Malaria Transmission and Deaths in the United States

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QUESTION

How do you get malaria? How does malaria come to the United States? How many people have died from malaria?

ANSWER

Malaria is caused by infection with single-celled parasites called Plasmodium. There are five different species which infect humans; of these, Plasmodium falciparum is the most deadly. The Plasmodium parasites are transmitted to humans through the bite of an infected female Anopheles mosquito; when these mosquitoes bite a human, they transfer some of the parasites in their saliva.

After undergoing one set of multiplications in the liver, the malaria parasites are released into the blood, where they repeatedly infect and destroy red blood cells, multiplying in the process. These cycles of infection and destruction of red blood cells cause the cyclical fever that is characteristic of malaria. Eventually, the parasite produces new types of cells, called gametocytes; if the patient is then bitten by another mosquito, the mosquito can take up gametocytes as well as blood when it feeds, and the cycle continues.

Malaria used to be relatively common in the United States, though usually only occurring during the summer months and restricted to the warm, wet south-eastern region. However, a concerted control campaign in the 1940s, focusing mainly on vector control (i.e. killing mosquito and reducing their opportunities for breeding) swiftly led to the eradication of malaria from the US.

Nowadays, virtually all malaria cases in the US are imported from abroad, whereby people get infected when traveling to other countries, but are only diagnosed as having malaria when they return home. There are about 1,000 cases of malaria reported each year in the US, and most of these were actually caught outside the US. Very rarely, an Anopheles mosquito will bite one of these travelers once they have returned from abroad, and therefore have the potential to transmit malaria within the US. However, due to a strong public health network and good access to malaria diagnosis and treatment, these events rarely lead to more than a handful of cases before they are quickly treated and transmission eliminated again.

The World Health Organisation estimates that last year, approximately 700,000 people died of malaria. Of these, 90% were in sub-Saharan Africa, and the vast majority were children under the age of 5.

Dangerous

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QUESTION

Why malaria so dangerous?

ANSWER

Malaria can be dangerous for a number of different reasons, some of which relate to each other. First of all, there are five different types of malaria that infect humans, and each varies in terms of its severity and potential for severe consequences. Even within these types, the severity of the disease caused (termed “virulence” by scientists and doctors), can even vary by strain or geography. Generally, the most dangerous form of malaria is caused by Plasmodium falciparum. One reason why this species of malaria is so dangerous is that is replicates very quickly in the blood. This means that infection levels can build up very quickly; if a person infected with P. falciparum does not get diagnosed and treated within a few days of feeling sick, the infection can progress to a point where the disease becomes very severe. This rapid accumulation of infection is also observed with P. knowlesi, a much rarer form of malaria found in south-east Asia. The parasites of P. knowlesi have a 24-hour reproductive cycle in the blood, the quickest for any type of malaria that infects humans. However, P. falciparum also has other characteristics which make it even more dangerous, and which do not occur with P. knowlesi. For example, when P. falciparum infects red blood cells, it causes their shape to change, and makes them “sticky”. This stickiness causes the red blood cells to become lodged in the blood vessels leading in to major organs, in a process known as sequestration. Sequestration creates blockages of these blood vessels, reducing blood flow and resulting in oxygen deprivation. When this process occurs in the blood vessels in the brain, the outcome is known as cerebral malaria, characterised by impaired consciousness, coma and even death. It is this pathology which is associated with most cases of severe malaria, and causes the most number of deaths.

However, if treated promptly with the correct drugs, even P. falciparum malaria is usually easily controlled. Therefore, one of the additional reasons why malaria is so dangerous is that in many places, and particularly sub-Saharan Africa, people do not have access to medication, or not the right types of medication. Many strains of P. falciparum have become resistant to chloroquine, once the first line treatment for malaria, and so this drugs is now ineffective in many cases. Instead, the World Health Organisation recommends now that artemisinin-based combination therapies (ACTs, such as Coartem) should be given as first-line treatment against all uncomplicated malaria, to prevent additional resistance from developing.

Malaria Parasite Picture

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QUESTION

Please show parasites pictures.

ANSWER

The first plate below shows various stages of the life cycle of Plasmodium falciparum, the most deadly form of malaria, and the most commonly found type in sub-Saharan Africa. Below that, a similar plate shows a series of images of Plasmodium vivax, the most widespread type of malaria.

A: The stages of P. falciparum. 1: Normal red cell; Figs. 2-18: Trophozoites (among these, Figs. 2-10 correspond to ring-stage trophozoites); Figs. 19-26: Schizonts (Fig. 26 is a ruptured schizont); Figs. 27, 28: Mature macrogametocytes (female); Figs. 29, 30: Mature microgametocytes (male). Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. “The Primate Malarias”. Bethesda: U.S. Department of Health, Education and Welfare; 1971. Reproduced here courtesy of the CDC (www.cdc.gov)