Contributed by Lirong Qu, MD, PhD and Lydia Contis, MDFINAL DIAGNOSIS: Malaria due to Plasmodium falciparum
Contributors' Note:
Introduction
Malaria (from the Italian mal'aria, meaning bad air) is an acute infection caused by four species of the protozoal genus plasmodium: falciprium, vivax, malariea and ovale (1). Today, malaria is a disease associated with poverty and underdeveloped countries (2). Globally, the numbers are staggering: there are 500 million clinical cases reported each year (10% of the world population), and more than 1 million, mostly children, will die as a result of this disease (3). This translates into a death from malaria every 30 seconds, rendering it an eminent disease in tropical countries and ranking it the third killer among communicable diseases (behind HIV/AIDS and tuberculosis) (2). Emerging drug-resistant organisms and insecticide-resistant mosquitoes are making malaria a resurging infectious disease.
Malaria was eliminated from the United States and most of Europe during the first half of the twentieth century. Although malaria remains an important public health problem in some parts of Asia and South America, it exacts its greatest toll in sub-Saharan Africa where 90% of deaths occur from malaria (4). With increased world travel, the problem of imported malaria is gaining notoriety in developed countries. MMWR Reports from CDC (www.cdc.gov) have shown a current annual detection rate of 1600 cases in the US (9). In this case report, we will review some fundamentals and developments in our current standing and understanding of this infection.
Plasmodium life cycle and pathogenesis
Plasmodium falciparum is the main cause of disease and death from malaria. The parasite is transmitted to humans through the bite of the infected female anopheles mosquito. Mosquitoes inject sporozoites into humans during a blood meal. Sporozoites travel to the liver to initiate the infection. The entry of sporozoites to hepatocytes is mediated through a specific receptor-ligand interaction. The co-receptor on sporozoites involves the thrombospodin domains on the circumsporozoites protein and on thrombospodin-related adhesive protein. These domains bind specifically to the heparan sulphate proteoglycans on the hepatocytes (5). Inside the hepatocytes, each sporozoite develops into a cyst-like structure containing thousands of merozoites (the exoerythrocytic stage). When the swollen liver cell eventually ruptures, mobile merozoites are released into the blood stream. The symptomatic stage of the infection starts at this point. In P. vivax and P. ovale infection, a portion of the hepatic form remains dormant - causing the relapse characterizing these two species (6).
In the blood stream, the merozoite invades erythrocytes through attachment to a specific cell surface receptor. The Duffy blood-group antigen serves as a receptor for P. vivax; the Duffy-negative phenotype is resistant to P. vivax. Glycophorin A serves as a receptor for P. falciparum. Once inside the erythrocyte, the merozoite develops into the trophozoite or "ring form", and the trophozoite develops into a merozoite (the intraerythrocytic stage). By the end of the 48-hour intraerythrocytic life cycle (72 hours for P. malariae), the parasite has used up all the hemoglobin (as nutrient) and taken up the entire volume of the erythrocyte. Eventually the infected cell bursts, releasing merozoites that can enter other erythrocytes and repeat the cycle. This cycle results in more and more RBCs being infected by the parasite (the asexual replication cycle). The paroxysms (cyclical fever) associated with malaria occur shortly before or at the time of erythrocyte rupture.
After invading erythrocytes, the parasite alters the red cell membrane by changing its transportation properties, exposing cryptic surface antigens and inserting new parasite-derived proteins. P. falciparum infected RBCs express a high-molecular weight, antigenically variant, strain-specific adhesive protein PfEMP1 (P. falciparum erythrocyte membrane protein 1). PfEMP1 mediates the attachment of infected RBCs to the lining of small blood vessels in various organs through different receptors (e.g. ICAM 1, CD36). Infected cells also adhere to uninfected cells to form rosettes. These adhesions result in the sequestration of parasite-infected RBCs in vital organs (particularly the brain), where they interfere with the microcirculation and metabolism. The pathogeneses of P. falciparum involve several basic processes: rapid expansion of infected RBCs, destruction of both infected and uninfected RBCs, microvascular obstruction and inflammatory processes (7). Sequestered parasites continue to develop and avoid being cleared by host defense mechanisms (splenic processing and filtration). As a consequence, only the young ring forms of the asexual parasites are seen in the peripheral blood of P. falciparum malaria (6). All human Plasmodium invade by a similar mechanism, but P. falciparum reaches high parasitemia because the flexibility of the receptor allows it to invade a large percentage of erythrocytes, whereas P. vivax is limited to reticulocytes (7).
