes but remain dormant (as hypnozoites) for prolonged periods. The average time to relapse is approximately 9 months, but it can range from weeks to years. The interval to relapse depends on the strain (earlier with tropical strains and later with temperate strains), the initial inoculum, and host factors (e.g., febrile illnesses can trigger relapse associated with P. vivax). None of the commonly used prophylactic agents (chloroquine, mefloquine, doxycycline, or atovaquone–proguanil) eliminate hypnozoites. Primaquine, the only effective drug against dormant hypnozoites, has not been approved by the Food and Drug Administration for primary prophylaxis, but the CDC endorses its use for prophylaxis in Latin American countries where P. vivax predominates, because the drug can prevent both primary attacks and relapses caused by all species that are a source of malarial infection.
How is acute or recurrent P. vivax infection treated?
In patients with acute or recurrent malaria infection, treatment depends on the species and the resistance status in the area where the infection was acquired. P. falciparum is resistant to chloroquine in most regions in which it is endemic and resistant to mefloquine in parts of Southeast Asia. In contrast, nonfalciparum malaria parasites do not have substantial resistance to mefloquine, and the distribution of chloroquine-resistant P. vivax malaria is limited, occurring primarily in Indonesia and Papua New Guinea. After treatment is initiated, peripheral-blood smears should be obtained daily for 4 days (parasitemia is typically eliminated by day 4), on days 7 and 28 to confirm eradication, and at any time symptoms recur, suggesting treatment failure. In areas other than those with known chloroquine resistance, chloroquine, followed by a 14-day course of primaquine to prevent subsequent relapses, remains the standard treatment for P. vivax parasitemia. Given the risk of hemolysis in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency who receive treatment with primaquine, potential recipients should be tested for G6PD deficiency. Among patients with a contraindication to primaquine therapy, treatment with chloroquine alone carries a 20% risk of relapse; extended chloroquine prophylaxis can be offered to patients who have frequent relapses.