Social Parasites: Trypanosomes Co-operate

We usually don’t think about the organisms that live on us, or in us, communicating or co-operating with each other. At least, I don’t. They use nutrients to grow, to reproduce, to spread. They may move around; they may mate, or simply divide by binary fission, but one hardly imagines them saying to each other “let’s go see what we can find over there,” or “we’ll work together to get past these host defenses.”

Of course they don’t literally have these conversations, but scientists are discovering that many organisms, even single celled ones, communicate with each other for the benefit of all. A recent article published by Medical News Today describes new research findings for Trypanosoma brucei, agent of African sleeping sickness.  Researchers have found that individuals of this species work together as a group to exploit their environment and likely do so to survive and invade tissues in the host.

That’s fascinating on several levels. First, it casts the enemy in a new light – it makes the invader somehow more easily understood from an anthropomorphic point of view (one should not attribute human qualities to protozoa, but it does feel comfortable - more comprehensible -  to think of them in these terms sometimes). It implies, too, that these life forms don’t get enough respect for their complexity and sophistication.

Second, it reminds us that there is still much we don’t know about many familiar species. African sleeping sickness has been a major health concern for well over a hundred years, and yet we’re only starting to understand the organisms that cause it. Finally, as the researchers have pointed out, knowledge like this may lead to better ways to prevent or treat the infection. Know the enemy.

Did I mention they’re beautiful? This is how they can look if they're co-operating on culture media.

[caption id="attachment_223" align="aligncenter" width="300" caption="Creative Commons Attribution 2.5, Oberholzer et al."][/caption]

Oberholzer M, Lopez MA, McLelland BT, Hill KL, 2010 “Social Motility in African Trypanosomes.” PLoS Pathog 6(1): e1000739. doi:10.1371/journal.ppat.1000739

Interesting Questions About Leishmaniasis in Peru

On a recent trip to Amazonia in Peru (near Puerto Maldonado in the southeast), I heard something interesting from a local guide: malaria and dengue fever were not the only things to worry about with respect to mosquito bites. A Leishmania species that infects sloths (or is it armadillos?), he said, can be transmitted to humans by mosquitoes.

[caption id="attachment_218" align="alignleft" width="225" caption="Three-toed Sloth, Stefan Laube, Wikimedia"][/caption]

Leishmania spp. cause dreadful chronic skin lesions, as well as disfiguring lesions of the mucous membranes and life threatening tissue destruction internally. They are, according to all the text books, transmitted by sand flies belonging to various genera.

I wasn’t sure if I believed in the mosquito vector, but I reinforced my insect repellent regime anyway. Once home, I took some time to look it up. Leishmania naiffi is a parasite of armadillos in Brazil, French Guyana, Ecuador and Peru. Humans are sometimes infected, but the insect vector, at least in Brazil, is Psychodopygus squamiventris, one of the sand flies. Several Leishmania spp. of sloths sometimes infect humans, but not, apparently in Peruvian Amazonia, and their documented vectors are also sand flies. Nowhere could I find any report of mosquitoes transmitting the parasite.

I concluded the information was wrong – but at least my fly repellent should have deterred both mosquitoes and sand flies from dining on me. A recent report, however, raised the question all over again: research described at the website for the Australian Government: Agriculture, Fisheries and Forestry has revealed that leishmaniasis is transmitted to kangaroos and wallabies by biting midges.

On the one hand, that publication confirms my earlier conclusion: “This was the first evidence anywhere in the world” they write, “of a vector other than phlebotomine sand flies transmitting Leishmania.” On the other hand, they’ve shown that other biting insects can transmit it, so the question remains open. What’s biting the sloths and armadillos in Peru?

Sources

“Field Surveillance and Monitoring – Leishmania in the Northern Territory.” Australian Government: Agriculture, Fisheries and Forestry daff.gov.au 2010

Gramiccia, Marina and Luigi Gradoni. “The Current Status of Zoonotic Leishmaniasis and Approaches to Disease Control.” International Journal of Parasitology 35, 2005

Lucas, Carmen M., Eileen D. Franke, Marlene I. Cachay et al. “Geographic Distribution and Clinical Description of Leishmaniasis Cases in Peru.” American Journal of Tropical medicine and Hygiene 59(2), 1998

A Vaccine Against Malaria

In 1991, Robert Desowitz wrote about the early efforts to develop a vaccine to protect against malaria - a tale of great expense, effort (even some scandal), and failure. Twenty years later a lot more effort and expense has been poured into the problem but the picture doesn’t look much better.

[caption id="attachment_207" align="alignright" width="300" caption="World Malaria Day Button, Malaria Consortium"][/caption]

A vaccine in human trials today may be ready by 2015, but if it comes to pass it will probably only prevent about 50% of malaria cases in vaccinated individuals in Africa. That’s not stellar performance, and it raises a serious concern that it might actually result in unnecessary cases of malaria if people feel a false sense of security and become careless about other prevention measures.

Another problem is cost: the vaccine is likely to be expensive, and it’s valid to ask where the money will come from and whether it wouldn’t be better spent elsewhere. We know that other prevention measures work against malaria when they are consistently applied: down from a high of 178 countries with endemic malaria in the early 1900s, 99 countries have it today. In some countries still affected, the annual death toll today is only in the single or double digits, compared with a death toll from AIDS in the hundreds of thousands (Kelland and Hirschler).

