CHAPTER 123: MALARIA

INTRODUCTION

Malaria is a protozoan disease transmitted by the bite of infected Anopheles mosquitoes. The most important of the parasitic diseases of humans, it is transmitted in 106 countries containing 3 billion people and causes approximately 2000 deaths each day; mortality rates are decreasing as a result of highly effective control programs in several countries. Malaria has been eliminated from the United States, Canada, Europe, and Russia; in the late twentieth and early twenty-first centuries, however, its prevalence rose in many parts of the tropics. Increases in the drug resistance of the parasite, the insecticide resistance of its vectors, and human travel and migration have contributed to this resurgence. Occasional local transmission after importation of malaria has occurred in several southern and eastern areas of the United States and in Europe, indicating the continual danger to nonmalarious countries. Although there are many successful new control initiatives as well as promising research initiatives, malaria remains today, as it has been for centuries, a heavy burden on tropical communities, a threat to nonendemic countries, and a danger to travelers.

ETIOLOGY AND PATHOGENESIS

Six species of the genus Plasmodium cause nearly all malarial infections in humans. These are P. falciparum, P. vivax, two morphologically identical sympatric species of P. ovale (as suggested by recent evidence), P. malariae, and—in Southeast Asia—the monkey malaria parasite P. knowlesi (Table 123-1). While almost all deaths are caused by falciparum malaria, P. knowlesi and occasionally P. vivax also can cause severe illness. Human infection begins when a female anopheline mosquito inoculates plasmodial sporozoites from its salivary gland during a blood meal (Fig. 123-1). These microscopic motile forms of the malaria parasite are carried rapidly via the bloodstream to the liver, where they invade hepatic parenchymal cells and begin a period of asexual reproduction. By this amplification process (known as intrahepatic or preerythrocytic schizogony or merogony), a single sporozoite eventually may produce from 10,000 to >30,000 daughter merozoites. The swollen infected liver cells eventually burst, discharging motile merozoites into the bloodstream. These merozoites then invade the red blood cells (RBCs) and multiply six- to twentyfold every 48 h (P. knowlesi, 24 h; P. malariae, 72 h). When the parasites reach densities of ~50/μL of blood (~100 million parasites in the blood of an adult), the symptomatic stage of the infection begins. In P. vivax and P. ovale infections, a proportion of the intrahepatic forms do not divide immediately but remain inert for a period ranging from 3 weeks to ≥1 year before reproduction begins. These dormant forms, or hypnozoites, are the cause of the relapses that characterize infection with these two species.