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.