Thursday, 30 December 2010

Interesting Questions About Leishmaniasis in Peru

On a recent trip to Amazonia in Peru (near Puerto Maldonado in the southeast), I heard something interesting from a local guide: malaria and dengue fever were not the only things to worry about with respect to mosquito bites. A Leishmania species that infects sloths (or is it armadillos?), he said, can be transmitted to humans by mosquitoes.

[caption id="attachment_218" align="alignleft" width="225" caption="Three-toed Sloth, Stefan Laube, Wikimedia"][/caption]

Leishmania spp. cause dreadful chronic skin lesions, as well as disfiguring lesions of the mucous membranes and life threatening tissue destruction internally. They are, according to all the text books, transmitted by sand flies belonging to various genera.

I wasn’t sure if I believed in the mosquito vector, but I reinforced my insect repellent regime anyway. Once home, I took some time to look it up. Leishmania naiffi is a parasite of armadillos in Brazil, French Guyana, Ecuador and Peru. Humans are sometimes infected, but the insect vector, at least in Brazil, is Psychodopygus squamiventris, one of the sand flies. Several Leishmania spp. of sloths sometimes infect humans, but not, apparently in Peruvian Amazonia, and their documented vectors are also sand flies. Nowhere could I find any report of mosquitoes transmitting the parasite.

I concluded the information was wrong – but at least my fly repellent should have deterred both mosquitoes and sand flies from dining on me. A recent report, however, raised the question all over again: research described at the website for the Australian Government: Agriculture, Fisheries and Forestry has revealed that leishmaniasis is transmitted to kangaroos and wallabies by biting midges.

On the one hand, that publication confirms my earlier conclusion: “This was the first evidence anywhere in the world” they write, “of a vector other than phlebotomine sand flies transmitting Leishmania.” On the other hand, they’ve shown that other biting insects can transmit it, so the question remains open. What’s biting the sloths and armadillos in Peru?


“Field Surveillance and Monitoring – Leishmania in the Northern Territory.” Australian Government: Agriculture, Fisheries and Forestry 2010

Gramiccia, Marina and Luigi Gradoni. “The Current Status of Zoonotic Leishmaniasis and Approaches to Disease Control.” International Journal of Parasitology 35, 2005

Lucas, Carmen M., Eileen D. Franke, Marlene I. Cachay et al. “Geographic Distribution and Clinical Description of Leishmaniasis Cases in Peru.American Journal of Tropical medicine and Hygiene 59(2), 1998

Thursday, 23 December 2010

A Vaccine Against Malaria

In 1991, Robert Desowitz wrote about the early efforts to develop a vaccine to protect against malaria - a tale of great expense, effort (even some scandal), and failure. Twenty years later a lot more effort and expense has been poured into the problem but the picture doesn’t look much better.

[caption id="attachment_207" align="alignright" width="300" caption="World Malaria Day Button, Malaria Consortium"][/caption]

A vaccine in human trials today may be ready by 2015, but if it comes to pass it will probably only prevent about 50% of malaria cases in vaccinated individuals in Africa. That’s not stellar performance, and it raises a serious concern that it might actually result in unnecessary cases of malaria if people feel a false sense of security and become careless about other prevention measures.

Another problem is cost: the vaccine is likely to be expensive, and it’s valid to ask where the money will come from and whether it wouldn’t be better spent elsewhere. We know that other prevention measures work against malaria when they are consistently applied: down from a high of 178 countries with endemic malaria in the early 1900s, 99 countries have it today. In some countries still affected, the annual death toll today is only in the single or double digits, compared with a death toll from AIDS in the hundreds of thousands (Kelland and Hirschler).

According to the WHO, more than 33 million people are living with AIDS, with 2.6 million newly infected in 2009 and 1.8 million deaths (Global Summary of the AIDS Epidemic , 2009 ). One third of humanity is infected with the organism that causes tuberculosis and TB killed 1.7 million in 2009 (Tuberculosis: Fact Sheet No. 104 )  In 2008, there were 247 million cases of malaria, with nearly a million deaths (Malaria: Fact Sheet No. 94 )

If there’s only so much money to fight infectious diseases in the developing world, where should we spend it?