To complete the parasitic life cycle, a small number of merozoites (asexual parasites) develop into gametocytes (female macrogametocytes and male microgametocytes) after several intraerythrocytic cycles. The gametocytes are essential for transmitting the infection to others through female anopheline mosquitoes. The male and female gametocytes form a zygote in the insect's midgut. The zygotes develop into myriad motile sporozoites through asexual division in the intestinal cells. These sporozoites migrate to the salivary glad to continue the Plasmodium life cycle by infecting the next host during the next feeding (6).
Clinical Manifestation
The early symptoms of malaria are nonspecific: malaise, fatigue, headache, abdominal discomfort, and muscle aches. The classic malarial paroxysms, in which fever spikes, chills and rigors occur at regular intervals, suggest P. vivax and P. ovale. The fever of falciparum malaria may never become regular. The temperature of nonimmune individuals and children often rises above 400C with tachycardia and sometime delirium. In patients with uncomplicated malaria, physical findings include fever, malaise, mild anemia, and (in some cases) palpable spleen.
Severe falciparum malaria is a complex disease. Reduced tissue perfusion and metabolic acidosis (lactic acidosis) are now recognized as the main underlying pathophysiological features of the classical clinical syndromes of cerebral malaria and severe malarial anemia (7). Parasite-infected individuals are often dehydrated and relatively hypovolemic; this could further exacerbate microvascular obstruction and, along with anemia, compromise oxygen delivery. Severe malaria (cerebral malaria, acidemia/acidosis, renal failure etc) carries significant mortality.
Diagnosis
Diagnosis rests on the demonstration of the parasite in the red blood cells of peripheral blood smear. In P. falciparum infection, the parasite is identified as a tiny ring form within normal sized erythrocytes. Multiple ring forms are commonly seen in P. falciparum. The gametocytes show a classical banana shape as we saw in this case. In P. vivas infection, the parasite is identified as large ring form (~ 1/3 RBC width) in an enlarged erythrocyte (1).
Simple, sensitive and specific antibody-based diagnostic stick or card tests that detect P. falciparum - specific histidine rich protein (HRP) 2 or lactate dehydrogenase antigens in finger prick blood samples have been introduced. Some of these tests carry a second antibody, which allows falciparum to be distinguished from the less dangerous malarias (6).
Treatment
The development of resistance of plasmodia to available drugs has created a serious problem. Infections due to P. vivax, P. malaria, P. ovale, and known sensitive strains of P. falciparum should be treated with chloroquine. In Africa, chloroquine-resistant strains are usually sensitive to sulfadoxine/pyrimethamine. When resistance to both occurs, either (a) quinine plus tetracycline or doxycycline (or clindamycin) or (b) mefloquinine should be used. Tetracycline and doxycycline cannot be given to pregnant women or to children <8 years of age (6). Common antibiotics (such as tetracycline, doxycycline and clindamycin) inhibit parasite growth and are being used increasingly in combination with other anti-malarial drugs (9). These antibiotics are all thought to inhibit parasite growth through the inhibition of prokaryote-like protein biosynthesis in the apicoplast - an organelle unique to apicomlexan parasites such as Plasmodium (8).
Nonimmune individuals on malaria treatment should have daily parasite count performed until negative thick films indicate clearance of the parasite. Quinine (or quinidine) and tetracycline should be reserved for multi-drug resistant infection. There is a growing belief among malariologists that, to prevent resistance, malaria should be treated with combinational use of two or more drugs with different modes of action. This rationale has been applied in the treatment of tuberculosis and HIV/AIDS (8, 10).
Malaria control
Currently no vaccine is available for use. Research has brought several candidate vaccines through phase 1 and in some cases phase 2 of vaccine development. The only vaccine to reach phase 3 was a multicomponent synthetic peptide, SPf66. It claimed to have protective effects in one trial, but offered no protection in another trial (10).
In the absence of a vaccine, the control of malaria involves both vector control and drug therapy. After decades of relative lack of attention, growing international awareness and funding have led to new efforts towards controlling the disease. A coalition involving both public and private enterprises is being formed to combat the diseased. Most notably, the World Health Organization's "Roll Back Malaria" campaign aims to reduce the burden of malaria by half by 2010. The pharmaceutical industry, researchers and philanthropy are joining in the effort (4). It is our hope that this parasitic disease will be tamed in the near future!
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Contributed by Lirong Qu, MD, PhD and Lydia Contis, MD
