Monday, 31 January 2011

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.

Friday, 21 January 2011

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” January 12, 2011

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

Tuesday, 18 January 2011

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