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.

Neospora caninum and Neosporosis

The sign on this public footpath in the UK reminds dog
owners to pick up after their pets. Cattle in the area have
aborted due to Neospora caninum.  Image by Peter Barr
 CC BY-SA 2.0

Neospora caninum, a parasite of dogs, causes abortion in cattle

Neospora caninum is a coccidian parasite of domestic dogs but it causes big problems for the cattle industry. We’re still learning about N. caninum and neosporosis.

In August 2007, dog owners in Somerset, UK were asked to be especially vigilant about picking up after their dogs. The reason was a spike in the number of cattle aborting fetuses in the area—some of the cattle tested positive for a parasite, Neosporum caninum, carried by domestic dogs.

The parasitic disease called neosporosis was first recognized in domestic dogs in Norway in the 1980s. It’s now known that the parasite is present in dogs, cattle, and other animals worldwide. A coccidian, it’s related to Toxoplasma gondii and Cryptosporidium, well known causes of parasitic disease in humans.

Life Cycle of Neospora caninum

The life cycle of N. caninum in dogs is very similar to that of T. gondii in cats. An infected dog has parasites multiplying sexually in the intestine and the infective oocyst is passed in the dog’s feces. Meanwhile, parasites are also multiplying asexually in other tissues.

Harbouring the sexual form of N. caninum in the intestine makes domestic dogs the definitive host of the parasite. It’s not known whether other animals are capable of serving as a definitive host, but wild dogs, such as foxes, wolves, coyotes etc. may do so.

Animals other than dogs that ingest either oocysts in dog feces or animal tissue in which the parasite is present become intermediate hosts – they have only the asexual stage, multiplying in the tissue. Thus, dogs pass on the parasite in their feces, in their tissue if they are eaten by other animals, or to an unborn fetus. No animals other than dogs have been known to spread the parasite in feces.

Neosporosis in Dogs

Neospora caninum infects domestic dogs worldwide with varying prevalence. Studies testing dogs for antibodies to the parasite suggest that more than 30% of dogs are infected in some areas, with the highest numbers in South American countries and in rural dogs, especially those living on cattle farms.

Most infected dogs have no symptoms. When symptoms occur, neosporosis is most severe in newborn puppies, infected during gestation when the parasites move from the bitch’s tissues to the fetus. Puppies suffer paralysis, particularly of the hind legs, and often do not survive. Adult dogs may suffer from an illness similar to toxoplasmosis in cats, or they may develop dermatitis.

Neosporisis in Cattle

Like dogs, cattle everywhere harbour N. caninum, and most show no signs of it. In some herds, close to 90% of cattle are infected and the parasite is thought to account for more than 40% of abortions – a significant cause of economic loss for cattle farmers. Many infected fetuses and calves appear normal, however, and it is still unclear what factors cause or prevent disease symptoms.

In cattle, N. caninum is transmitted only from a pregnant cow to her fetus—the parasite does not pass between cows in a herd. Some cows, then, must acquire the parasite from dogs, consuming oocysts while grazing where dogs have defecated. It’s easy to imagine how farm dogs and livestock (sheep, goats, and horses can also be infected) may have increased the prevalence of the parasite, with dogs eating the remains of aborted young, becoming infected, and then passing infective oocysts in feces deposited where livestock graze. Calves born without symptoms, meanwhile, pass the parasite on to their own young.

Whether the abortions occurring in Somerset in England resulted from infected dogs defecating on cattle farms, or whether they came from silent infections already present in the cattle remains unknown; however, picking up after your dog is always good practice, and will lessen the risk of spreading not only neosporosis, but other diseases as well.

Sources:

Foundations of Parasitology 8th Ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2009.

Review of Neospora caninum and Neosporosis in Animals. Dubey, J. P. The Korean Journal of Parasitology 41:1 Mar 2003, 1-16.

Will Eating Oranges Kill Parasites?

Oranges are a popular and relatively
affordable fruit. It would be great if
claims that they kill parasites were
true. Image by Rosino: CC BY-SA 2.0

Have you read that oranges have antiparasitic properties, and that eating them can help keep parasites away? Is there any truth to this claim? It’s certainly true that many substances derived from plants are antiparasitic, so what’s the evidence for oranges?

