Introduction
Malaria is a key public health predicament and cause of much distress and untimely death in the poorer areas of tropical Africa, Asia and Latin America. In many endemic areas it is becoming increasingly complex to control because of the resistance of the parasite to anti- malarial drugs and the failure of vector control measures. Malaria is mainly caused by plasmodium species of the Coccidia family of Parasites. The most pathogenic species of the plasmodium species is plasmodium falciparum which is responsible for almost 90% of malarial infections in the world. The Coccidia are intracellular parasites that reproduce asexually by a process called schizogony and sexually by sporogony. They are normally found in the liver and red cells, and are transmitted by anopheline mosquito vector.
Characteristics of the pathogen
Four distinct Plasmodium species infect humans: P. falciparum, P. vivax, P. malariae, and P. ovale. P. falciparum is the most pathogenic of the human malaria species, with untreated infections causing severe disease and death, particularly in young children, pregnant women and non-immune adults (Caraway, 1959).
The pathogenicity of P. falciparum is mainly due to: The cytoadherence of falciparum parasitized red cells, causing the cells to adhere to one another and to the walls of capillaries in the brain, muscle, kidneys and elsewhere and in pregnant women, in the placenta. The Sequestration of parasitized cells in the microcirculation causes congestion, hypoxia, blockage and rupturing of small blood vessels. And Due to high levels of parasitaemia, the activation of cytokines prompts and the destruction of many red cells occur (WHO, 2000). Falciparum malaria parasitaemia can go beyond more than 250 000 parasites per liter of blood. And about up to 30–40% of red cells may become parasitized resulting to severe falciparum malaria which is associated with cerebral malaria, haemoglobinuria, severe anemia, hypoglycaemia, and complications in pregnancy (2000).
P. falciparum is found mainly in the hotter and humid regions of the world. It is the main species found in tropical and subtropical Africa and parts of Central America and South America, Bangladesh, Pakistan, Afghanistan, Nepal, Sri Lanka, South East Asia, Indonesia, Philippines, Haiti, Solomon Islands, Papua New Guinea and many islands in Melanesia (WHO, 2000). It also occurs in parts of India, the Middle East, and eastern Mediterranean. According to WHO, the species Plasmodium falciparum contains several varieties which show differences in geographical distribution, vector susceptibility, human infection pattern, drug susceptibility, morphology and antigenic composition (2000).
Transmission
Malaria parasites are transmitted when an infected female Anopheles mosquito bites a host. Sporozoites contained in the saliva of the mosquito are inoculated into the blood of a human host when the mosquito takes a blood meal (Webster, 2003). Infection can also occur by transfusion of infected donor blood, by injection through the use of needles and syringes contaminated with infected blood, and very occasionally congenitally, usually when a mother is non-immune. Following inoculation, the Sporozoites rapidly, within 8 hours, leave the blood and enter liver cells. Within 5–15 days, depending on species, they develop into liver schizonts and are referred to as pre-erythrocytic (PE) schizonts. Mature PE schizonts contain many merozoites (2003). When mature, a PE schizont ruptures from the liver cell, releasing its merozoites into the blood circulation. The merozoites infect red cells by binding to receptors on the red cell membrane. Entry of the parasites into red cells starts a cycle of schizogony in the blood which to complete takes 48 hours for P. falciparum.P vivax and P. ovale and 72 hours for P. malariae (2003).
During this time the intracellular merozoites develop into trophozoites which feed on the contents of the red cells. As the trophozoite feeds, malaria pigment, known as haemozoin, is produced as an end product of hemoglobin breakdown. This accumulates in the trophozoite, appearing as brown-black granules (Webster, 2003). When the trophozoite is fully developed, the nucleus begins to divide, followed by a division of cytoplasm, resulting in the formation of a schizont containing 8–24 merozoites (2003). The mature schizont ruptures from its red cell releasing merozoites, malaria pigment, and toxins into the plasma which is the cause of a typical malaria attack. Merozoites released from schizonts enter the blood circulation and those which are not destroyed by the host’s immune system infect new red cells, beginning a further cycle of schizogony with more red cells being destroyed. After several erythrocytic schizogony cycles, some of the merozoites entering red cells develop into male and female gametocytes.
