Dodging Mosquito-Borne Diseases

Malaria, dengue fever, Japanese encephalitis, yellow fever, chikungunya, West Nile and other unpleasant diseases are mosquito-borne. The WHO estimates that, each year, there are:
•    390 million cases of dengue (2015)
•    198 million cases of malaria (2013)
•    200 thousand cases of yellow fever (2014)
•    68 thousand cases of Japanese encephalitis (2014)

Those are big numbers. Travelers need to be alert to health risks from mosquito bites in some parts of the world, but mosquito borne diseases can be avoided.

Who Has the Most Dangerous Mosquitoes?

You might not know you've been bitten by a mosquito
until you become ill. User: Ngari.norway CC BY-SA 3.0

Mosquito bites in North America, Europe, northern Asia and most of Australia are unwelcome but usually not considered life threatening. They’re painful, itchy, and can become infected, and the sound of a bloodthirsty mosquito whining around your head can drive you mad. Mosquito-borne diseases, however, aren’t usually a big concern. In recent years, a slight risk of contracting West Nile virus has heightened the level of concern about mosquito bites in North America.

In the tropics, it’s different. Mosquito bites can be deadly. Day flying mosquitoes transmit dengue fever and yellow fever: the night flyers bring malaria and Japanese encephalitis. Some locations as well as some seasons mean higher risk. Be careful. The first line of defense is to know the enemy – travelers should take time, before they go, to find out what the risks are at their destination.

Preventing Mosquito-Bourne Diseases

Travelers can be immunized against yellow fever and Japanese encephalitis, and antimalarial drugs are advised to prevent malaria, but none of these work 100 percent of the time, and there’s no protection against dengue or chikungunya. The only way to be sure you don’t catch any of the mosquito borne diseases is to be sure you don’t get any mosquito bites. It’s not as easy as you might think.

A trip to Southeast Asia – my first trip to a destination where mosquitoes were a particular concern – taught me many useful things:

•    Even in the dry season, there are mosquitoes virtually everywhere you go; I saw my first Asian mosquito in the taxi from the Bangkok airport.

•    Many hotels and guest houses do not have intact window screens and tight doors – mosquitoes get in through grates, tears in screens, and under doors. Perhaps this shouldn’t have surprised me, but it did.

•    Mosquito bed netting often isn’t provided either. Mosquito bed netting is indispensable, preferably netting that has been soaked in mosquito repellent. Buy it at home and take it with you. Don’t assume there will be a hook available to hang bed netting – carry a hook that can be screwed into wood and a long piece of cord that you can stretch across a room, say from a curtain rod to a closet rod or some other fixture. You may have to get creative.

•    Don’t expect the local people to warn you about mosquitoes or help you take precautions. Their level of concern can be surprisingly low and they may deny the presence of mosquitoes even when you have just seen one checking out your ankles.

•    Bring lots of mosquito repellent with a minimum concentration of 30% DEET (check recommendations for children). This can be hard to find in Asian shops, though, surprisingly, we had better luck in the small markets used by local people than in the shops aimed at tourists. Remember that insect repellent is only effective for a certain number of hours and will be removed by swimming, sweating, and showering. Carry repellent with you and reapply it diligently – at a minimum, morning and evening.

•    Light colored clothing is recommended, as well as long sleeves and long pants or skirts. Don’t be too quick to rule out the more extensive clothing – long sleeved shirts and pants made of light cotton can be surprisingly cool and they’ll protect you from both insects and sunburn.

I got through my Southeast Asian vacation without any mosquito bites that I was aware of, though one black fly did sample my blood in northern Thailand. I say “that I was aware of” because I found the mosquitoes in Thailand, Cambodia and Laos to be both smaller and quieter than the ones I’m used to; if you don’t react to the bite you might never know you had a visitor.

Malaria Killed Half the People Who Have Ever Lived

Malaria killed half the people who have ever lived. This surprising statement pops up from time to time; in fact, I found a variation of it in a 2002 Nature article by John Whitfield, and Nature is usually a trustworthy source. But is this true? Could it be true?

How Many People Have Ever Lived?

In many human conflicts, the casualties due to
malaria have exceeded those of battle. Image
courtesy of U.S. National Archives and Records
Administration.

Of course we can’t know for sure how many people have ever lived - we don’t have census data going back 52,000 years – but fortunately, someone has tried to guesstimate the answer to this question. Carl Haub, of the Population Reference Bureau came up with a number, despite the fact that “any such exercise can be only a highly speculative enterprise:" roughly 108 billion people have been born.

Therefore, working with Haub’s numbers, if malaria has killed half the people who ever lived, 54 billion people have died from malaria.

How Many People Die From Malaria?