Antiparasitic Oranges - The Science

A search for papers on the antiparasitic properties of oranges yields very little, but when I looked at orange essential oil and parasites, I found some research. Studies have used orange oil to treat parasites in fish, sheep, gerbils, and in the lab, but none (at least none that I could find) have used human subjects.

A study by Squires and colleagues investigated whether an orange oil emulsion would kill a roundworm parasite in gerbils and sheep. These researchers found that, at a high enough dosage, the treatment killed a significant number of the worms, suggesting that it might be useful in animal parasite control.

In contrast, in a 1990 Japanese study, “many essential oils were found to be nematocidal to the larvae of dog-roundworm, Toxocara canis,” but essential oil of oranges wasn’t one of the star examples. In the lab, only about a third of larvae were killed by orange oil, and only after 24 hours of exposure.

Hirazawa and colleagues studied whether plant essential oils could be used to kill a flatworm that infests puffer fish. They looked at caprylic acid (derived from palm and coconut oil, and human milk), orange oil, peppermint oil, and cinnamon oil, exposing the parasites to the oils in the lab. The researchers found that orange oil is the only one of the four that does not affect this parasite.

Other studies found orange oil to be ineffective at killing honey bee mites, and ineffective at killing kissing bugs, the vector of Chagas' disease. So it looks like orange oil may kill some parasites, under specific conditions, but it’s no panacea. The good news is that it does appear to be useful, if used properly, in the control of subterranean termites.

Oranges vs Orange Essential Oil

Essential oils are derived from plants using a distillation process. They are highly concentrated essences, and orange oil is generally made from the peel of the fruit. Obviously, eating an orange is very different from ingesting orange oil (and I’m not sure that this would be safe). I wonder how many oranges one would have to consume (including the peel) to get any benefit, assuming you had a parasite that they would kill.

I believe it’s safe to say that there is no convincing evidence that orange oil is a good way to keep parasites at bay, and there is no evidence at all for oranges.

Pity.

Sources

Hirazawa, N., T. Ohtaka, and K. Hata. 2000. “Challenge trials on the anthelmintic effect of drugs and natural agents against the monogenean Heterobothrium okamotoi in the tiger puffer Takifugu rigripes.” Aquaculture, 188, 2000

Nakamura N., F. Kiuchi, Y.Tsuda et al. 1990 "Nematocidal and bursting activities of essential oils on the larvae of Toxocara canis." Shoyakugaku Zasshi: 44(3).

Squires J., J. Foster, D. Lindsay et al. 2010. "Efficacy of an orange oil emulsion as an anthelminticagainst Haemonchus contortus in gerbils (Meriones unguiculatus) and in sheep."  Am.Vet Parasitol. Aug 27;172(1-2):95-9. doi: 10.1016/j.vetpar.2010.04.017. 

The Rise and Fall of the Guinea Worm, Dracunculus medinensis

As of May 2013 there's a bounty on the Guinea worm (Dracunculus medinensis) in Nigeria. Anyone reporting a confirmed case of Guinea worm infection, or dracunculiasis, will receive a cash reward of N25,000, or about US158.00. This marks another milestone in the battle of humanity versus the Guinea worm that's been going on for thousands of years.

Even today, the only effective way to get rid
of a Guinea worm is to slowly wind it around a
stick. The process can take weeks.
 Public Health Image Library.

Fortunately for anyone hoping to claim the cash, a case of Guinea worm infection is fairly easy to recognize. A description from Persian physician Avicenna, written a thousand years ago, is as good as any we'd write today:
“A pustula first appears and swells up, but afterwards contracts down again to a mere bleb. ...the bleb perforates and dark red matter is continuously exuded. ...movement can be distinguished beneath the skin as though some living thing were there, and indeed... a worm is present... For the most part it is the legs that are involved...” (translation quoted in Hoeppli, Parasites and Parasitic Infections in Early Medicine and Science).

