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.

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.

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.