Manure and Latrines = Farming and Parasites

Pig sties are notoriously smelly places, but aged
pig manure makes excellent fertilizer.
 Image by MontagZen; CC BY-SA 3.0.

Almost twenty years ago, Jared Diamond wrote about the domestication of plants in Discover  ("How to Tame a Wild Plant," Sept 1994). "Human latrines," he wrote, "may have been a testing ground for the first crop breeders."

How so? Many plants get spread around when their seeds are eaten, passed through the digestive tract of the forager intact, and deposited in feces. Because humans tend to use latrines - designated outdoor toilets that they return to again and again - the seeds of the plants that early humans liked to eat got deposited in a concentrated area, and the wastes deposited with them provided fertilizer. Voila! Latrines would have effectively selected the choicest food items of early humans and aided their reproduction.

Because I'm interested in parasites, I couldn't help thinking that this very process also aided the transmission of intestinal parasites: choice food plants growing in areas used as latrines means people gathering food there, with resulting exposure to the eggs and larvae of intestinal worms (and probably protozoa as well).

"Our ancestors' garbage dumps," Diamond pointed out, "undoubtedly joined their latrines to form the first agricultural research laboratories." This, because larger seeds, roots and other plant parts that might reproduce would end up in the midden, or garbage dump, along with all the other food waste (think compost) and animal manure (if they had livestock), gaining a survival advantage in virtually the same way.

And now, in his article "Early Farmers and Manure: Stone Age Europeans Were More Advanced Than We Thought," (Decoded Past, July 3013) Frank Beswick reports on evidence that stone age people used animal manure as fertilizer as long ago as 6000BC. Beswick wonders "is it possible that the use of manure was a precondition of the development of agriculture?"

And I wonder how much it contributed to the parasites we share with domestic animals, particularly cattle and pigs.

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."

Parasites and Networks - Food Webs, Epidemiology

When we think about how parasites fit into nature - their close interaction with their hosts, their reliance on specific hosts in a specific sequence, their ubiquitous presence in the environment - a network (or web) context makes sense. Recently, two articles  have come to my attention; they represent parasites in webs in similar ways but for different reasons.

Parasites Affect Food Webs

This food web for a mosquito would be much
more complex - and of much more relevance
to humans - if it included parasites.
Illustration by Tyler Rubley. CC BY-SA 3.0

 

 

The first is "Parasites Affect Food Web Structure Primarily Through Increased Diversity and Complexity." by J. Dunne and co-authors (PLoS Biol 11(6): e1001579), published on June 11, 2013. These researchers added parasites to aquatic food webs and asked whether the changes that resulted (a much more complex food web) could have been caused by the addition of any large group of organisms, or whether food web structure was altered by parasites in unique ways.

Beyond the central findings of that study, the article raised (or re-raised) a question for me: if we know that parasites are an important component of an ecosystem, if we know that they affect food webs in both generic and unique ways, why do they never appear on endangered species lists? For every endangered species, there should be a list of parasites that depend on that host species for survival. And why do conservationists routinely rid endangered animals in captivity of their parasites, knowing that if that species ever returns to the wild, it will do so without its specialist parasites, potentially to its detriment?

Parasites Predict Disease

 

The other paper is "Centrality in Primate–parasite Networks Reveals the Potential for the Transmission of Emerging Infectious Diseases to Humans" by José María Gómez and co-authors , published in PNAS (110:19 2013).  Like the paper by Dunne et al, this research builds a food web, but this web has parasites as the main consumers - a network of nonhuman primates that share parasites (or that are consumed by the same parasites, to look at it from the parasites' viewpoint).

The point is to examine whether this web can predict where diseases of primates are likely to jump to humans. The authors "found that primate species having higher values of centrality in the primate–parasite network harbored more parasites identified as EIDs [emerging infectious diseases] in humans and had parasite communities more similar to those found in humans." These species, then, are more likely to be the source of emerging infectious diseases in humans. Another good reason to include parasites in food webs.

Food webs tell us where parasites fit, and I think that is something we really need to understand, for our own sake and for theirs.

Disseminated Strongyloidiasis - Interesting Things to Note

A recent case of disseminated strongyloidiasis, reported by the New England Journal of Medicine highlighted a couple of intriguing features of this catastrophic disease: in the absence of an effective immune response, worms can mature in the lungs as well as the intestine, and, there's something special about corticosteroid drugs that makes an unfortunate success of the worm.

Adult Strongyloides stercoralis in the Lungs

The case report of disseminated strongyloidiasis by Schroeder and Banaei describes adult worms, both rhabditiform and filarifom larvae, and ova containing active larvae in a tracheal aspirate. A similar case is reported by Bava et al. Typically, you'd find only filariform larvae in the lungs, and adults are almost never seen, even in the stool, because the adult females spend their time migrating through the tissues of the intestinal lining (and there are no parasitic males).