The Origin and Historic Range of the Guinea Worm

Guinea worm may be gone from Nigeria, and the bounty is part of an effort to prove that. In the last quarter century the parasite has been pushed back, and back, and back, to its last bridgehead in Sub-Saharan Africa north of the equator.

As recently as 80 years ago, however, the worm was a torment in present day Uzbekistan, Turkmenistan, Pakistan and India, throughout the Arabian Peninsula, and in much of the northern half of the African continent. In its heyday, about 120 million people lived in its range and were at risk of infection. More than three million people suffered this extremely painful, debilitating, and basically untreatable parasitic infection every year.

When Did the Guinea Worm First Infect Humans?

We don't know where the Guinea worm first infected a human. It's tempting to think that it evolved somewhere in Africa because similar parasites of other vertebrates occur in both the Americas and Australia, both of which, with Africa, were part of the supercontinent Gondwana 100 – 125 million years ago.

Whatever its origins, the earliest written records of dracunculiasis come from North Africa and the Middle East, with the area around the Red Sea being particularly notorious.

Early Records of Guinea Worm Disease

Early references to dracunculiasis are uncertain, as people referred to the disease by different names, wrote in ancient languages and described things in various ways. The Papyrus Ebers may contain the oldest written reference to D. medinensis: the Egyptian document is dated to about 1550 BC, but it's thought to contain information copied from texts that were from earlier dates, perhaps a thousand years earlier or more.

The reference in the Papyrus Ebers that scholars think relates to Guinea worm describes “a swelling... in any limb of a man... it goes and comes, piercing through the flesh which is under it...” (translation quoted in Hoeppli).

Others describe the parasite as well, some with various remedies for dracunculiasis, though writers were unsure exactly what they were dealing with. Even as late as the 1700s, scientists disagreed as to whether D. medinensis was a worm, a vein, a nerve, an abscess, or even a piece of plant material.

Everyone agreed on one thing however: the condition caused untold misery and hardship.

The widespread misery may have been enough to give the worm a lasting place in history and Western culture. One theory suggests the Guinea worm inspired the traditional medical symbol still used today - the rod of Asclepius, which features a serpent wound around a stick.

Dracunculus medinensis may also be the “fiery serpent” that inhabited the land around the Red Sea – the serpent that bit and killed so many of the Children of Israel in the Old Testament book of Numbers.

Guinea Worm's Spread and Retreat

Step wells in South Central Asia were built to allow water
levels to change drastically. People walked right into the
water to access it. Wells like this one supplied many people
and encouraged the spread of Guinea worm.
 Image by Chetan. CC BY-SA 3.0

How this fiery serpent spread so far and wide is clear once one understands the Guinea worm's life cycle: it relies upon a dry environment where relatively few sources of water draw people together to drink, wash, and sooth their excruciating Guinea worm lesions in cool water. There also the intermediate host, a tiny crustacean, flourishes; it eats the worm's larvae and in turn is swallowed by thirsty humans, who may then move on to some other locale, some other waterhole.

How the fiery serpent has been driven back is also clear once one understands that two things will break the life cycle: people with Guinea worm lesions must not immerse their lesions in drinking water, and people who take drinking water from ponds and wells that might harbour the parasite must filter out the tiny water crustaceans before drinking.

Extinction of the Guinea Worm

Since the 1930s, cultural changes, chemical treatment of water sources, health education, and simple water filtration have had the worm in retreat from the north and east of its range. A Guinea worm eradication program, spearheaded by the Carter Center and in progress since 1984, has now pushed the parasite to near extinction. Less than 600 cases were reported in 2012, most of them in the African country of South Sudan. It's virtually certain that the story of the dreadful Guinea worm is about to end, forever.

Resources

Beaver, Paul Chester; Jung, Rodney Clifton et al. Clinical Parasitology 9th ed. (1984). Philadelphia: Lea and Febiger.

WHO Collaborating Center for Research, Training, and Eradication of Dracunculiasis, Center for Global Health, Centers for Disease Control and Prevention. "Guinea Worm Wrap-Up #216." (Jan 17, 2013). Accessed June 19, 2013.