According to the WHO, more than 33 million people are living with AIDS, with 2.6 million newly infected in 2009 and 1.8 million deaths (Global Summary of the AIDS Epidemic , 2009 ). One third of humanity is infected with the organism that causes tuberculosis and TB killed 1.7 million in 2009 (Tuberculosis: Fact Sheet No. 104 )  In 2008, there were 247 million cases of malaria, with nearly a million deaths (Malaria: Fact Sheet No. 94 )

If there’s only so much money to fight infectious diseases in the developing world, where should we spend it?

Sadly, perhaps it’s still not the right time for a malaria vaccine.

Sources

Desowitz, Robert S. The Malaria Capers. New York: Norton, 1991

Kelland, Kate, and Ben Hirschler. “Special report: The Cost of a Malaria-Fee World.” Reuters Health Information Dec 22, 2010

Interview: Quirks & Quarks

I've been interviewed by Bob McDonald of CBC's national radio program Quirks & Quarks. The interview will air on Saturday Dec 18, 2010. You can check air times on the CBC website, CBC.ca, or listen to the podcast later!

Human Scabies, Dog Mange and the Chupacabra - Is There a Connection?

Recent news stories about the legendary chupacabra, or goat sucker, identified the scabies mite, Sarcoptes scabiei, as the indirect culprit.

One artist's idea of what a chupacabra looks like.
Illustration by Alvin Padayachee

What is a Chupacabra?

A flurry of news stories appeared reporting on the identification of dead chupacabras as coyotes with severe mite infections. The same mite, reports said, causes scabies in humans and mange in dogs.

While providing a plausible explanation for a puzzling mystery, these stories may have caused some anxiety in readers who worry about catching things from household pets. What is the likelihood that these mites, which can apparently transform a coyote into a hairless, grotesque, and desperate livestock killer, could spread to a human and have a similar effect?

The Mange Mite and its Hosts

A little research reveals that, as reported, the same species of mite, Sarcoptes scabiei, infects people and a wide variety of animals. However, they don’t tend to jump from one type of host to another all that successfully. In fact, one 2007 study found that mite populations on chamois and red foxes in Italy were genetically distinct, suggesting that even closely related host species don’t cross infect each other (D. Soglia et al, “Microsatellites as markers for comparison among different populations of Sarcoptes scabiei.” Ital J. Anim Sci 6 (Suppl 1).

A coyote with mange is a sad-looking creature,
 and a sick one. USDA image.

Another study, reported in the American Journal of Tropical Medicine and Hygiene in 1999, compared mites from dog and human infestations. The researchers collected samples in the United States, Australia, and Panama and found that “genotypes of dog-derived and human-derived scabies cluster by host species rather than by geographic location” (S. F. Walton et al. “Genetically Distinct Dog-derived and Human-derived Sarcoptes scabiei in Scabies-endemic Communities in Northern Australia.”  Vol 61 [4]).

In other words, humans and dogs apparently aren’t sharing their mites much. Dogs and coyotes probably aren't either.

So while there are reports of people catching Sarcoptes scabiei from the family dog, this appears to be the exceptional circumstance. We don’t have to worry about becoming goat suckers any time soon.

A Review of Parasites

www.rosemarydrisdelle.com

Here’s an excerpt from the most recent review of Parasites: Tales of Humanity’s Most Unwelcome Guests:

“Drisdelle treats her subject objectively. We can't help but respect these unpleasant but marvelously well-adapted organisms. She even gives some credibility to the theory that parasites can actually be beneficial by endowing their hosts with health benefits…”

Read the rest of the review by Philip McIntosh in "Parasites by Rosemary Drisdelle" on Suite101.com

How Do You Get Chinese Liver Fluke?

To someone in the industrialized world, the chances of getting Chinese liver fluke, Clonorchis sinensis, often seem remote, especially if you consider the life cycle of the worm.



[caption id="attachment_162" align="aligncenter" width="300" caption="Clonorchis sinensis life cycle. CDC"][/caption]

Eating raw fish will do it, if the fish has the parasite in its tissues, and with sushi becoming more and more popular, this is believable. But how does the fish get Clonorchis? In order for everything to come together for the worm, an infected person has to defecate in fresh water where the correct species of snail is resident. If the snail eats the worm eggs in the feces it, in turn, is infected. After multiplying in the snail, the parasite leaves of its own accord and burrows into the flesh of a fish. The fish must then be caught and eaten raw by a human (you) in order for the parasite to infect another person (who then has to defecate in fresh water…).

That life cycle is so complicated, one wonders how the fluke can possibly continue to exist. In fact, it lives in some 19 million human livers today. Humans owe it all to ourselves: behind every successful human parasite, there’s a human behavior that makes it all possible. In Asia, a cultural love affair with eating raw fish, and the historical habit of building toilets over fish-rearing ponds (to encourage algae growth to feed the fish) helped turn this fluke into a successful fluke.

And did you know that C. sinensis in fish can survive pickling, salting, drying, and smoking?