Malaria, the ancient disease thought to have killed King Tut, is a tricky one. It kills millions of people worldwide each year, mainly children in sub-Sahara Africa. In order to understand symptoms, diagnosis, treatment, and prevention of malaria, you must first understand what causes it and how it comes to be in a person’s body. It is not caused by bad air (mal aria) or bacteria as it was once thought. The complicated, complete relationship between the pregnant female Anopheles mosquito and a person answers the question “How does malaria get into and infect people?”.
Plasmodia is the parasite that causes malaria. A parasite needs to feed off somebody in order to live. Without a host like a monkey, ape, bird or a person, it dies. There are hundreds of species of Plasmodium. There are mainly four species of Plasmodia that infect people, and they vary in geography, the way in which they cause disease, and the type of symptoms they cause.
The Plasmodia that causes the most severe malaria (Plasmodia falciparum) is the most common type in sub-Sahara Africa, but is also found in other tropical and subtropical areas (i.e., Haiti or South America). This species accounts for most of the mortality associated with malaria.
One sentence of terminology–Think of 7 different phases of the Plasmodia life cycle in humans: sporozoites, hepatic (liver) schizont, hypnozoites, merozoites, blood cycle (ring form, trophozoite, erythrocytic schizont), immature gametocytes, and male and female gametocytes. Basically you have the starting form in the person, the first location where it reproduces and is stored for a short while, the location where it is stored for a long while, the form that enters red blood cells and reproduces, the form that another mosquito pulls out and carries to some other location after it is processed and packaged within this mosquito. Think of it in these simplified terms. The terminology need not trip you. Spore-liver-blood-mosquito-spore-liver-blood-mosquito-spore…….an incredibly simplified cycle.
Sporozoites come from the female Anopheles mosquito (through its saliva) and enter the blood stream during feeding. Very quickly, these sporozoites enter the person’s liver and form two different things (depending on the species of Plasmodium)–either hepatic schizonts or hypnozoites. The hepatic schizonts mature and rupture, releasing merozoites into the blood stream. The hypnozoites remain dormant or “hypnotized” for months or years (depending on various factors) before releasing merozoites. This delayed release is related to malaria relapses you may have heard about. The merozoites circulating in the blood are the ones that enter red blood cells and cause them to swell (if person is susceptible–some people are immune or partially immune). During this blood cycle, the form of Plasmodium within the red blood cell shifts from ring form to trophozoite to erythrocyte schizont to merozoites. Like in the liver cells, the merozoites rupture from the red blood cell. The merozoite in the blood can either go on to enter more red blood cells (thus increasing the amount of parasite in the blood) or become immature gametocytes (cells capable of becoming either male or female). The immature gametocytes eventually mature into male or female gametocytes. It is these mature gametocytes that get drawn up by the Anopheles mosquito and enter its life cycle there. Many steps take place within the mosquito, but the final stage in the female Anopheles mosquito consists of sporozoites in its saliva that gets injected into the blood stream of its host (i.e., a person).
One cycle of the life cycle of malaria is completed, only to go on and on until stopped either with medication, preventative measures (i.e., netting, clothing), and/or mosquito eradication. This life cycle is the key to understanding malaria, its signs and symptoms, its diagnosis and treatment, and what preventative measures should be undertaken. The way malaria gets into and infects people is the first level of understanding malaria.
Mandell: Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases, 7th ed..