For the life cycle to be continued, the gametocytes must be ingested by a female Anopheles mosquito in a blood meal. If they are not taken up by a mosquito they die.
Symptoms
The characteristic feature of malaria is fever caused by the release of toxins, when erythrocytic schizonts rupture, which stimulate the secretion of cytokines from leucocytes and other cells. In the early stages of infection the fever is irregular or continuous. As schizogony cycles synchronize, fever begins to recur at regular intervals particularly in quartan malaria, every 72 hours, vivax and ovale malaria, every 48 hours (Beales, 2002). Splenomegaly occurs in all forms of malaria with repeated attacks causing a greatly enlarged spleen. Anemia and jaundice are also features of malaria, particularly falciparum malaria. Malaria caused by P. falciparum is referred to as falciparum malaria, formerly known as sub tertian (ST) or malignant tertian (MT) malaria. It is the most widespread, accounting for up to 80% of malaria cases worldwide (2002).
Diagnosis
The diagnosis of malaria is by done routinely though detecting and identifying malaria parasites microscopically in blood films, concentrating parasites in venous blood by centrifugation when they cannot be found in blood films, using a malaria rapid diagnostic test (RDT) to detect malaria antigen (WHO,2000). Measurement of hemoglobin or packed cell volume (PCV) is done due to presence of malaria with heavy parasitaemia particularly in young children and pregnant women. The measurement of blood glucose to detect hypoglycaemia is usually done particularly to diagnose young children and pregnant women off severe falciparum malaria.
Also with suspicion of Falciparum Malaria, the total white cell count and platelet count is done. Coagulation tests if abnormal bleeding is suspected in falciparum malaria.
A thick blood film is the most suitable for the rapid detection of malaria parasites, particularly when they are few. In areas where P. malariae is found, unless a thick film is examined, infection is likely to be missed because parasitaemia is normally low with this species. In a thick film the blood is not fixed. The red cells are lysed during staining, allowing parasites and white cells to be seen in a much larger volume of blood .A thin blood film is required to confirm the Plasmodium species if this is not clear from the thick film. The blood cells are fixed in a thin film, enabling the parasites to be seen in the red cells. Parasitized red cells may become enlarged, oval in shape, or stippled. These features can help to identify Plasmodium species. Examination of a thin film greatly assists in the identification of mixed infections. By counting the percentage of parasitized red cells before and after treatment, thin films are also of value in assessing whether a patient with falciparum malaria is responding to treatment in areas where drug resistance is suspected. Examination of a thin film also gives the opportunity to investigate anemia and white cell abnormalities, and in the absence of malaria parasites, suggest an alternative diagnosis, e.g. sickle cell disease (WHO, 2000).
Treatment
The treatment of Malaria parasites is now tricky due to intense drug resistance. Artemisinin-based combination therapies are combinations in which one of the components is artemisinin and its derivatives, artesunate, artemether, dihydroartemisinin. The artemisinins produce rapid clearance of parasitaemia and rapid resolution of symptoms, by reducing parasite numbers 100- to 1000-fold per asexual cycle of the parasite, which is more than the other currently available anti- malarials can achieve (Webster, 2003).
Prevention
Reducing the suffering and loss of life caused by malaria is possible, providing the financial, political, and technical commitment to achieve this is strengthened. The WHO/UNICEF/UNDP and World Bank Roll Back Malaria Partnership, the Global Fund to fight AIDS, Tuberculosis, and Malaria, the Medicines for Malaria Venture, the Gates Malaria Partnership and the Multilateral Initiative on Malaria have been established to reduce the burden of malaria by: implementing malaria control strategies, improving health infrastructures, raising awareness of malaria and its effects on poverty and development, mobilizing communities to combat malaria, raising and monitoring funds to effect and sustain malaria control programmes and the development of anti-malarial drugs and vaccines.
References
Beales, P. F, and Gilles, H .M, (2002). Rationale and technique of malaria control. Ch. 6 in 4th ed. Essential Malariology, pp 107–190
Caraway, W.T., (1959). American Journal of Clinical Pathology, 32.
Webster D et al., (2003) Progress with new malaria vaccines. Bulletin World Health Organization, 81(12), pp 902–909
WHO, (2000). Severe falciparum malaria. Transactions Royal Society Tropical Medicine and Hygiene, 94, Supplement
