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.

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.

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 WICN.org, 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.

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.

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 (6^th 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.