Drisdelle, Rosemary. Parasites: Tales of Humanity's Most Unwelcome Guests. (2010). University of California Press.

Hoeppli, R. Parasites and Parasitic Infections in Early Medicine and Science. (1959). University of Malaya Press.

Premium Times. Nigeria Seeks Support on Guinea Worm Eradication. (May 17, 2013). Accessed May 21, 2013.

The Carter Center. "Guinea Worm Disease Eradication." Accessed May 22, 2013.

Symptoms of Elephantiasis (Lymphatic Filariasis)

“Elephantiasis… drew the attention of the physicians and laymen from early times on account of the often grotesque and horrible disfigurement of the patient” (Hoeppli, 33).

Lymphatic filariasis, or elephantiasis, is one of those diseases that manages to convey both its best known symptom and its special horror in a single word: its common name. Elephantiasis. Limbs as thick and gnarled as three trunks, a scrotum so enlarged that it has to be carried around in a wheelbarrow, dreadfully swollen heavy breasts.

That’s what elephantiasis is, but when it’s caused by parasites – worms living in the lymph vessels – the underlying infection is lymphatic filariasis. This is a disease of the tropics: Africa, much of Asia, Australia and islands in the Pacific, even parts of South America and the Caribbean. It is transmitted by mosquitoes.

This map shows areas of the world where lymphatic filariasis had a
severe impact in 2004. The darker the colour, the greater the impact.
Map by Lokal_Profil CC BY-SA 2.0

Asymptomatic Lymphatic Filariasis

Many people who harbour filarial worms never have any symptoms. Fetal exposure to the worm antigens in infected women appears to result in some level of tolerance in children.

It’s thought that elephantiasis only develops after many years and repeated infection; thus, though children are infected at an early age in areas where the disease is found, symptomatic lymphatic filariasis is a disease of adults.

Acute Lymphatic Filariasis

Researchers are not sure what causes an asymptomatic infection to progress to inflammation of the lymph vessels and lymph nodes, fever and chills, and swelling of tender affected limbs. It may be the death of the adult worms, which live for perhaps a decade. At this stage, bacteria are present in inflamed tissues, and these may originate from the victims skin or from the worms themselves: filarial worms are known to carry symbiotic bacteria that the human immune system reacts to.

The acute phase often also brings enlargement and inflammation of the testes, however symptoms typically subside in about a week.

Elephantiasis

The slide toward elephantiasis begins when the lymph vessels become enlarged and blocked as a result of the presence of adult worms. Lymph accumulates in the tissues instead of flowing through the vessels back to the bloodstream, and permanent swelling occurs with thickening and folding of the skin. Scar tissue forms, the urine becomes milky with lymph fluid, and bacterial infection is common.

Interestingly, when visitors to an affected region acquire the infection the immune response is different because they have not been exposed since infancy. For these people, symptoms may include inflammation of the lymphatic system that waxes and wanes over many years.

Roberts and Janovy point out that the name elephantiasis is nonsense, since it literally means “a disease caused by elephants!” I suppose, however, that if it seemed like you were turning into an elephant, the name would seem about right.

Further reading:

Hoeppli, R. Parasites and Parasitic Infections in Early Medicine and Science. University of Maylaya Press, 1959.

Hotez, Peter J. Forgotten People, Forgotten Diseases. Washington: ASM Press, 2008, 39 – 47.

Roberts, Larry S., and John Janovy Jr. Gerald D. Schmidt & Larry S. Roberts’ Foundations of Parasitology 8^th ed. Boston: McGraw Hill, 2009, 463 - 468.

Toxoplasma gondii - Andrew Alexander’s Terrifying Parasite

“I felt like Frankenstein,” Alice Smellie quotes Andrew Alexander in the MailOnline. The Downton Abbey star apparently picked up a “terrifying” parasite more than a year ago and waited months for a diagnosis. When that diagnosis came, it was disturbing, no doubt, but “terrifying” is a bit over the top. (Admittedly, the prevailing dramatic and negative response to anything parasitic could account for the terror.)

