Clinical:Malaria

Causes

Malaria is a disease that afflicts 300-500 million people annually and claims the lives of over one million people per year, most of whom are children under the age of five. The causative agents of malaria are obligate, intracellular protozoa belonging to genus Plasmodium. Though malaria develops in a number of mammalian and non-mammalian organisms, there are four species of Plasmodium that only infect humans—P. ovale, P. malariae, P. vivax, and P. falciparum. Of the four species, P. falciparum is responsible for the greatest morbidity and mortality.^[1] P. falciparum dominates in sub-Saharan Africa, Hispaniola, and Papua New Guinea. P. vivax is most common to Asia, Central and South America, and Eastern Europe. P. ovale is endemic to West Africa, while P. malariae has a low prevalence in various places throughout the world.^[2] Parasite infections are not mutually exclusive. In fact, mixed Plasmodium species and polyclonal single species infections are not uncommon in endemic regions.^[3]

Life cycle of Plasmodium parasites. Sexual development occurs within female Anopheles mosquitoes. Parasites are transferred to the asexual host through the bite of infected mosquitoes. Source:CDC

Plasmodia are characterized by a complex life cycle involving two hosts. Sexual development occurs within female mosquitoes of genus Anopheles. During the sexual phase, ookinetes develop within the digestive tract of the mosquito following the fertilization of female gametes (macrogametes) by male gametes (microgametes). Ookinetes then mature into oocysts within the mosquito midgut. Oocyst development is succeeded by sporozoite release. Female anopheline mosquitoes are hematophagous arthropods that require blood for egg production and development. Parasite sporozoites are transferred from the salivary glands of infected females to the asexual host (e.g. humans, rodents, birds) during blood meal acquisition.^[4]

Upon entry of the verbrate host, the parasite travels to the liver where it undergoes asexual amplification within hepatocytes. Following hepatic release, red blood cell (RBC) invasion commences. Blood stage development proceeds along a cyclic route, where the parasite adopts a number of morphologically distinct forms-- ring, trophozoite, and schizont. Upon schizont rupture, upwards of twenty merozoites are released, each with the potential to invade additional RBCs.^[1][5] Each merozoite follows one of two fates: continued asexual amplification through reinvasion of erythrocytes or gametogenesis. The life cycle begins anew when an anopheline mosquito feeds upon the blood of an infected individual, ingesting microgametocytes and macrogametocytes.^[1][6][7]

The asexual, blood stage of Plasmodium development is characterized by ring, trophozoite, and schizont forms of the parasite. Housed within the schizonts are numerous merozoites, each of which has the potential to invade a red blood cell. Source:CDC

Signs and Symptoms

Clinical manifestations of malaria are attributable to the blood stage component of the parasite life cycle.^[5] Symptoms and symptoms of malaria include non-specific flu-like symptoms, malaise,abdominal pain, anemia, splenomegaly, chills, and fever. The fevers of malaria have a periodic onset that is commisserate with asexual parasite development. This periodic fever, however, is frequently not seen clinically, and therefore is not often helpful in distinguishing malaria from other causes of fever. For example, fevers arising from infection with P. falciparum emerge every 48 hours. This corresponds to the time needed for P. falciparum to proceed from the ring stage to rbc rupture and invasion of other rbcs, a point at which the parasite once again adopts the morphologically-ascribed ring form. Anemia results from the rupture of infected erythrocytes, splenomagaly, and vascular sequestration .^[8][9] More severe manifestations of malaria include cerebral malaria, severe anemia, renal failure, respiratory distress, and death. The death rate from Falciparum malaria may be as high as 40% in non-immune individuals.^[10]

Epidemiology

Malaria is prevalent in sub-Saharan Africa, as well as other tropical and sub-tropical regions such as Central and South America, Asia, and the Middle East ^[11]. The geographic distribution of malaria is coordinate with the mosquito vector. As a result, malaria is generally not found in high altitude places. Though Anopheles mosquitoes can be found in the United States, public health interventions have disrupted parasite transmission.^[12] Most cases of malaria in the United States are therefore "imported malaria," or malaria acquired by a person traveling from an endemic region to the United States.