Sadly, perhaps it’s still not the right time for a malaria vaccine.


Desowitz, Robert S. The Malaria Capers. New York: Norton, 1991

Kelland, Kate, and Ben Hirschler. “Special report: The Cost of a Malaria-Fee World.” Reuters Health Information Dec 22, 2010

Monday, 13 December 2010

Interview: Quirks & Quarks

I've been interviewed by Bob McDonald of CBC's national radio program Quirks & Quarks. The interview will air on Saturday Dec 18, 2010. You can check air times on the CBC website,, or listen to the podcast later!

Tuesday, 7 December 2010

Human Scabies, Dog Mange and the Chupacabra - Is There a Connection?

Recent news stories about the legendary chupacabra, or goat sucker, identified the scabies mite, Sarcoptes scabiei, as the indirect culprit.

One artist's idea of what a chupacabra looks like.
Illustration by Alvin Padayachee

What is a Chupacabra?

A flurry of news stories appeared reporting on the identification of dead chupacabras as coyotes with severe mite infections. The same mite, reports said, causes scabies in humans and mange in dogs.

While providing a plausible explanation for a puzzling mystery, these stories may have caused some anxiety in readers who worry about catching things from household pets. What is the likelihood that these mites, which can apparently transform a coyote into a hairless, grotesque, and desperate livestock killer, could spread to a human and have a similar effect?

The Mange Mite and its Hosts

A little research reveals that, as reported, the same species of mite, Sarcoptes scabiei, infects people and a wide variety of animals. However, they don’t tend to jump from one type of host to another all that successfully. In fact, one 2007 study found that mite populations on chamois and red foxes in Italy were genetically distinct, suggesting that even closely related host species don’t cross infect each other (D. Soglia et al, “Microsatellites as markers for comparison among different populations of Sarcoptes scabiei.” Ital J. Anim Sci 6 (Suppl 1).

A coyote with mange is a sad-looking creature,
 and a sick one. USDA image.

Another study, reported in the American Journal of Tropical Medicine and Hygiene in 1999, compared mites from dog and human infestations. The researchers collected samples in the United States, Australia, and Panama and found that “genotypes of dog-derived and human-derived scabies cluster by host species rather than by geographic location” (S. F. Walton et al. “Genetically Distinct Dog-derived and Human-derived Sarcoptes scabiei in Scabies-endemic Communities in Northern Australia.”  Vol 61 [4]).

In other words, humans and dogs apparently aren’t sharing their mites much. Dogs and coyotes probably aren't either.

So while there are reports of people catching Sarcoptes scabiei from the family dog, this appears to be the exceptional circumstance. We don’t have to worry about becoming goat suckers any time soon.

Friday, 3 December 2010

A Review of Parasites
Here’s an excerpt from the most recent review of Parasites: Tales of Humanity’s Most Unwelcome Guests:

“Drisdelle treats her subject objectively. We can't help but respect these unpleasant but marvelously well-adapted organisms. She even gives some credibility to the theory that parasites can actually be beneficial by endowing their hosts with health benefits…”

Read the rest of the review by Philip McIntosh in "Parasites by Rosemary Drisdelle" on

Monday, 8 November 2010

How Do You Get Chinese Liver Fluke?

To someone in the industrialized world, the chances of getting Chinese liver fluke, Clonorchis sinensis, often seem remote, especially if you consider the life cycle of the worm.

[caption id="attachment_162" align="aligncenter" width="300" caption="Clonorchis sinensis life cycle. CDC"][/caption]

Eating raw fish will do it, if the fish has the parasite in its tissues, and with sushi becoming more and more popular, this is believable. But how does the fish get Clonorchis? In order for everything to come together for the worm, an infected person has to defecate in fresh water where the correct species of snail is resident. If the snail eats the worm eggs in the feces it, in turn, is infected. After multiplying in the snail, the parasite leaves of its own accord and burrows into the flesh of a fish. The fish must then be caught and eaten raw by a human (you) in order for the parasite to infect another person (who then has to defecate in fresh water…).

