Malaria

Cause of Malaria and Natural Cycle

The four species of the genus Plasmodium that cause malarial infections in humans are P. falciparum, P. vivax, P. ovale, and P. malariae. Human infection begins when the malaria vector, a female anopheline mosquito, inoculates infectious plasmodial sporozoites from its salivary gland into humans during a bloodmeal. The sporozoites mature in the liver and are released into the bloodstream as merozoites. These invade red blood cells, causing malaria fevers. Some forms of the parasites (gametocytes) are ingested by anopheline mosquitoes during feeding and develop into sporozoites, restarting the cycle.

Manifestations

The complex interrelationships of the malaria parasite, the female Anopheles mosquito vector, and the human target, along with environmental factors and control measures, determine the expression of disease manifestations and epidemiology.

The first symptoms of malaria are nonspecific: Patients are unwell and have headache, fatigue, abdominal discomfort, and muscle aches followed by fever—all similar to a minor viral illness. Later, fever spikes, chills, and rigors occur. Anemia, hypoglycemia, cerebral manifestations, and low birthweight newborns result frequently from malaria as do neurocognitive sequelae after severe illness. Those who have been exposed to malaria develop partial immunity, but not protection from infection, such that they have parasitemia but not illness: This condition is called premunition and is the reason that adults living in malarious areas have much less illness, despite being bitten by infected mosquitoes. In addition, many persons may have comorbidity—malaria parasitemia or illness at the same time that they have other diseases, complicating the diagnosis.

Case Fatality Rates and Sequelae

Correctly and promptly treated, uncomplicated falciparum malaria has a mortality rate of approximately 0.1%. Once vital organ dysfunction occurs or the proportion of erythrocytes infected increases to more than 3%, mortality rises steeply. Coma is a characteristic and ominous feature of falciparum malaria and, [Page 624]despite treatment, is associated with death rates of some 20% among adults and 15% among children. Convulsions, usually generalized and often repeated, occur in up to 50% of children with cerebral malaria (CM). Whereas less than 3% of adults suffer neurological sequelae, roughly 10% to 15% of children surviving CM—especially those with hypoglycemia, severe malarial anemia (SMA), repeated seizures, and deep coma—have some residual neurological deficit when they regain consciousness. Protein-calorie undernutrition and micronutrient deficiencies, particularly zinc and vitamin A, contribute substantially to the malaria burden.

Figure 1 Natural Cycle of Malaria

Where, When, and Why Malaria Occurs

P. falciparum predominates in Haiti, Papua New Guinea, and sub-Saharan Africa. P. vivax is more common in Central America and the Indian subcontinent and causes more than 80 million clinical episodes of illness yearly. The prevalence of these two species is approximately equal in the Indian subcontinent, eastern Asia, Oceania, and South America. P. malariae is found in most endemic areas, especially throughout sub-Saharan Africa, but is much less common than the other species. P. ovale is unusual outside Africa, and where it is found accounts for less than 1% of isolates.

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