Geographic distribution of malaria. Blue denotes malaria-endemic regions. Source:CDC

Diagnosis

Malaria is diagnosed upon microscopic examination of parasite infected RBCs. Both thin and thick preparations are usually done, with the thin smear most helpful in determining the species of plasmodium responsible for disease and the thick smear helpful with determining degree of infection . Ring stage-infected RBCs are most often seen upon visual examination. Trophozoite-infected RBCs may be sequestered within blood vessels, as a result of the up-regulation of cell surface markers that mimic endogenous cellular adhesion molecules. As a consequence, it may be necessary to acquire multiple blood smears to make a firm diagnosis. With regard to infection with P. falciparum if we assume that on Day 1 that the parasite population is at the ring-stage of development, then infections of sufficient parasitemia will be readily visible. However, if blood smears were prepared 24-hours later, which is roughly equivalent to the time needed for a parasite to proceed from the ring to the trophozoite stage, few, if any, parasite-infected RBCs are likely to be observed. Polymerase chain reaction (PCR) may also be used to determine infection. PCR facilitates diagnosis of low parasitemia infections and permits species identification.

Treatment

Prophylactic Treatment

There exist a number of measures that target the mosquito. These methods disrupt vector transmission, thereby preventing infection with Plasmodium and the onset of malaria^[13]

1. Insecticide-Treated Bednets-- Holes must be of sufficiently small size (25 holes/cm²) to impede mosquito passage. Bednets must be treated every 6-12 months with insectide (e.g. pyrethrinoids). Becuase of increasing resistance of the mosquito to insecticides, there is a trend toward treating bednets with more than one type of insecticide. Major impediments to the widespread use of bednets include discomfort due to poor air circulation and elevated temperatures and cost. Their utility is also limited by mosquito feeding times, which may occur during the day or prior to bedtime.

2. Indoor Residual Spraying-- The spraying of homes with insectides will disrupt mosquito feeding cycles and consequently parasite transmission. Coverage must be sufficiently high to be effective. However, many species of Anopheles feed outdoors.

3. Biocontrol-- This method interrupts mosquito development by eliminating mosquito breeding grounds, which center around stagnant water. Another form of biocontronl involves Bacillus thuringiensis israelensis (Bti), a bacterium that secretes a toxin lethal to many species of insect larvae.

Medications

Chloroquine is the choice anti-malarial, but due to widespread drug resistance by P. falciparum parasites, alternate drug regimens have been established. The second-line treatment options include (1) quinine plus an antibiotic (e.g. a tetracycline or clindamycin), (2) atovaquone-proguanil, or (3) mefloquine.^[2]
Quinoline-containing compounds Quinoline-containing compounds include chloroquine, quinine, and mefloquine. During asexual development, Plasmodium digests hemoglobin, the oxygen-binding component of red blood cells. Hemoglobin is processed to heme, a toxic moiety that the parasite then converts to the non-toxic derivative hemozoin. While their mechanism of action is not fully understood, the quinoline-containing compounds are thought to kill the parasite by diffusing into its digestive vacuole and disrupting the conversion of heme to hemozoin. In the presence of these drugs, non-resistant parasites are poisoned by their own waste products.^[14]

1. Chloroquine—Chloroquine is the first-line anti-malarial. However, chloroquine-resistant P. falciparum has emerged in Southeast Asia, sub-Saharan Africa, South America, and the Pacific Islands, necessitating the use of alternate therapies.^[2] Chloroquine-resistant parasites have evolved transporters that neutralize the drug by pumping it out of the digestive vacuole.^[15]

2. Quinine- Quinine has a rapid onset of action and when combined with an antibiotic such as tetracyline, doxycycline, or clindamycin is effective in the treatment of chloroquine-resistant parasites.^[2] Quinine is added to tonic water for flavor.
3. Mefloquine (Lariam^®)- Mefloquine may be used for treatment of existing infection or as a prophylaxis. The side-effects of mefloquine are notable and include hallucinations and nightmares. Mefloquine-resistant parasites have been reported in Thailand, Cambodia, and Burma.^[2]