That life cycle is so complicated, one wonders how the fluke can possibly continue to exist. In fact, it lives in some 19 million human livers today. Humans owe it all to ourselves: behind every successful human parasite, there’s a human behavior that makes it all possible. In Asia, a cultural love affair with eating raw fish, and the historical habit of building toilets over fish-rearing ponds (to encourage algae growth to feed the fish) helped turn this fluke into a successful fluke.

And did you know that C. sinensis in fish can survive pickling, salting, drying, and smoking?

Monday, 25 October 2010

Screwworm eradication in South America

Aside from the effects of the fly’s depredations, one of the most compelling reasons to continue with screwworm eradication (throughout South America) is that if we don’t, there will always be a dire threat of Cochliomyia hominivorax spreading to other areas – not just in the Americas, but to many other parts of the world. Continually increasing travel, and transport of goods and livestock says it will. It already has (Vargas et al).

[caption id="attachment_145" align="alignleft" width="300" caption="Cochliomyia hominivorax; COMEXA (Public Domain, Wikimedia Commons)"][/caption]

But now the countries of South America are banding together to eradicate it there, presumably the place where it evolved. Could there be any negative consequences of deliberately exterminating this nasty fly? The Mexican American Commission for the Eradication of Screw-Worm (COMEXA) believes that, to date, in the eradication zones, “there has been no evidence of any impact at all on biodiversity, perhaps because a multitude of fly species occupy the same biological niche” (qtd. in Godoy).

There’s no fly like the screwworm fly for causing mortality among livestock and other animals. The insect must play a role in controlling populations of wild animals. Granted, human livestock rearing has been a bonanza for C. hominivorax, but once upon a time it must have been an important part of the balance of nature in its home territory. Is it possible that eradicating it to extinction will have NO impact on biodiversity?

People who investigate the environmental impact of screwworm eradication invariably see many positive effects. I suspect the question of negative impact is one that no one wants to examine very closely. In an era where we’re just beginning to understand how nature balances itself, I hope that doesn’t come back to haunt us.

Godoy, Emilio, and Raúl Pierri. 2010. “Latin America: Radioactive Attack on Flesh-Eating Screw-Worm.” Inter Press Service

Vargas-Terán, M., H. C. Hofmann, and N. E. Tweddle. 2005 “Impact of Screwworm Eradication Programmes using the Sterile Insect Technique.” In: V.A. Dyck, J. Hendrichs and A.S. Robinson (eds.), Sterile Insect Technique. Principles and Practice in Area-Wide Integrated Pest Management, Netherlands: Springer.

Tuesday, 12 October 2010

Black lice? Do head lice change colour?

According to the research of R. Hoeppli, ancient Chinese medicine used lice to cure “high fever and severe headaches as if the skull is cracking.” A paste made from 300 – 500 black lice, spread on the head, was said to do the trick. I must admit, I’d rather have 500 pureed lice on my head than 500 live ones, but the question this raised for me was where one would find black lice. Is there such a thing?

[caption id="attachment_135" align="alignleft" width="300" caption="CDC/James Gathany"][/caption]

Head lice that I’ve seen have ranged from pale ivory through a golden – sometimes reddish – brown; magnified, they are transparent. Lice that have fed have a black mass inside – presumably blood in the process of being digested – but are not, themselves, black. There is a colour range, to be sure. But black? A literature search turned up some interesting things:

  • Head lice may have a natural way of blending in without actually changing colour. Ibarra and Hall wrote: “Eggs and lice are well camouflaged, reflecting the colour of their surroundings.”

  • Newly hatched lice that have not fed are transparent (Meinking) and do not have colour until after they’ve fed.

  • Colour that matches the background has been shown to have evolved in other species of lice (Bush et al.). This, however, refers to colour change over generations, not within the life span of a single organism.

