Epidemics Throughout History

Malaria

A thin-film Giemsa stained micrograph of ring-forms, and gametocytes of Plasmodium falciparum.

Brief Overview

* Malaria is one of the most common infectious diseases and an enormous public-health problem.

* Malaria is a vector-borne infectious disease that is widespread in tropical and subtropical regions

* It infects between 300 and 500 million people every year and causes between one and three million deaths annually, mostly among young children in Sub-Saharan Africa.

* The disease is caused by protozoan parasites of the genus Plasmodium.

* The most serious forms of the disease are caused by Plasmodium falciparum and Plasmodium vivax, but other related species can also infect humans.

* This group of human-pathogenic Plasmodium species is usually referred to as malaria parasites.

* Malaria is not just a disease commonly associated with poverty, but is also a cause of poverty and thus a major hindrance to economic development.

* This sometimes fatal disease can be prevented and cured. Bednets, insecticides, and antimalarial drugs are effective tools to fight malaria in areas where it is transmitted. Travelers to a malaria-risk area should avoid mosquito bites and take a preventive antimalarial drug.

* During 1988, there were 1,023 malaria cases reported in the U.S., 991 of them were introduced cases. In 1992, 910 malaria cases were reported in the United States

* Most Americans today don't realize that just a few generations ago, Malaria was a big problem in the United States...discussed later

Discussing malaria in general...

History

* Malaria has infected humans for over 50,000 years, and may have been a human pathogen for the entire history of our species.

[1] Indeed, close relatives of the human malaria parasites remain common in chimpanzees, our closest relatives.

[2] References to the unique periodic fevers of malaria are found throughout recorded history, beginning in 2700 BC in China during the Xia Dynasty.

[3] The term malaria originates from Medieval Italian: mala aria -- "bad air"; and the disease was formerly called ague or marsh fever due to its association with swamps

* Scientific studies on malaria made their first significant advance in 1880, when a French army doctor working in Algeria named Charles Louis Alphonse Laveran observed parasites inside the red blood cells of people suffering from malaria.

o He therefore proposed that malaria was caused by this protozoan, the first time protozoa were identified as causing disease.

* A year later, Carlos Finlay, a Cuban doctor treating patients with yellow fever in Havana, first suggested that mosquitoes were transmitting the disease to humans.

* However, it was Britain's Sir Ronald Ross working in India who finally proved in 1898 that malaria is transmitted by mosquitoes.

He did this by showing that certain mosquito species transmit malaria to birds and isolating malaria parasites from the salivary glands of mosquitoes that had fed on infected birds.

* Although the blood stage and mosquito stages of the malaria life cycle were established in the 19th and early 20th centuries, it was not until the 1980s that the latent liver form of the parasite was observed.

* The discovery of this latent form of the parasite finally explained why people could appear to be cured of malaria but still relapse years after the parasite had disappeared from their bloodstreams. Discussed more later...

Distribution & Impact

* Malaria causes about 350-500 million infections in humans and approximately one to three million deaths annually -- this represents at least one death every 30 seconds.

* The vast majority of cases occur in children under the age of 5 years; pregnant women are also especially vulnerable.

* Despite efforts to reduce transmission and increase treatment, there has been little change in which areas are at risk of this disease since 1992.

* Indeed, if the prevalence of malaria stays on its present upwards course, the death rate could double in the next twenty years.

* Precise statistics are unknown because many cases occur in rural areas where people do not have access to hospitals or the means to afford health care. Consequently, the majority of cases are undocumented.

* Where malaria is found depends mainly on climatic factors such as temperature, humidity, and rainfalls. Malaria is transmitted in tropical and subtropical areas, where:

o Anopheles mosquitoes can survive and multiply

o Malaria parasites can complete their growth cycle in the mosquitoes ("incubation period").

* Even within tropical and subtropical areas, transmission will not occur:

At high altitudes

During cooler seasons in some areas

In deserts (excluding the oases)

In some islands in the Pacific Ocean, which have no local Anopheles species capable of transmitting malaria

In some countries where transmission has been interrupted through successful eradication.

* Generally, in warmer regions closer to the equator:

Transmission will be more intense and Malaria is transmitted year-round

* Malaria is presently endemic in a broad band around the equator, in areas of South America, South and Southeast Asia, parts of the Middle East, and much of Africa; however, it is in sub-Saharan Africa where 85- 90% of malaria fatalities occur.

* Endemic--> Common diseases that occur at a constant but relatively high rate in the population

* The geographic distribution of malaria within large regions is