Aminoquinoline-containing compounds
1. Primaquine- Primaquine targets hypnozoites, which are dormant, liver-stage parasites that can persist in the host liver for up to 229 days after initial sporozoite exposure. Hypnozoites can induce relapsing malaria. Not all Plasmodium species can form hypnozoites. As a result, primaquine is effective against P. vivax and P. cynomolgi (a non-human primate malaria), but not P. falciparum.^[16]

2. Amodiaquine- Amodiaquine is no longer recommended as a prophylaxis because of its potential to induce hepatotoxicity and agranulocytosis (a loss of granulocytes, particularly neutrophils).^[17]

Anti-folate inhibitor-based drugs
A number of enzymes are involved in the generation of folic acid (folate), a B-vitamin. Folate is needed for the synthesis of nucleotides, namely the purines, but also pyrimidine.^[18] Dihydropteroate synthetase plays a critical role in the generation of folic acid, while dihydrofolate reductase is essential to de novo purine biosynthesis.^[19]

1. Atovaquone-proguanil (Malarone^®)- Atovaquone-proguanil combination therapy may be used not only in the treatment of malaria, but also as a chemo-prophylaxis. Atovaquone disrupts the parasite's mitochondrial electron transport chain, which Plasmodium requires for de novo pyrimidine biosynthesis. Unlike mammalian species, Plasmodium is unable to salvage pyrimidine.^[20] Proguanil has been used in the treatment of malaria since the 1940s and was identified as the optimal anti-malarial for combined use with atovaquone. Proguanil is effective against exoerythrocytic, liver stage parasites.^[17] Proguanil inhibits the activity of dihydrofolate reductase. Atovaquone-proguanil therapy therefore interferes with nucleic acid biosynthesis and ultimately DNA replication. Atovaquone-proguanil is effective in killing multidrug-resistant parasites.^[21]

2. Sulfadoxine-pyrimethamine (Fansidar^®)- Pyrimethamine, like proguanil, is an inhibitor of the enzyme dihydrofolate reductase and active against exoerythrocytic, liver stage parasites.^[17] Sulfadoxine belongs to the sulfa class of antibiotics.^[19] Sulfa drugs act as competitive inhibitors of dihydropteorate synthetase. Parasite resistance to sulfadoxine-pyrimethamine is increasing.^[2]

Artemisinin
Artemisinin refers to a group of drugs based on a naturally occuring compound found in Artemisia annua. A. annua is a plant that belongs to the same genus as wormwood, the plant from which absinthe is derived. Artemisinin derivatives include dihydroartemisinin, artesunate, and artemether. The mechanism of action of artemisinin-based chemotherapies is not understood. In light of increasing parasite resistance to many mainstay anti-malarials, the use of artemisinin-based combination therapies (ACTs) has been instituted. The parasite is less likely to develop resistance to artemisinin if its use is combined with drugs with distinct mechanisms of action. However, ACT is not intended to restore the efficacy of failing anti-malarials, for the cytotoxic capabilities of artemisinin-based therapies is compromised when used in combination with chemotherapies of reduced efficacy. Despite knowledge of artemisinin's pharmacokinetic properties, the World Health Organization (WHO) recommends use of artusenate-amodiaquine (a chloroquine analog) and artusenate-sulfadoxine-pyramethamine as second and third-line malaria treatment options, respectively.^[22]

Photo of Artemisia annua, the plant from which artemisinin is derived. Source:http://www.usaid.gov

Antibiotics
Doxycycline- Doxycycline belongs to the tetracycline class of antibiotics. The tetracyclines interfere with translation of apicoplast-derived genes. Doxycycline is slow-acting, affecting the progeny of doxycycline-exposed parasites. These daughter parasites are unable to form merozoites. Recall that merozoites are the invasive forms of blood stage parasites. Therefore, doxycycline impedes the amplification of blood stage parasites.</ref>Tetracyclines specifically target the apicoplast of the malaria parasite Plasmodium falciparum. Dahl EL, Shock JL, Shenai BR, Gut J, DeRisi JL, Rosenthal PJ.</ref> The function of the apicoplast has not been definitively defined, but it is an essential sub-cellular organelle.^[23] Antibiotics should never be used as a monotherapy for the treatment of malaria, but only in combination with an anti-malarial.^[2]

