Pinworms and Human Immunity

Thoughts on pinworm infections...

I had a strongly worded email from someone who had apparently read, and taken exception to, one of my articles about pinworm (Enterobius vermicularis). I was a little surprised by the tone of the email - who knows why someone would become attached to this particular soapbox - but the vehement claim that the human immune system will eradicate a case of pinworm (enterobiasis) got me thinking.

One female pinworm can produce 10,000 sticky eggs.

Mentnafunangann; CC BY-SA 3.0

Will Pinworm Infection Resolve Without Treatment?

I’m of the understanding that a pinworm infection will resolve on its own eventually if left untreated – and I never said otherwise – but what role does the immune system play? Can the immune system kill the adult worms? We know that the immune system responds to parasites, but many parasites are good at evading immune attacks, and prompting moderation of the immune response. Enterobius vermicularis should be particularly good at this because it is one of our heirloom parasites: it’s been with us hundreds of thousands of years.

Add to this the frequently reported problem of reinfestation through swallowing eggs in the home/school environment and one begins to suspect that waiting for the immune system to do the job could be a long process. It’s clear that, for many people, the immune system is ineffective at eliminating the adults (they likely die of old age after a month or so), and unable to effectively kill larvae over the long term after initial exposure.

Having said all that, it probably doesn’t matter all that much, except for those unfortunate individuals that experience bad symptoms.

A Realistic Approach For Pinworm Infection

Further thoughts:

• Texts tell us that the majority of pinworm infections are asymptomatic, so lots of people have pinworms, don’t suffer any unpleasant symptoms, and eventually eliminate the worms on their own. (And if they have no symptoms, they’re less likely to spread it around.)


• The ability to fight off enterobiasis likely varies from one person to the next, as it appears to with other parasitic worm infestations.


• An overzealous approach to eradicating pinworms is probably unrealistic.  They’re so good at spreading themselves around, and we’re so poor at fighting them off, that actually getting rid of them is next to impossible.


• When symptoms are severe and persistent, however, only the most stoic of patients would refuse treatment. I’ve never had the pleasure myself, but I’ve heard first hand accounts that were not pretty.

I suspect most people would choose to treat a pinworm infection but it's up to the individual.

Onchocerca volvulus and Wolbachia

In my book, Parasites: Tales of Humanity’s Most Unwelcome Guests, I discuss the efforts to treat people for river blindness - the difficulty of treating enough people for long enough to eradicate the disease. I also explore the fascinating relationship between the worm Onchocerca volvulus, and a genus of bacteria, Wolbachia. Wolbachia lives literally inside the cells of the worm, even within the embryos.

[caption id="attachment_231" align="alignleft" width="300" caption="Wolbachia inside a cell, Creative Commons Attribution 2.5 Generic"][/caption]

We know that O. volvulus can’t live without Wolbachia - that if you kill the bacteria with antibiotics, the worms die as well. Obviously Wolbachia does something for O. volvulus that it can’t do for itself. But what? This seems counterintuitive to our ideas of germs: a bacterial infection you can’t live without? But it is not so foreign really: even humans have bacteria living in their intestines that help to digest food and provide nutrients, and protect us from infection caused by less friendly species. Perhaps Wolbachia produces some vital nutrient for the worm that the worm can’t produce alone.

But here’s where the relationship gets more complex and more fascinating. Research indicates that the symptoms of river blindness are actually caused by the response of the human immune system to Wolbachia, not O. volvulus. While most of the bacteria are inside the worm, and therefore protected from the immune system’s attack, enough are exposed to keep the attack going, causing long term damage to host tissues but never wiping out the bacteria.

New evidence reveals that, meanwhile, our own immune cells targeted at Wolbachia shield the worm from the immune system like an invisibility cloak. The immune system doesn’t see the worm for the bacteria. So in essence, this is Wolbachia’s game: it uses Onchocerca to evade our immune defenses and causes river blindness. What we have here is not a horrible worm that uses human bodies and bacteria to provide its every need while unleashing dreadful disease on millions. What we have is a bacterium that uses a worm like a fortress to protect it while IT causes dreadful disease. It looks like the worm might be innocent.

University of Liverpool “Study sheds new light on river blindness parasite” Physorg.com January 12, 2011

Welsh, Jennifer. “River Blindness Parasite Relies on Bacteria to Fool Host” LiveScience Jan 19, 2010

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.