A larva of Strongyloides stercoralis. At a later stage, this larva would
be capable of penetrating and migrating through tissues like skin, or the
lining of the intestine. Image: CDC

I assumed that the adults found in this case had actually matured in the lungs rather than migrating there from the intestine. A 2004 paper in Clinical Microbiology Reviews agrees. Keiser and Nutman write “...findings suggest that filariform larvae develop into adults in the lungs... This hypothesis is supported by... autopsy studies showing adult worms in lung tissue.” In this scenario, the parasite could be multiplying very rapidly with new worms originating not only in the intestine, but in the lungs as well. One can only imagine the numbers of parasites that could be present within a short period of time.

Corticosteroids and Strongyloides stercoralis

We know that corticosteroids can initiate disseminated strongyloidiasis. But do they just give the worms a green light by suppressing immune response, or do they actually favor the parasite? Corticosteroids prevent production of eosinophils and cause the rapid destruction of eosinophils that already exist; these cells are part of the body's immune response to parasites. But it's thought that corticosteroids actually contribute to the success of S. stercoralis in another way. Gary Simon writes in Medical Parasitology that “they may stimulate female worms to increase larval output and promote molting of rhabditiform larvae into the invasive filariform larvae.”

Eosinophils in the blood are part of the
immune response to parasites. Image by
Iceclanl. (cropped)  CC BY-SA 3.0

So it looks like giving corticosteroids to a patient with S. stercoralis expands the “territory” in which the parasite can reproduce, hobbles the immune system's attempts to control it, and boosts the worms' fecundity and maturation. Given all of this, it's easy to see why it might be difficult to save a patient suffering from disseminated strongyloidiasis, unless the problem is discovered quickly. It also sheds some light on why other types of immunosuppression are relatively less catastrophic.

 

Sources

Bava  BAJ, Cecilia D et al. “Adult Female of Strongyloides stercoralis in Respiratory Secretions.”, Asian Pacific Journal of Tropical Biomedicine 3:4, April 2013, Pages 311–313

Keiser PB, and Nutman TB. “Strongyloides stercoralis in the Immunocompromised Population.” Clinical Microbiology Reviews. 17:1, January 2004, 208–217.doi:10.1128/CMR.17.1.208-217.2004

Castelletto ML, Massey HC Jr et al. "Morphogenesis of Strongyloides stercoralis Infective Larvae Requires the DAF-16 Ortholog FKTF-1." , PLoS Pathogens 5(4): e1000370. doi:10.1371/journal.ppat.1000370

Schroeder L, and Banaei N. “Strongyloides stercoralis Embryonated Ova in the Lung.” New England Journal of Medicine: March 21, 2013; 368:e15 http://www.nejm.org/doi/full/10.1056/NEJMicm1204579

Simon, G. “Strongyloidiasis.” In: Medical Parasitology. Satoskar AR et al eds. Austin: Landes Bioscience; 2009, pg 31

Beauveria bassiana - A Fungus That Kills Bed Bugs

Beauveria bassiana is a fungus that is well known for killing insects. Spores of B. bassiana adhere to the cuticle (the outer protective covering) of the insect, begin to grow, and work their way through to the inner tissues. There the fungal growth continues, taking nutrients from the host's body until the insect dies. Because B. bassiana is lethal to many insects, including their larvae, it has been grown and distributed commercially for use in agricultural control of insect pests. It makes sense to wonder whether it could be used to control bed bugs as well.

Beauveria bassiana is grown for control of agricultural
pest insects. Image courtesy of Keith Weller.

Beauveria bassiana Kills Bed Bugs

A study by Alexis Barbarin et al tested B. bassiana against bed bugs, and the results indicate that the fungus is lethal to the pests: not only does it kill virtually all bugs that come in contact with it, infected bugs can carry it back to daytime hiding places and pass it on to other bugs that have not been otherwise exposed. Barbarin el al propose that B. bassiana might rid a bed bug infested building of its bugs.

Bed Bug Traps Using Beauveria bassiana

In their study, Barbarin et al exposed bed bugs to a mixture of oil and fungal spores on various surfaces, and found that jersey knit cotton transmitted the infection most effectively. Though further research is required, they propose that a fabric bed skirt impregnated with B. bassiana spores might be an efficient means of infecting a resident bedbug population. Presumably any trap designed so that all bedbugs climbing onto or leaving the bed would have to pass through it could be used to infect them with the fungus.

Is Beauveria bassiana Safe for Humans?

Beauveria bassiana is generally regarded as safe for humans and it's already being used for insect control applications all over the world without dire consequences for human health. This fungus is already naturally occurring in the environment. However, there's reason to be cautious with this approach. A study that tested fungi isolated from poultry barns found that B. bassiana has several virulence factors that potentially “increase [its] survival, growth, and propagation... in animal tissue.” Authors Taira el al comment, quite correctly, that otherwise harmless fungi can cause serious infections in people whose immune systems are already compromised. Such people include AIDS patients and organ donor recipients among others.

These grasshoppers were killed by Beauveria bassiana.
Fungal growth is visible on the insects' remains.
Image courtesy of Stefan Jaronski.

