Good evening. This evening we’re going to be talking about a rather unusual topic – something that is rather close to my heart. Actually, it’s rather close to YOUR heart, as well as mine. We’re going to be talking about parasites.
Now why on earth would I choose a topic like this to discuss? When I was at university, I had three weeks of lectures on parasites and our lecturer started off by telling us how ubiquitous parasites are. It was a description he gave that initially captured my imagination, but as I studied these creatures I began to realise how incredible and amazing they are. If I had not opted to become a teacher, I think I might have become a parasitologist and spent my days marvelling at them.
The description my lecturer used was this: If all living tissue apart from parasites was instantly removed from the planet, you would still be able to see the outline and form of everything you see now – humans, animals and all plants. Think about that for a second. You would still see ‘people’ walking around, you would still see ‘trees’, ‘plants’, ‘dogs’, ‘cats’ and ‘butterflies’. If our success as a species is measured by our numbers, or our proliferation, then we don’t even rate on the scale, let alone come close to winning the prize. Rather, it’s the parasites that do. Humans have more DNA than most other species, we are much more technologically advanced and like to think that we have the greatest impact upon the earth. Yet, the humble parasites are more successful than we are in many respects.
Many people think that when discussing human parasites, we are discussing a Third World problem. This is not so. With the rise in international travel, increasing pollution and water contamination, increasing sexual promiscuity, and with many parasites becoming immune to the treatments we currently use (such as DDT to control parasite vectors), many people in the First World are infected and are not even aware of it. These parasites may enter their hosts’ bodies through any contaminated food or water they consume, by burrowing through their skin (either while swimming or while walking barefoot over contaminated soil), through the inhalation of contaminated dust, through sexual contact with an infected person, or through a vector like a house fly or mosquito.
Talk to any international traveller and they will probably be able to tell you a horror story of some sort. In Ecuador mosquitoes can deliver a Bot fly egg onto your skin, which will hatch and the larva will burrow into your body. In Malawi, swimming in the water will expose you to another microscopic burrowing parasite that will use any of your hair follicles as an entry point to your blood. In Nepal, drinking mountain water could result in a leech entering your nasal passages. Closer to home, in the Kruger Park, a mosquito bite could give you malaria.
Every human being has several parasites, they simply aren’t aware of it. Just in our guts, there are about 3200 different parasites we can have, from unicellular through to multi-cellular organisms. That number accounts for only about 30% of all human parasites, with the other 70% living in our other organs. Parasites really are ubiquitous, and as such, they deserve our special attention, and our grudging praise for the manner in which they manage to survive despite our best efforts to eliminate them. They are the ultimate Survivors!
For the benefit of the non-biologists present, let’s just spend a few moments reviewing some basic information about parasites.
Basic parasite info
Parasites exist on and in all living organisms, not just in animals or humans. In multi-cellular organisms they can be found externally – ectoparasite, internally in body spaces – extracellular parasite, or internally inside cells – intracellular parasites. In the life cycle, parasites have at least 3 different forms they adopt: eggs, larvae, adult; but many have several different larval stages. Often the larval and adult stages have different hosts and are unable to survive if they wind up inside the wrong host. If they do survive, then they are unable to reproduce. In other words, each parasite and each parasite stage is specially adapted to a specific host. This adaptation includes specialisation to:
- gain entrance to the host (or attach to the host in the case of an ectoparasite),
- bypass the host's defences or immune system, and
- avoid detection by the host that would lead to its destruction
This indicates that parasites have evolved alongside their hosts. It’s a pas de deux as first the host species evolves to try to protect itself and then the parasite evolves to bypass the host’s new adaptations, and then the host evolves again, etc. All of this begs the question though – what exactly is a parasite?
So what exactly is a parasite?