References

1. ↑ ^{1.0 1.1 1.2} Centers for Disease Control: Malaria
2. ↑ ^{2.0 2.1 2.2 2.3 2.4 2.5 2.6} Griffith KS, Lewis LS, Mali S, Parise ME. Treatment of malaria in the United States: a systematic review. JAMA. 2007 May 23;297(20):2264-77. Abstract | Full Text | PDF
3. ↑ McKenzie FE, Bossert WH. Multispecies Plasmodium infections of humans.J Parasitol. 1999 Feb;85(1):12-8. Abstract
4. ↑ Spielman, A. and M. D'Antonio (2001). Mosquito. New York, Hyperion.
5. ↑ ^{5.0 5.1} Miller LH, Baruch DI, Marsh K, Doumbo OK. The pathogenic basis of malaria. Nature. 2002 Feb 7;415(6872):673-9. Abstract
6. ↑ Campbell, N. (1996). Biology. California, The Benjamin/Cummings Publishing Company, Inc.
7. ↑ Barnwell, J. and M. Galinski (1998). Invasion of Vertebrate Cells: Erythrocytes. Malaria: Parasite Biology, Pathogenesis, and Protection. I. Sherman. Washington, DC, ASM Press: 93.
8. ↑ Curtis, H. and N. Barnes (1989). Biology. New York, Worth Publishers, Inc.
9. ↑ Heymann, D., Ed. (2004). Control of Communicable Diseases Manual. Washington, DC, ASM Press.
10. ↑ Heymann, D., Ed. (2004). Control of Communicable Diseases Manual. Washington, DC, ASM Press.
11. ↑ Trigg, P. and A. Kondrachine (1998). Malaria: Parasite Biology, Pathogenesis, and Protection. I. Sherman. Washington, DC, ASM Press.
12. ↑ http://www.cdc.gov/malaria/distribution_epi/distribution.htm
13. ↑ Heymann, D., Ed. (2004). Control of Communicable Diseases Manual. Washington, DC, ASM Press.
14. ↑ Foley M, Tilley L. Quinoline antimalarials: mechanisms of action and resistance and prospects for new agents. Pharmacol Ther. 1998 Jul;79(1):55-87.
15. ↑ Sanchez CP, Wünsch S, Lanzer M (1997). "Identification of a chloroquine importer in Plasmodium falciparum. Differences in import kinetics are genetically linked with the chloroquine-resistant phenotype". J. Biol. Chem. 272 (5): 2652–8.
16. ↑ Cogswell FB. The hypnozoite and relapse in primate malaria. Clin Microbiol Rev. 1992 Jan;5(1):26-35.
17. ↑ ^{17.0 17.1 17.2} Bia FJ. Malaria prophylaxis: taking aim at constantly moving targets. Yale J Biol Med. 1992 Jul-Aug;65(4):329-36. Abstract | PDF
18. ↑ Smith C, Lieberman M, Marks DB, Marks AD (2007). Marks' essential medical biochemistry. Hagerstwon, MD: Lippincott Williams & Wilkins.
19. ↑ ^{19.0 19.1} Nzila A. The past, present and future of antifolates in the treatment of Plasmodium falciparum infection. J Antimicrob Chemother. 2006 Jun;57(6):1043-54. Abstract | Full Text | PDF
20. ↑ Looareesuwan S, Chulay JD, Canfield CJ, Hutchinson DB. Malarone (atovaquone and proguanil hydrochloride): a review of its clinical development for treatment of malaria. Malarone Clinical Trials Study Group. Am J Trop Med Hyg. 1999 Apr;60(4):533-41.
21. ↑ Looareesuwan S, Chulay JD, Canfield CJ, Hutchinson DB. Malarone (atovaquone and proguanil hydrochloride): a review of its clinical development for treatment of malaria. Malarone Clinical Trials Study Group. Am J Trop Med Hyg. 1999 Apr;60(4):533-41.
22. ↑ Davis TM, Karunajeewa HA, Ilett KF. Artemisinin-based combination therapies for uncomplicated malaria. Med J Aust. 2005 Feb 21;182(4):181-5.
23. ↑ Ralph SA, D'Ombrain MC, McFadden GI. The apicoplast as an antimalarial drug target. Drug Resist Updat. 2001 Jun;4(3):145-51.