 

The series Downton Abbey is set in the early 1900s.
No one was worried about toxoplasmosis then, but
other parasites were much more common in England
  than they are today.
      Image by JB + UK_Planet; CC BY 2.0

Toxoplasma gondii, a Terrifying Parasite

Alexander had Toxoplasma gondii – often simply called toxoplasma. Smellie writes that the not-so-terrifying toxoplasmosis is actually “an incredibly common bacterial infection.” She’s right that it’s common; in fact, as many as a third of people living in the UK are infected with the parasite. But T. gondii is not a bacterium, it’s a protozoan.

What qualifies T gondii as a disturbing, and concerning, parasite? A handful of things:

• It lives in your tissues, including the brain, and once you have it, you have it forever. 
• A first exposure during pregnancy can have truly terrifying implications for the fetus. 
• If you have it and your immune system falters (with AIDS for instance), reactivation of the infection can be fatal. 
• There is evidence that T. gondii can change behavior, and it is implicated in some forms of mental illness. 
• Acute infection can make you look like Frankenstein. (Well, it can cause swollen lymph nodes but it usually doesn’t).

Andrew Alexander’s Forever Parasite

Most people who walk around with T. gondii in their tissues never know they have it. The acute infection is typically mild and goes undiagnosed. Perhaps one or two in ten infected people have more severe symptoms like Alexander: flu-like symptoms with fever, headache, sore throat, achy muscles, swollen lymph nodes. Within a few weeks, even these people usually feel better, though it can take much longer.

When good health returns, it means that the immune system has won the battle – at least for now. The parasite is no longer multiplying rapidly and spreading; it has formed tiny cysts and basically gone dormant. In the parasite’s perfect world, a carnivore eventually eats the host and the parasite gets to infect that carnivore next. I suspect, however, that Andrew Alexander’s bradyzoites have reached a dead end.

How Did Andrew Alexander Catch Toxoplasma gondii?

Most people catch T. gondii in one of two ways:

• By eating meat that is not fully cooked. Lots of people eat rare, or raw, meat; domestic and wild animals are often infected with T. gondii. The meat you buy from the butcher might well contain bradyzoites. 
• By swallowing infective oocysts that have been passed in cat feces. Cats are the only host in which T. gondii multiplies sexually, and during acute infection, a cat passes millions of oocysts.

Smellie’s article relates that Alexander became ill after returning from Africa. Did he catch T. gondii there? Possibly. But he could have picked it up just about anywhere. This parasite is ubiquitous. Where was he one to three weeks before becoming ill? That’s where he got it.

Further reading:

Drisdelle, Rosemary. Parasites: Tales of Humanity's Most Unwelcome Guests. University of California Press. 2010

Smellie, Alice. "I looked like Frankenstein... the cast gathered round as if I was about to expire: Downton's Andrew Alexander reveals his battle with a terrifying parasitic infection." MailOnline; Oct 12, 2013.

"Toxoplasmosis: Toxoplasma Infection." The Center for Food Security and Public Health, Institute for International Cooperation in Animal Biologics. May 2005

Elephantiasis and Wolbachia: The Bacteria Behind the Worms

Horrific parasites sometimes turn out to be mere vessels.

Filarial worms, nematodes that are transmitted by insects and live in the tissues, cause horrible disfigurement, disability, blindness. The common names for the diseases they cause are descriptive and apt: elephantiasis, and river blindness.

Wucheria bancrofti, and Brugia malayi nematode worms live in human lymph vessels. Prolonged presence of the worms there often results in the affected part – often a limb or the scrotum - becoming grossly enlarged and deformed. Microfilaria, the worm’s young, circulate in the blood.

Elephantiasis results in almost
unbelievable disfigurement of
various parts of the body.
Imagine living with this.
Tropenmuseum of the Royal
Tropical Institute (KIT)
CC BY-SA 3.0

Onchocerca volvulus adults live together in nodules in the skin. Their young migrate through the skin and, over the long term, can cause discoloration, aging, and sagging of the skin. Worse, microfilariae in the eyes cause blindness.

