Flatworm

Chapter 6 Flatworm
 
page 87 various phyla of worms...among the phyla not mentioned in the book is Acanthocephala, the Thorny-headed worms.  Their spikes have inspired better sticky medical tape.

 page 91: parasites. See Carl Zimmer on Blood flukes (parasitic flatworms) and the fountain of youth.  A kind of parasitic flatworm, Ribeiroia, can cause a frog to grow debilitating extra legs.

page 93: death. philosophical tangles and reflections from Jeff McMahan and John Broome and Stephen Cave.  

This is the seventh in a new series of notes and comments on chapters in The Book of Barely Imagined Beings. It appears around the time of the US publication, and adds to an earlier series that appeared around UK publication. 

A many legged beast

Extra legs sprouting on frogs, first observed in North America in the 1990s, were found to be caused by the parasitic flatworm Ribeiroia, writes Carl Zimmer:

Ribeiroia starts out life in snails. It grows and reproduces inside the snails, which it castrates so that they don’t waste time on making eggs or looking for a mate. In its castrated host, the parasite produces a new generation of flatworms that can escape the snail and swim in search of a vertebrate host. They typically infect fish or tadpoles. When they invade tadpoles, the parasites bury themselves in the tiny buds that will eventually grow into legs.

An apparent increase in the rate of these deformities may be due to increased concentrations of pesticides in the ponds in which the frogs live. Pesticides can kill off the parasites but they also lower the defenses of the frogs, which may lead to higher infections.

A paper in Nature suggests another factor in the success of leg-deforming parasites: declining biodiversity. In ponds with 'high' biodiversity – up to six species of amphibians – the parasites do much worse at getting transmitted than in low diversity ponds. This is not a small difference: there is a 78.5% decline in deformed frogs in high-diversity sites.

Parasite rex

When you look at normal rats, and expose them to cat urine, cat pheromones, exactly as you would expect, they have a stress response: their stress hormone levels go up, and they activate this classical fear circuitry in the brain. Now you take [Toxoplasmosis]-infected rats, right around the time when they start liking the smell of cat urine, you expose them to cat pheromones, and you don't see the stress hormone release. What you see is that the fear circuit doesn't activate normally, and instead the sexual arousal activates some. In other words, Toxo knows how to hijack the sexual reward pathway. And you get males infected with Toxo and expose them to a lot of the cat pheromones, and their testes get bigger. Somehow, this damn parasite knows how to make cat urine smell sexually arousing to rodents, and they go and check it out. Totally amazing...

...On a certain level, this is a protozoan parasite that knows more about the neurobiology of anxiety and fear than 25,000 neuroscientists standing on each other's shoulders, and this is not a rare pattern.

-- Robert Sapolsky.

One in the eye

They always mean beautiful things like hummingbirds. I always reply by saying that I think of a little child in east Africa with a worm burrowing through his eyeball. The worm cannot live in any other way, except by burrowing through eyeballs. I find that hard to reconcile with the notion of a divine and benevolent creator.

-- David Attenborough on the hate mail he receives from Christianists.

P.S. 28 Jan: I was puzzled as to which organism Attenborough might be referring, and asked a medical friend. He wrote:

Onchocerciasis or river blindness does fit the bill although it's West African and not East. Plus it’s a systemic disorder as you say [i.e. the worm can travel through almost any part of the body]. Humans are the intended host. There is an East African entity known as Dicofilaria conjunctiva- it’s a parasitic worm of the Filaridae family, which usually lives in dogs intestines and accidentally infects humans.

It looks as if Attenborough is overstating the case when he says the worm "cannot live in any other way, except by burrowing through eyeballs". But his point that much is ugly and horrible in nature stands.

My medical friend (who is not a specialist in tropical parasites so concedes he could have missed something) sent a link to photos of Thelazia, the oriental eye worm.

Invasion of the body snatchers!

Parasites that control the minds of their hosts -- whether those hosts be fish, 'zombie' caterpillars, 'brainwashed' grasshoppers or even humans -- make for great headlines, resonanting all kinds of uncomfortable thoughts on the edge of consciousness. [1] In general, 'parasite' is a term of abuse in our culture, and scientists, notes Carl Zimmer [2], took a long time to get beyond this. Konrad Lorenz, for example, saw their only virtue as a warning to humans. "A retrogression of specific human characeristics and capacities conjures up the terrifying specter of the less than human, even of the inhuman". [3] But, argues Zimmer,

parasites are complex, highly adaptive creatures at the heart of the story of life. If there hadn't been such high walls dividing scientists who study life --the zoologists, the immunologists, the mathematical biologists, the ecologists -- parasites might have been recognised sooner as not disgusting, or at least as not merely disgusting.

Parasites find themselves a vast number of ecological niches, he notes; they even find out a way to parcel out the human eye: one species of worm in the retina, one in the chamber, one in the white of the eye, one in the orbit. Zimmer also suggests that an assertion made back in 1845 by Johann Steenstrup, a pioneer in the study of flukes, is still relevant:

I believe that I have given only the first rough outline of a province of a great terra incognita which lies unexplored before us and the exploration of which promises a return such as we can at present scarcely appreciate.[4]

Footnotes:

1. Humans fear parasitic infestations, of course, and often with good reason. At least as disturbingly, we apply the term to other people and groups of people, sometimes with horrendous political consequences. And there is sometimes for some people a lurking sense that we ourselves as a species are parasitic (the borderland between parasite and predator not being well defined).

2. Parasite Rex (2000). The book is sensationally subtitled Inside the Bizarre World of Nature's Most Dangerous Creatures.

3. According to Kenan Malik's account (Man, Beast and Zombie. 2000), Lorenz saw civilisation as a degradation of the natural state of Man, the great hunter of the Upper Paleolithic. Lorenz was, notes Malik, highly sympathetic to Nazi ideology.

4. Just one example of a recent step to better understand and manage a remarkable protozoan is the work by Hugo D. Luján et al reported here. Unlike most of its fellow eukaryotes, giardia has no mitochondria. Even stranger, each giardia cell has two nuclei.

Image: hookworm