Global Warming Heats Up Need for Malaria Vaccine

The link between climate change and the spread of infectious diseases has been a matter of controversy for several years. While the issue has been hotly debated, recent evidence strongly suggests that global warming will, at the very least, be one of several factors that profoundly affect global public health.

Sylvain Fleury

A 2006 study by University of Michigan ecologist Mercedes Pascual, published in the Proceedings of the National Academy of Sciences, found that climate change, in addition to drug and pesticide resistance, changing land use patterns, and human migration, was responsible for the resurgence of malaria (1).

A 2006 study by University of Michigan ecologist Mercedes Pascual, published in the Proceedings of the National Academy of Sciences, found that climate change, in addition to drug and pesticide resistance, changing land use patterns, and human migration, was responsible for the resurgence of malaria (1).

Pascual studied malaria in highland regions and desert fringes, which had previously been spared outbreaks of the disease. A 2002 study had dismissed the role of climate change in malaria's emergence in these areas. But a better appreciation of the sensitivity of the mosquito life cycle to even slight temperature changes has begun to change the thinking of leading scientists. Pascual, for example, found that even small increases in temperature lead to the proliferation of mosquitoes in regions that were previously inhospitable to malaria-bearing mosquitoes.

Most experts agree that warmer temperatures will change the delicate balance of flora and fauna in areas lying at the borderlines of major climate regions. We have already begun to observe such climatic changes in Europe, where the average temperature has increased by nearly 1 °C during the past century.

Malaria has been called the "forgotten epidemic" by Bill Gates, whose foundation has poured tens of millions of dollars into malaria vaccine research. Between 350 and 500 million cases of malaria occur each year, causing more than one million deaths. While most cases occur in sub-Saharan Africa (and most mortalities are in children), the rise in global temperatures will affect developing and developed countries alike in the years to come.

More than 40% of the human race— approximately 2.3 billion people—lives in areas of high malaria transmission. Because Europe, North America, and North Asia are now significantly colder than regions of high malaria incidence (Africa, Asia, the Middle East, Central and South America), developed nations feel immune from the threat of malaria. That complacency could turn tragic, as borderline regions are learning.

For example, malaria has begun to spread to the African highlands, where before 1970, cold temperatures greatly limited populations of malaria-bearing mosquitoes. And three countries that were once considered outside the "malaria belt"—Azerbaijan, Tajikistan, and Turkey—are now deemed by the World Health Organization to lie within an emerging danger zone.

More than 64,000 cases of malaria were reported in Peru in 2007, despite the eradication of malaria from that country 40 years ago. Even the United States, another country where malaria supposedly does not exist, reported 1337 cases of malaria, including eight deaths, in 2002.

Today, industrialized nations produce by far the largest quantities of greenhouse gases but the economic and public health consequences of global warming is felt most strongly in developing countries. That will not always be the case.

Efforts to curb global warming have become mired in economics and geopolitics, with no acceptable solution in sight. Even if the most drastic measures such as severely limiting the use of carbon fuels, were adopted today, the earth would continue to warm for several decades. While some consequences of global warming such as rising sea levels, may be unavoidable, we can take measures to soften the impact of habitat expansion for insect-borne infectious diseases.

A malaria vaccine, which has been the focus of dozens of academic and corporate research programs, would save lives today and prevent the worst consequences of habitat expansion for mosquitoes that carry Plasmodium facilparum, the malaria parasite that causes the more serious forms of the disease and is responsible for about 90% of deaths.

Previous attempts at vaccine development have failed for numerous reasons. The malaria parasite's life cycle is complex, with multiple stages in human hosts, and multiple antigens at each stage. The parasite evades the human immune system by seeking refuge inside cells, where it multiplies.

Unlike viruses and bacteria, the Plasmodium genome encompasses more than 6000 genes encoding hundreds of surface antigens, many of which could serve as vaccine targets. Unlike most pathogens, which display a finite and characteristic set of surface antigens, Plasmodium varies its antigens, similar to the way human beings change their hair color. Finally, the parasite occurs in a wide variety of strains, each with its own unique antigenic components.

Experts believe that a successful malaria vaccine must target the two principal forms of the Plasmodium parasite: sporozoites, the stage present immediately after the mosquito bite, and merozoites, which are released from infected liver cells and cause typical malaria symptoms such as fever and anemia.

An ideal malaria vaccine must be inexpensive enough to administer to a billion (or more) at-risk individuals, and should be stable without refrigeration but robust in its ability to treat all strains, including those that may emerge during epidemics in northern climates.

Mymetics is one of the companies exploring this two-pronged approach, which ideally will induce protection at both stages by preventing sporozoites from infecting the liver cells where they mature into highly infectious merozoites, and preventing merozoites from infecting red blood cells where they propagate and cause disease.

With one death every 30 seconds, malaria remains one of humankind's deadliest diseases, which climate change could make even worse. Regardless of one's position on global warming, a vaccine is our best hope to rid the world of this scourge once and for all.

Sylvain Fleury is chief scientific officer at Mymetics, European Office, 14, rue de la Colombière, Nyon, Switzerland, tel. +41.21.692.57.75, sylvain.fleury@mymetics.com.

Reference

1. M. Pascual et al., "Malaria Resurgence in the East African Highlands: Temperature Trends Revisited," Proceed. of the Nation. Acad. of Scien., 103 (15) 5829–5834 (2006).