Cases of both deep tissue infection and skin infection caused by B. bassiana have been reported in the medical literature (Figueira et al). The possibility of skin infection, in particular, prompts second thoughts. Beauvaria bassiana does not wipe out bed bugs on contact: it takes time for the infection to kill. Therefore, bugs that have contacted the fungus will still visit the sleeping host to feed. And while feeding, they will create a break in the skin, often with severe irritation resulting, and possibly introduce fungal spores. This does not seem like a good plan.

Beyond the possibility of skin infection arising from contact with the bugs, a spore impregnated bed skirt would presumably contaminate a living space with fungal spores pretty thoroughly, and fungal spores are as hardy and as hard to eliminate as bed bugs are. Under the right conditions, they could remain viable for a very long time, possibly protecting against reinfestation by bed bugs, but also a potential hazard for the immunocompromised occupant of, or visitor to, the space.

Sources

Barbarin AM, Jenkins NE et al. “A Preliminary Evaluation of the Potential of Beauveria bassiana for Bed Bug Control.” Journal of Invertebrate Pathology 111 (2012) 82–85

Figueira L, Pinheiro D et al. “Beauveria bassiana Keratitis in Bullous Keratopathy: Antifungal Sensitivity Testing and Management.” European Journal of Ophthalmology 22:5 (2012) 814-818

Taira CL, Marcondes NR et al. “Virulence Potential of Filamentous Fungi Isolated From Poultry Barns in Cascavel, Paraná, Brazil.” Brazilian Journal of Pharmaceutical Sciences 47:1 Jan./Mar. 2011

Every Living Thing: A Book Review

Every Living Thing: Man's Obsessive Quest to Catalog Life, from Nanobacteria to New Monkeys
by Rob Dunn
Harper 2010 ISBN 978-o-06-143031-2

“The biological world does not revolve around us... life is smaller than we imagined, ...we are a smaller part of life than we imagined,” and we occupy a “marginal position in the biological universe” (Every Living Thing p. 59 - 60). With Rob Dunn, you know where we stand. But from that marginal position, he's written a fascinating book about the life we share our planet with, and the key people who've helped us learn what we know about that life.

I found Every Living Thing to be a eye-opener in two ways. First, it reminded me forcefully that in spite of all that we know, we still don't know much. For someone who once believed (albeit a very long time ago) that all the answers were out there – you just had to go to the right library and ask the right question – it was both bewildering and liberating to realize that what we don't know is actually in the majority. To contemplate the vast quantity of scientific knowledge stored in academic libraries and realize that it is only the beginning is truly shocking.

'Animalcules' illustrated by
Anton van Leeuwenhoek; c1795.

Second, Dunn has illuminated the lives and accomplishments of some people who are frequently mentioned only briefly, even in the most interesting science books. I knew that Antonie van Leeuwenhoek discovered Giardia lamblia when he observed it in his own stool by looking through his own glass microscope lenses. I did not know that he was the first person known to observe microscopic life of any kind; that he discovered two entire kingdoms of life; that he was a family man with five children. I didn't know that two scientists who changed the world in very different ways – Lynn Margulis and Carl Sagan – were actually married to each other at one time. The book is full of surprises like that.

If there is anything that makes Every Living Thing a challenge, it's frequent jumps through time and space, and from third to first person narrative. Far from being chronological, the narrative draws unexpected connections throughout. The reader who is fully engaged and paying close attention will be fine. Others may feel suddenly disoriented and have to retrace their steps to find out how they got from 1960s California to 1848 Brazil; from first person contemplation of becoming a father, to endosymbiogenesis. The connections are relevant; they clarify and enlarge the picture, but it is, at times, quite a ride.

Dunn has a talent for explaining complex science in a way that the general reader can understand and he combines this talent with great storytelling. This grounds the people he writes about in a context of everyday life we can all relate to, and results in a very interesting and relevant science book. It's easy to imagine that Every Living Thing will end up in the reference section of a lot of personal libraries, including mine.


At one time, I was a great fan of Stephen Jay Gould, but today I'd have to say that both in print and online, Rob Dunn is my favourite science writer. I hope this strong beginning is just the first of many fascinating science books.

Was Joshua the Hero Of Jericho? (Schistosoma and Jericho)

The Jordan Valley near Jericho.
From the OSU Special Collections
and Archives.

Reform Judaism Magazine has published an adapted excerpt from my book Parasites: Tales of Humanity's Most Unwelcome Guests. The excerpt is a shortened version of my retelling of the story of Rahab, Joshua, and the Battle of Jericho, with the blood fluke Schistosoma hematobium as a central character.
 
Reform Judaism Magazine is an interesting publication. The online version features a number of articles that would appeal to any reader: titles that caught my eye include "Were the Jews Slaves in Egypt?," "The Divinity of Dementia," and "A Paragraph that Changed History" ("Why Jesus' last supper could not have been a Passover meal..."). Fascinating.

It's my pleasure to be published in this magazine. You can read Was Joshua the Hero of Jericho? on the internet.