Despite mankind's longstanding struggle to control mosquito populations, the World Health Organization estimates that in 2018, 228 million people developed malaria globally and 405,000 people died from it, most of them children under age 5 in Africa.
Malaria is perhaps even tougher to combat than HIV or tuberculosis because of its multiple distinct life-cycle stages and its genetic complexity, which allows Plasmodium to adapt rapidly to drugs and to our immune system's efforts to render it ineffective. With billions of parasites circulating in a single human host, Plasmodium species are poised to resist immunologic and chemotherapeutic attacks. With the aid of mosquito vectors, a single infected individual can transmit to hundreds of other individuals within months, far outstripping the infectiousness of HIV or tuberculosis.
What is Malaria?
Malaria is a febrile, mosquito-borne infection, classically characterized by periodic chills, rigors, and high fevers followed by profuse sweating, which occur at regular intervals of 48 to 72 hours. Infection in humans begins when the infected female anopheline mosquito injects the sporozoite parasitic form from its salivary glands into the bloodstream during a blood meal. The sporozoites are carried to the liver, where they undergo asexual reproduction. When these infected liver cells burst, merozoites are released into the blood, where they invade red blood cells. The intraerythrocytic parasite develops through ring forms into schizonts that produce more infectious merozoites that affect additional red cells. The periodic fever is the result of synchronization of red cell lysis and release of more merozoites. Some of the organisms develop into distinct sexual forms (gametocytes) which, if ingested by the Anopheles mosquito during a feeding, can undergo sexual reproduction that starts the cycle over again.
Malaria is prevalent throughout most of the tropical world, producing a situation in which 50% of the world's population is at risk for acquiring this disease. Affecting 212 million people yearly, malaria is considered one of the most common infectious diseases and the most important of the parasitic diseases. Infections in humans are caused by four different species of the genus Plasmodium. Plasmodium falciparum, Plasmodium malaria, Plasmodium vivax, and Plasmodium ovale are associated with different clinical presentations, progression, prevalence, and antimalarial resistance patterns. P. falciparum infection results in the highest morbidity and mortality, accounting for almost all of the 429,000 deaths caused by malaria annually.
Why should the developed world care?
Although cases of malaria around the world have decreased in recent years, the parasite is rapidly becoming resistant to the drugs we use for its prevention and treatment. In addition, the mosquito is becoming resistant to the sprays and treated nets we use to prevent transmission to humans.
Many U.S. citizens travel to malarious areas and are at severe risk of the disease when they return home if not properly diagnosed and effectively treated. The Centers for Disease Control and Prevention, which reports between 1,500 cases in the United States annually, considers this to be about half the actual number of cases occurring in the country.
Anopheles mosquitoes, which transmit malaria, are common in most populated regions of the world, including almost everywhere in the United States. In the United States, occasional outbreaks transmitted locally by indigenous mosquitoes have occurred in the past five years in California, Texas, Michigan and near New York City.
Anopheline mosquitoes are cold-blooded, which means their body temperature and their ability to nurture the Plasmodium parasite is affected by the temperature of their surroundings. With increasing temperatures due to global climate change, most new areas where there is a potential risk for malaria transmission will be temperate regions where lower temperatures presently limit the mosquito populations and the parasite's ability to develop in them.