For a long time, we thought that the worms themselves, and the microfilariae, caused these terrible responses to infection – that the battle between the human body and the worms caused blindness and disfigurement. But now it’s becoming clear that the real culprit is not the worms, but a type of bacteria living inside the worms. The problem is Wolbachia pipientis.

Wolbachia and Filarial Worms

Wolbachia is best known for living in insects, where it is typically parasitic and sometimes causes fatal disease. In nematodes, the relationship is different. In nematodes, Wolbachia is a symbiont: the bacterium actually provides nutrients to the parasitic worm that the worm can’t otherwise get, and without the Wolbachia, the worm can’t survive.

Researchers now have compelling evidence that what the human immune system responds to is actually the Wolbachia, not the filariae or the microfilariae. It’s the Wolbachia that causes elephantiasis and river blindness.

Wolbachia, Filarial Nematodes’ Achilles Heel

Treatment of the diseases caused by filarial nematodes has proven difficult for many reasons, not the least of which is that few drugs kill the adult worms. The discovery of their reliance on Wolbachia reveals a weakness that may be the worm’s undoing: instead of finding a drug to kill the worms, we can use a drug to kill the Wolbachia.

Ironically, if it’s the Wolbachia that’s actually the culprit, the worms end up as collateral damage.

Schistosomes: The Snail, the Prawn, and the Dam

It’s a story that has played out in Africa all too often. A river is dammed to control flooding, or for hydroelectric power, or to create a water reservoir in case of drought (or all of the above at once). The dam, of course, creates a large lake and lots of good habitat along the shore for aquatic plants, which in turn support a large snail population.

Schistosome Life Cycle

People come to live near the water’s edge, to use the water and to fish. Generally, human contact with water is high, particularly for children and fishers. If schistosomes are present, along with the right snail species, escalating levels of schistosomiasis soon follow. This is how it works:

• People infected with schistosomes pass the eggs in urine or feces, and if those eggs find their way into fresh water, they hatch.

• The emerging larva seeks out an aquatic snail – only the right snail will do – and penetrates the snail, after which it multiplies asexually in the snail’s tissues.

• Larvae leave the snail host and, if a human enters the water, penetrate human skin, maturing to adult worms that reside in the tiny blood vessels around the bladder or intestine.

• Adult worms produce eggs which are passed in the urine or feces, and so on.

It makes perfect sense that water resource projects can result in rapidly increasing incidence of schistosomiasis because they bring together the two hosts of the parasites – the human and the snail – in circumstances ideal for transmission. We hardly need a more complicated explanation. It seems, however, that there’s at least one other factor that’s been left out: the African river prawn.

River Prawns and Schistosomes

River prawns are not small!
This is Macrobrachium carcinus (molt).
Clinton & Charles Robertson. CC BY 2.0

Fascinatingly, the adult river prawn lives in fresh water, but the young require brackish water – a mixture of river water and sea water – in order to survive. When we dam an African river, we cut off access to the river’s mouth where the water is brackish. Adult prawns cannot migrate downriver; young prawns cannot migrate upriver. The prawn population upstream of the dam is doomed. And what do prawns eat? Lots of things, but one of those things is snails.

So just as the dam provides beautiful habitat for the proliferation of snails, it also wipes out a key predator of those snails. Instead of tasty prawns, people living on the shores of the reservoir get schistosomiasis. That’s a bad deal.

At least one project – Project Crevette - is now rearing the prawns and releasing them into an affected river, effectively reducing the level of schistosomiasis and providing food and marketable prawns for the people in the area. That’s a good deal.

Michael B. New writes that postlarvae (the stage of prawn young that migrate upstream) of at least one species “can climb vertical surfaces and cross land, provided there is abundant moisture available” (Farming Freshwater Prawns: A Manual for the Culture of the Giant River Prawn Macrobrachium rosenbergeii. Food and Agriculture Organization of the United Nations, 2003, pg 10). One wonders how difficult it would be to re-establish the natural life cycle of the river prawn.