Friday, 11 September 2015

Mysteries of Entamoeba histolytica and Amebic Dysentery

A beautiful Entamoeba histolytica
trohpozoite; Image by Stefan Walkowski,
CC BY-SA 3.0

Entamoeba histolytica has always had us guessing. It’s the only Entamoeba species that causes disease. Why? It causes amebic dysentery in some people and not in others. Why? How does it invade tissue? Fedor Lösch was the first to describe the organism (1875), which he found while looking for the cause of dysentery in a Russian patient. Lösch guessed that the organism he called Amoeba coli was not the cause of the illness, but he guessed wrong.

Prevalence of Entamoeba histolytica

For more than a hundred years after its discovery, we thought E. histolytica was much more common than it actually is. During that time, researchers gradually separated out the look-alikes: Entamoeba coli (1879), E. hartmanii (1912), E. polecki (1912), E. moshkovskii (1941), and finally E. dispar (1978). We now know that many organisms formerly identified as E. histolytica were actually the morphologically identical, but nonpathogenic E. dispar. Estimates that 10% of the population is infected with E. histolytica have been revised downward to 50 million symptomatic cases (although many infections are asymptomatic) (Medscape). In reality, nobody really knows.

How Did Entamoeba histolytica Become Pathogenic?

Entamoeba histolytica differs from the other Entamoeba species in that it can invade tissue. It can cause amebic dysentery, with ulcers and abscesses in the intestine, and it can travel further to set up abscesses in other places - usually the liver, sometimes the lung or the brain.

I can see that causing diarrhea might be to the amoeba's advantage: this would result in organisms being rapidly expelled from the body to contaminate the environment, with other potential hosts being exposed. But diarrhea usually means the release of the vulnerable trophozoite stage, not the hardier infectious cyst stage, and organisms in deeper abscesses would never reach the outside world. How does tissue invasion benefit the parasite?

Maybe it doesn’t. One theory suggests that the ability of E. histolytica to invade and feed on human tissue is coincidental: the same thing that enables it to feed on bacteria and other organisms, and compete for food in the intestine, just happens to enable it to feed on our cells as well. That would make abscesses in the liver and other organs simply bad luck for both parasite and host.

Another theory explores the possibility that virulence develops when hosts are crowded together. When diseases are easily passed from one host to another due to crowding, the organisms that quickly make people sick and contagious do better than the slower moving ones. This might explain how E. histolytica might have evolved as a pathogen in the cooler climate of Europe.

Where Did Entamoeba histolytica Evolve?

The recent discovery of E. histolytica in a Jerusalem cesspit dating back to Medieval times (15th and 16th century) is suggested evidence that people were visiting the city from Europe. Interestingly, E. histolytica has been confirmed in a number of European coprolites dating back as far as 500 BCE. But very few coprolites from tropical locations have been tested for E. histolytica and the evidence might be gone after so long in a tropical climate.

Did E. histolytica spread around the world from Europe? Europeans certainly had it and spread it, but I find it difficult to believe that E. histolytica evolved in what is today Europe for a number of reasons:
  • It is an ancient and predominantly human parasite, and humans didn’t evolve in Europe.
  • It is vulnerable to cold, dying at temperatures below 5C. This is in contrast to other parasites such as Giardia and Cryptosporidium, which survive much colder temperatures and are more common in cooler climates, even today.
  • It is relatively uncommon in Europe and other temperate climates today, but very common in the tropics.
  • A number of early accounts from other parts of the world appear to describe amoebic dysentery. The earliest of these came from 1000 BCE in India, and Avicenna’s writings from the Middle East around 1000 CE have good descriptions of both amebic dysentery and liver abscess.

The mysteries remain unsolved. I’m interested to see whether further research can resolve them. Meanwhile, E. histolytica continues to cause an estimated 100,000 deaths annually.

Further Reading

Cox, F. E. G. 2002. History of Human Parasitology. Clinical Microbiology Reviews 15(4) 595-612 doi: 10.1128/CMR.15.4.595-612.2002

Harrison, Alan. 2015 “500 Year Old Toilet in JerusalemReveals Clues About Long-distance Trade and Travel in the Medieval Period" Bugbitten

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

Samie A, ElBakri A, and AbuOdeh R. 2012 "Amoebiasis in the Tropics: Epidemiology and Pathogenesis.” Current Topics in Tropical Medicine, Dr. Alfonso Rodriguez-Morales (Ed.), ISBN: 978-953-51-0274-8, InTech

Sargeaunt PG, Williams JE, and Grene JD. 1978 “The Differentiation of Invasive and Non-invasive Entamoeba histolytica by Isoenzyme Electrophoresis.” Trans R Soc Trop Med Hyg 72 (5): 519-521 doi:10.1016/0035-9203(78)90174-8

Singh U, Huston CD. 2012 “Evolution of Entamoeba histolytica Virulence.” Evolution of Virulence in Eukaryotic Microbes, L. David Sibley, Barbara J. Howlett, Joseph Heitman (Eds.) Chapter 22.