The ‘wisdom’ that human head lice change colour depending on the hair colour of the host is oft repeated on websites and in non-academic publications. Published scientific information to back it up, however, appears nonexistent. Similarly, parasitology texts and laboratory identification references do not mention it.

I remain highly skeptical that our head lice can change colour within one generation, or that black head lice actually exist. I conclude that the Chinese remedy called for human head lice that had fed and had blood in their guts.

Bush, Sarah E et al. 2010 “Evolution of Cryptic Coloration in Ectoparasites.” The American Naturalist 176: 4

Hoeppli, R.  1959. Parasites and Parasitic Infections in Early Medicine and Science. Singapore: University of Malaya Press, pg 181.

Ibarra, J., and DM Hall. 1996 “Head Lice in Schoolchildren.” Archives of Disease in Childhood, 75.

Meinking, Terri. 2004 “Clinical Update on Resistance and Treatment of Pediculosis Capitis.” American Journal of Managed Care. 10:9, Sup.

Tuesday, 28 September 2010

Origins of our hookworms - who's an heirloom?

Hookworms are probably one of our heirloom parasites – parasites that have been with us for millions of years. It seems, however, that Necator americanus, or New World hookworm, has been a parasite of humans longer than our other species: Ancylostoma duodenale, or Old World hookworm (which is exactly the opposite of what you would expect if you were to draw conclusions from the names of these worms).

[caption id="attachment_118" align="alignleft" width="300" caption="Image by Avsa. Numbers represent thousands of years BP. Creative Commons Attribution-Share Alike 3.0 Unported license."][/caption]

Both humans and Necator americanus are thought to have evolved in Africa and moved out to spread throughout the rest of the world later. Migrating humans would have taken N. americanus with them – an intestinal worm that had coevolved with them after early hominids split from a primate ancestor.

Most references name Asia and Southeast Asia as the original home of Ancylostoma duodenale, and either canids or primates as its original host. If this is correct, humans encountered it only after arriving in that part of the world. Ancylostoma duodenale, then, jumped from another host rather than coevolving, making this hookworm a souvenir, albeit a very early one, rather than an heirloom.

Does this explain why hookworm disease can be more severe when caused by A. duodenale? Though it’s been with us a long time, perhaps we haven’t achieved a comfortable relationship with this souvenir compared with our old friend Necator.

Friday, 17 September 2010

Heirloom Parasites, and Souvenirs

More than 100 different species of parasites infest humans. Where did they all come from?


Heirloom Parasites

Homo erectus, a human ancestor, had some of
the same parasites we have today. Reconstruction
by John Gurche; photographed by Tim Evanson.
CC SA 2.0
Some parasites that infest humans are uniquely ours: they only infect humans and have done so for hundreds of thousands of years. Though we may not like to think about it, these parasites are only here because their ancestors have inhabited an unbroken line of human bodies for countless generations – all the way back beyond our distant ancestors who walked out of Africa. Surely no other species on Earth can claim such a close association with people – not even our long-domesticated crops and animals.  They are our “heirloom parasites” – some call them “old friends”  - and they can tell us a lot about our past.

Souvenir Parasites

Other parasites are generalists: they infect humans regularly but they can also infect other animals and do so whenever the opportunity presents itself. Some of these may be heirloom parasites that spread from humans. Others likely infected other species first and crossed to humans later. These last, along with parasites that only infect humans rarely and by accident, are our “souvenirs.”

Both of these things are intriguing when you think about it: that over the course of , perhaps millions of years, we never failed to – unintentionally - give our heirloom parasites what they needed to survive; and that we are still picking up new souvenirs today, in numerous unplanned, unobserved, unexpected ways. It says a lot about the devilishly unbreakable life cycles of parasites.

Interesting reading:

Araújo, Adauto, Luiz F. Ferreira, Karl J. Reinhard et al. 2008 “Parasites as Probes for Prehistoric Human Migrations?" (galley proofs) Natural Resources, School of Papers in Natural Resources, University of Nebraska – Lincoln.

