Physiological Determinants of Global Health

Malaria

A worrisome change from the point of view of human health is the potential expanded range of the either the Anopheles mosquito or the Plasmodium parasite, the vector and infectious agent of human malarial transmission. Malaria is a deadly disease that leads to over one million deaths in the developing world annually. Areas currently free of malaria are at risk of becoming more hospitable to the Anopheles mosquito or more favorable to the Plasmodium parasite, which could lead to an increase in cases of malaria.

Warmer temperatures lead to longer seasons for mosquito breeding, and expanding ranges into areas that were previously too cold to allow for the mosquitoes to survive. Areas that receive more water also provide areas for mosquito breeding.⁵

Malaria is a global killer that affects between 200 and 300 million people every year,⁶ the majority in poor and undeveloped countries in tropical regions. The disease is caused when a mosquito infected with parasitic protozoa of the genus Plasmodium bites a human and infects the bite victim. Over 1 million people die of malaria annually, due to lack of medical access to lifesaving medicines such as cholorquine and mefloquine. Both of these are orally administered medications that can be used prophylactically or for treatment.

Plasmodium is a single celled, eukaryotic microorganism with a complex lifecycle. It is an obligate parasite, which means it is only able to live inside of a vector and host organism, in this case the Anopheles mosquito and vertebrates. The genus Plasmodia comprises over 200 recognized species, of which approximately 100 species are pathogenic and cause some form of malaria.

The symptoms of malaria include fever, fatigue, nausea and respiratory distress. A worst-case scenario can lead to coma or death. Malaria is curable if it is properly diagnosed and treated with medication. Sadly these medicines are lacking throughout much of the developing world where malaria is found today. Malaria can be diagnosed through microscopic analysis of a blood smear by identifying the parasite in the blood cells or serum.

Due to a lack of adequate medical facilities and trained personnel, often the only recourse is to focus on the prevention of transmission through the use of bed nets to prevent mosquitos from biting, and pesticide spraying to kill the vector mosquitoes.

Malaria was once endemic to the United States; it was found all along the west coast, eastern seaboard and especially in the South, with thousands of cases reported annually well into the 20^th century. In 1947 a government campaign was begun to eradicate the disease, and by 1951 it was declared successful.⁷ This amazing endeavor was a result of attacking the disease on several fronts. Mosquito population reduction was performed by changing agricultural land use to minimize standing water, where the mosquito breeds. Pesticides such as DDT were used to kill adult mosquitos upon contact. Improved diagnosis and treatment with newly available drugs prevented the spread of the disease by killing the parasite and thereby reducing the chances for a mosquito to become infected with the parasite.

Most species of mosquitos in the United States were never implicated in malaria transmission, Texas leads the nation with 85 native species of mosquitos and Florida is second with 80. Four species of Anopheles mosquitos that were responsible for malaria transmission are still found in the United States. Between 1957 and 2014, there were 63 cases of local malaria transmission reported in the United States.⁸ An infected person, infected in an endemic area, brought the parasite back to the U.S. where local mosquitos were infected upon biting the affected person.

The cost of malaria can be measured in human life and suffering, as well as in the economic and societal costs to countries with endemic malaria. To this end, The Bill and Melinda Gates foundation has dedicated over $1.3 billion to combat this deadly disease. Their goal is nothing less than the complete global elimination of malaria.⁹

Malaria infection begins when an infected female Anopheles mosquito bites someone. The malaria infection cycle can be broken into three locations. The first location is in the infected person’s liver. Once bitten by an infected mosquito, Plasmodium parasites, in the form of sporozoites, are injected into the bloodstream. Once in the host, they quickly migrate to the human liver. There, they multiple asexually within 1-2 weeks. This phase is asymptomatic. The next location is the bloodstream. In this phase of the lifecycle, the parasite is called merozoites. It is released by the liver in vesicles into the circulatory system that then migrate to the lung capillaries. Once lodged in the lungs, the vesicles disintegrate allowing the parasite to travel freely in the bloodstream where they invade red blood cells. Once inside the blood cells they again multiply asexually, leading the enlarged cells to burst, and releasing more parasites to repeat the cycle. Some of the merozoites will progress to the next phase called gametocytes, where they reproduce sexually. The gametocytes circulate in the bloodstream where they can reinfect a biting mosquito. The third location is inside of an Anopheles mosquito, where they continue their lifecycle into another phase called ookinetes. The ookinetes develop into sporozoites and then migrate to the mosquito salivary glands where they can infect a new victim starting the cycle anew.