Friday, 10 July 2015

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.

Friday, 26 June 2015

Crazy – Cats, Toxoplasma, and Your Mental Health

Is your cat making you crazy? If you’ve been reading the many recent articles about a theoretical link between Toxoplasma gondii infection and schizophrenia (and other mental health issues) you may think that your cat is a significant threat to your mental health.

Infected rodents pass Toxoplasma gondii
to wild and domestic cats. Yintan, CC BY-SA 3.0

But to say that your cat could literally make you mentally ill is a bit disingenuous. It’s not the cat itself, it’s the parasite Toxoplasma gondii that’s linked to mental illness. And while it’s true that T. gondii is a parasite of cats, it’s equally true that it infects more than 300 other species of animals, and there are a number of ways to catch it without a cat being anywhere in sight. Don’t be too hard on your pampered kitty which, in all likelihood, is not capable of infecting you with T. gondii.

Cats and Toxoplasma gondii

 Cats typically catch T. gondii by eating infected rodents. In the cat, T. gondii multiplies in the intestine and millions of microscopic oocysts (you might think of them as eggs, or germs) are released into the outside world in the cat’s feces. But an infected cat will usually only shed these oocysts for one to two weeks of its entire life - about 0.2 percent of its life if it lives to be sixteen. Cats that don’t hunt, don’t eat raw meat, and particularly cats that don’t go outside, are unlikely to be infected, and extremely unlikely to infect you with T. gondii.

Toxoplasma gondii in the Environment

There’s a bigger picture of course. Domestic cats have been instrumental in spreading T. gondii around the world and contaminating the environment everywhere with oocysts. But this parasite can survive without cats for a very long time, if not indefinitely. This is because the parasite multiplies asexually (clonally) in the tissues of any warm blooded vertebrate (and even fish can be infected). It can multiply in human tissues, in chickens, sheep, dogs, rats, mice, horses, and 300+ other animals. If a pig eats an infected rat, the pig will catch toxoplasmosis. If a wild or feral dog eats the pig, the dog will catch toxoplasmosis. So blaming it all on the cats at this point is like closing the barn door after the horses have left.

How Do Humans Get Toxoplasma

The cat is the only animal in which Toxoplasma gondii
can reproduce sexually, but it is widespread in the
tissues of other species. CDC image.

Humans become infected with Toxoplasma gondii in various ways:
•    Eating undercooked meat
•    Consuming food or water contaminated with oocysts
•    Exposure to oocysts in soil (gardening etc.)
•    Congenitally, from mother to fetus
•    Through blood transfusion or organ donation
•    Direct contact with an acutely affected cat
•    Sexual transmission (theoretical)

In the end, it’s clear that while it is possible to catch T. gondii from contact with your beloved domestic cat, if the cat has an acute case of toxoplasmosis, this is an unlikely source of infection. And while cats certainly bear responsibility for spreading this parasite far and wide, it’s the parasite that’s linked to mental illness, not the cat.

Further reading

Centers for Disease Control and Prevention. “Toxoplasmosis (Toxoplasma infection).”

Flegr, Jaroslav (2015). “Schizophrenia and Toxoplasma gondii: an undervalued association?”
Expert Review of Anti-infective Therapy 13:7 , 817-820

Flegr, J., Klapilová, K., & Kaňková, Š. (2014). “Toxoplasmosis can be a sexually transmitted infection with serious clinical consequences. Not all routes of infection are created equal.” Medical Hypotheses, 83(3), 286-289.

McAuliffa, Kathleen. (2012). “How Your Cat is Making You Crazy.The Atlantic

Zhang, M., Yang, Z., Wang, S., Tao, L., Xu, L., Yan, R., ... & Li, X. (2014). “Detection of Toxoplasma gondii in shellfish and fish in parts of China.” Veterinary Parasitology, 200(1), 85-89.

Thursday, 3 April 2014

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

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.


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.

Monday, 31 March 2014

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.


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.

Thursday, 27 February 2014

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.



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. 

Thursday, 20 February 2014

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.


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.

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