Araújo, Adauto and Luiz Fernando Ferreira. 2000 “Paleoparasitology and the Antiquity of Human Host-parasite Relationships.” Memórias do Instituto Oswaldo Cruz On-line. Suppl. 1 Nov.

Tuesday, 14 September 2010

Podcasts About "Parasites"

There are now two podcasts online in which I discuss Parasites: Tales of Humanity's Most Unwelcome Guests.

In one, posted on, I talk to Mark Lynch, the engaging host of Inquiry. Inquiry airs on WICN Public Radio on Sundays from 9 - 10PM.

... and here's Part II with Mark Lynch.

The other podcast is available on the University of California Press website.

Friday, 10 September 2010

Parasite - Origin of the Word

The mitochondria that produce energy in our cells were
 once free-living organisms. Diagram by Boumphreyfr
     CC BY-SA 3.0
Parasite comes from the Greek word meaning “one who eats at the table of another.” The term is a social one, used in ancient Greece - and still today - to refer to people who get what they need by exploiting others. I’ve seen parasite used to describe various freeloaders, politicians, government employees, tax evaders, control freaks and pimps. It’s never meant as a complement.

At some point people must have recognized the similarity between the social parasites that plagued them and the animal ones that they observed living off other animals. They saw this as a negative thing – a way of life to be despised, and they gave it a negative label. Little did they know that parasitism is a time honoured way of life, a very successful way of life, and something to which they owed their very existence.

Wednesday, 1 September 2010

What is a Parasite?

I am often asked “What is a parasite?” “Are bacteria parasites?” “Are viruses parasites?”

Few parasites have a worse reputation
   than the scabies mite. Image by
    Kalumet CC BY-SA 3.0
     (background edited)
A working definition of a parasite is “an organism that lives on or in another organism and gets what it needs for survival and reproduction from that host organism.” In Foundations of Parasitology (6th ed., McGraw Hill, 2000) Roberts and Janovy define a parasite as an organism that “either harms its host or in some sense lives at the expense of the host” (p. 6).  Robert S. Desowitz wrote that parasites “have evolved from free-living forms who through opportunity, mutation, and selection have come to live in or on another organism” (The Malaria Capers, Norton, 1991, p. 94).

By these definitions, bacteria and fungi living in us, and on us, are clearly parasites. Whether viruses are parasites or not depends on whether one believes that viruses are alive – but that’s a discussion for another time. These definitions provide an objective, biological view of what a parasite is. From a medical point of view, parasites include protozoa, worms, and things like lice, fleas, and bedbugs. They do not include bacteria, fungi, and viruses. It’s a division of labour: microbiology gets the bacteria, mycology gets the fungi, virology gets viruses, and parasitology gets everything else that wriggles, jumps, crawls, swims, or squelches along.

And from a medical point of view, parasites are bad: they harm the host and are no earthly good whatsoever. Usually they do seem bad. We are only just beginning to realize that human parasites may have some important redeeming qualities – and any definition of parasite is a long way from acknowledging that.

Parasite: the word itself denotes something bad.

Sunday, 22 August 2010

Parasites: Tales of Humanity's Most Unwelcome Guests. The Book.

This month, my book, Parasites: Tales of Humanity’s Most Unwelcome Guests, is officially launched. Described as a natural and social history of parasites, the book is a marriage of my interest in parasites and my fascination with the way the human world works.

Despite thousands of years of disentangling ourselves from nature, walling it off, controlling it, parasites are still there, getting mixed up in our affairs. Many of them, in fact, have far more impact on human lives than they did millennia ago. Repeatedly, our determined efforts to modify our environment have created opportunities for parasites that allow them to interfere with (or aid) everything from agriculture to murder. That’s what fascinates me, and that’s what my book is about.

Parasites: Tales of Humanity’s Most Unwelcome Guests is available through booksellers, and on the internet from the University of California Press, and

ISBN 978-0-520-25938-6