Schistosomes: The Snail, the Prawn, and the Dam

It’s a story that has played out in Africa all too often. A river is dammed to control flooding, or for hydroelectric power, or to create a water reservoir in case of drought (or all of the above at once). The dam, of course, creates a large lake and lots of good habitat along the shore for aquatic plants, which in turn support a large snail population.

Schistosome Life Cycle

People come to live near the water’s edge, to use the water and to fish. Generally, human contact with water is high, particularly for children and fishers. If schistosomes are present, along with the right snail species, escalating levels of schistosomiasis soon follow. This is how it works:

• People infected with schistosomes pass the eggs in urine or feces, and if those eggs find their way into fresh water, they hatch.

• The emerging larva seeks out an aquatic snail – only the right snail will do – and penetrates the snail, after which it multiplies asexually in the snail’s tissues.

• Larvae leave the snail host and, if a human enters the water, penetrate human skin, maturing to adult worms that reside in the tiny blood vessels around the bladder or intestine.

• Adult worms produce eggs which are passed in the urine or feces, and so on.

It makes perfect sense that water resource projects can result in rapidly increasing incidence of schistosomiasis because they bring together the two hosts of the parasites – the human and the snail – in circumstances ideal for transmission. We hardly need a more complicated explanation. It seems, however, that there’s at least one other factor that’s been left out: the African river prawn.

River Prawns and Schistosomes

River prawns are not small!
This is Macrobrachium carcinus (molt).
Clinton & Charles Robertson. CC BY 2.0

Fascinatingly, the adult river prawn lives in fresh water, but the young require brackish water – a mixture of river water and sea water – in order to survive. When we dam an African river, we cut off access to the river’s mouth where the water is brackish. Adult prawns cannot migrate downriver; young prawns cannot migrate upriver. The prawn population upstream of the dam is doomed. And what do prawns eat? Lots of things, but one of those things is snails.

So just as the dam provides beautiful habitat for the proliferation of snails, it also wipes out a key predator of those snails. Instead of tasty prawns, people living on the shores of the reservoir get schistosomiasis. That’s a bad deal.

At least one project – Project Crevette - is now rearing the prawns and releasing them into an affected river, effectively reducing the level of schistosomiasis and providing food and marketable prawns for the people in the area. That’s a good deal.

Michael B. New writes that postlarvae (the stage of prawn young that migrate upstream) of at least one species “can climb vertical surfaces and cross land, provided there is abundant moisture available” (Farming Freshwater Prawns: A Manual for the Culture of the Giant River Prawn Macrobrachium rosenbergeii. Food and Agriculture Organization of the United Nations, 2003, pg 10). One wonders how difficult it would be to re-establish the natural life cycle of the river prawn.

The Candiru vs Peer Review

This is reportedly a candiru.
       It's a big one.
Image by em_j_bishop: CC BY 2.0

I think sometimes our faith in a system, and our love of a colourful story, deadens our common sense. Take the candiru, for example – Vandellia spp., the vampire catfish.

“This little charmer normally lives inside the gills of other fish to suck their blood, but is attracted to urine and reputedly able to wriggle up human's urinary tracts, where it lodges itself with sharp spines and can only be removed by surgery.”

Hogwash.


The quote above came from the 2002 Lonely Planet guide to Brazil (pg. 75). First of all, these little catfish do not live in the gills of their fish hosts. They visit there to feed for all of about two minutes – more like a mosquito, really, than a parasite.

There is no scientific proof that they are attracted to urine. If one ever wriggled up a human urethra it wouldn't lodge itself so much as become stuck as a result of those spines, designed for a different situation entirely. Finally, there is only one published case – semi-documented and unconfirmed – and it was reportedly resolved by cystoscopy rather than surgery. (Samad, Anoar. "Candiru Inside Urethral." Urology Clinic.)

The Candiru in Academic Literature

While we'd like to expect better of Lonely Planet, it's only fair to excuse this case of bad information – the candiru has been reported widely and legitimized in publications that normally hold themselves to an even higher standard. For example:

• From The New Encyclopedia Britannica 15^th ed. Vol 2. (1990): “... parasitizes man and has been known to enter the urethras of bathers and swimming animals. Once inside the passage, it erects the short spines on its gill covers and may thereby cause inflammation, hemorrhage and even death...” (“Candiru”).

• From a 1991 article by J. L. Breault in the Journal of Wilderness Medicine (2): “Forced extraction may cause lacerations of the urethral mucus membranes, which has caused death by exsanguination. Remedies have ranged from penile amputation and suprapubic cystostomy to application of a native herb that softens the spines” (“Candiru: Amazonian Parasitic Catfish”).

Judging by these sources and others published by academic presses and peer reviewed journals, subject as they are to editorial scrutiny, peer review, and fact checking, one would think that the consequences of a personal encounter with the candiru were well documented medical knowledge.

The library website at the University of Victoria says “Peer review ensures that an article-and therefore the journal and the scholarship of the discipline as a whole-maintains a high standard of quality, accuracy, and academic integrity. When you consult peer-reviewed sources, you are tapping into a wealth of established, verified knowledge” ( library.uvic.ca, accessed July 1, 2013).

A 2013 Review of the Evidence for Candiru Attacks

Nonetheless, Dr. Irmgard Bauer, in her 2013 investigation of two centuries of literature describing the candiru, found little in the way of verification. “...most reports are...repeated again and again,” she writes, “based on the same stories already described elsewhere... After careful distillation, very little [evidence] remains and of that little, even accounts sounding like first-hand descriptions become suspect.” (“Candiru – A Little Fish With Bad Habits: Need Travel Health Professionals Worry? A Review.” Journal of Travel Medicine; 20:2.)

What happened here? In 200 years, did no one go to the trouble to verify candiru reports until now? The candiru is like the Sasquatch's rich city cousin: in contrast to the Sasquatch, consistently disinherited by science because there is no evidence for it, the candiru has been repeatedly legitimized by science and reputable publishers despite the lack of evidence. All we really have for sure is a translucent little fish that takes a blood meal from a fish larger than itself, and a bunch of anecdotes. It really isn't even a parasite.

And we wonder why it's important to consult original sources?

I wrote about what we know about the Candiru for Decoded Science: "Candiru- A "Don't Pee in the Water" Horror Story Debunked."