Estimates of the number of annual deaths from malaria differ quite widely. One of the reasons for this is that we still don’t have accurate records. One statistic often quoted is that, today, malaria kills about one million people every year, most of them children. This prompts reflection on a couple of points that are worth acknowledging:
•    Most people who die from malaria are killed before they have a chance to reproduce, which means that the disease will have had a very significant impact on population, even if it hasn’t literally killed half of all humans.
•    Malaria is especially serious during pregnancy, leading to septic abortion, still birth, and poor health in newborns. The WHO states that low birth weight due to malaria in pregnancy contributes significantly to infant mortality; therefore, babies conceived but born dead, and infant deaths indirectly due to malaria are significant as well.

The World Health Report 1999 (WHO) states that “during the first half of the 20th century, the world sustained around 2 million deaths from malaria each year,” so for that period alone, there were a hundred million deaths.

After that, mortality was halved by better treatment and eradication efforts, so we’ll add another 64 million deaths. This is the only part of human history for which we have even roughly accurate numbers.

Malaria Deaths Before 1900

Malaria is the only one of the major epidemic killers of humans that is thought to have been with us throughout our entire evolution, so the death toll started right at the beginning, though numbers were likely low until we had permanent settlements with larger populations.

According to Carl Haub, more than 96 billion people, out of 108 billion, lived before 1900, with more than 85 billion born between 8000BC and 1650. During most of this time, there were essentially no cures for malaria and people would have settled down in permanent settlements, allowing for high transmission of the disease. How many did malaria kill? We still have most of our 54 billion deaths to account for.

If most of these people died between 8000BC and 1650, that would mean an average death rate of 5 ½ million per year. Is that possible?

Again, there are a few other important points to note:

•    Until relatively recently, Plasmodium falciparum, the species that kills, was nearly cosmopolitan (Roberts and Janovy).
•    Before the days of modern medicine, infectious disease was the major killer of humans, and the reason why life expectancy was much lower.
•    Malaria does cause epidemics, but where it is endemic, it is typically present continuously and all year round. Other diseases, such as smallpox, flu, measles, cholera, and plague strike as epidemics and then disappear, often for long periods of time.

R. S. Bray writes: “Morbidity and mortality due to malaria worldwide were greater than any other disease… a world-wide epidemic of enormous proportions (89).”

Other historians tell us that

•    “The Portuguese... had imported so many African slaves into Portugal by the late fifteenth century that their falciparum malaria ignited a series of epidemics so intense that the Tagus valley was almost depopulated (Desowitz, 77).”
•    “Malaria throughout the tropical Old World, cholera in tropical Southeast Asia, and Yellow Fever in tropical Africa were (and still are) the most notorious of tropical killers (Diamond, 214).” (my emphasis)
•    Accounts of European settlements in the tropics and other populations report greater than 90% death toll from malaria and other tropical diseases.
•    “For centuries, malaria has outranked warfare as a source of human suffering (John F. Kennedy).”

Desowitz quotes the U.S. Public Health Service, 1919: “for the South as a whole it is safe to say that typhoid fever, dysentery, pellagra, and tuberculosis, all together, are not as important as malaria.” Desowitz notes that this would be equally valid for the preceding 200 years.

Has Malaria Killed Half the People Who Have Ever Lived?

So did malaria kill between 53 and 54 billion of the 96 billion who lived before 1900? I’m neither an epidemiologist nor a statistician, and I had limited (secondary) sources to work with. We’ll never know for sure, but based on my reading I think it’s possible.

If we also consider deaths indirectly due to malaria – and here I’ll also invoke the parasite theory of human values/morals, which suggests that much war and other violence is, at its root, due to infectious disease – I think it’s indisputable.

Resources

Bray, R. S. 1996. Armies of Pestilence: The Impact of Disease on History. New York: Barnes & Noble.

Desowitz, Robert S. 1997. Who Gave Pinta to the Santa Maria?: Torrid Diseases in a Temperate World. New York: W. W. Norton and Co.

Diamond, Jared. 1997. Guns, Germs, and Steel: The Fates of Human Societies. New York: W. W. Norton and Co.

Haub, Carl. 2011. “How Many People Have Ever Lived on Earth?” Population Reference Bureau.

Roberts, Larry S., and John Janovy Jr. 2009. Gerald D. Schmidt & Larry S. Roberts’ Foundations of Parasitology. Boston, McGraw Hill.

Whitfield, John. 2002. “Portrait of a Serial Killer: A Roundup of the History and Biology of the Malaria Parasite.” Nature. doi:10.1038/news021001-6

WHO. “Rolling Back Malaria.” The World Health Report 1999. p. 49 – 63.