At school one learns that the definition of a parasite is any organism in relationship with another, in which the parasite benefits while the host is harmed. The parasitic relationship is more fully defined as a long-term one, one in which the parasite is highly specialised and adapted to suit their host, and one in which the parasite reproduces more quickly and frequently than the host. The parasite depends upon the host for protection, nutrients or some other life-supporting function and therefore the parasite does not usually kill the host, although it does reduce the host’s overall fitness. In truth, parasitism is much more complicated than that. In fact, it can be very difficult to prove or demonstrate what harm has been done to the host.
As a biology teacher who happens to be a Christian, I am often asked by my students why God created parasites. My students want to know what ecological role parasites play. Until recently, the only answer I could give was that they help to control population sizes by being the top predators or by being the keystone species (the species that has the greatest impact on the environment), or occasionally provide some small benefit. For example, as you sit here, you all have the parasite E.coli in your gut. Most strains of this parasite are fairly harmless, and in fact, most strains actually benefit their host by producing Vit K for you and by preventing other parasites or infections that could potentially kill you from being able to flourish.
However, the latest research has given me another, better answer. The latest research has shown that some parasites actually benefit their hosts in such a way that removing them would have a detrimental effect on their host’s survival. Specifically, it is now understood that many parasites help to prevent auto-immune diseases and allergies. Imagine that!
What is an auto-immune disease? This is any disease in which the body begins to attack itself, because it mistakes some organ or tissue as being foreign. Asthma and Multiple Sclerosis are both examples of auto-immune diseases.
The human parasite Schistosoma mansoni is a flat worm trematode, or fluke. It normally causes two different infections. In the larval stage it travels in the blood, feeding on your blood, and causing Bilharzia. In the adult worm stage it infests the intestines where it mates and lays eggs. These eggs can become trapped in the intestinal tissues instead of passing out of the host’s body. If this happens it causes an immune response that blocks the colon, leads to blood loss, increases blood pressure and causes swelling of the spleen and oesophagus which can tear and cause internal bleeding. All round, not a pleasant character to have living inside you.
Type 1 diabetes is caused when a person’s own immune system attacks the beta cells in the pancreas. These cells are responsible for producing insulin, which is essential for regulating blood sugar levels. Amazingly, research has shown that being infected with the Schistosoma eggs prevents the person from developing diabetes. This is because the parasite eggs release a soluble enzyme that stops the host’s immune system from producing the specific T Helper cells of the immune system that destroy the beta cells of the pancreas. So – having the parasite (which causes all those nasty side effects) can actually save the life of the host. Isn’t that a thought?!
Of course, this complicates matters when trying to define a parasite. If parasites actually benefit the host, can they still be called parasites? It would appear that for our immune systems to develop properly, they need to be exposed to these parasites so that they can learn to react against them, but also so that they are regulated by these parasites. Thus, for the moment, the answer is yes. Although parasites do bring benefits, those benefits are only fringe benefits. Their main effect on their host is still assumed to be a negative one. As our understanding and knowledge of parasites grows though, there may come a time when we change our definition, but that is the nature of science – constantly changing and adapting to accommodate new knowledge.
I am particularly fascinated by Schistosoma. The adults consist of a double layer of membranes. The outer one is continually shed and replaced as a protective mechanism against the host’s defences and digestive system. It’s only 1cm long, but the male has about 8 testicles. The Schistosoma is also one of the few parasitic worms that actually has a separate male and female worm.
Interestingly, if an infection only contains female worms, they don’t mature properly and remain unable to reproduce. However, if an infection contains only male worms, they still develop properly. In other words, the females need the males to develop, but the males don’t need the females.
When both males and females are present together though, they form mating pairs that last for life. This is because the females actually live inside the males. Well, sort of. Even though the worms are only 1cm long, the males are about 10x the width of the females. They develop a special groove on their side, and the much slimmer female crawls into it and spends the rest of her life there. She will lay about 300 eggs a day, which is basically the same as her own body weight. (Some parasites can lay 1 million eggs a day, so 300 per day is pretty insignificant.)
Another reason I’m fascinated by this particular parasite is that I was infected with it back in 2000, after going on holiday to Lake Malawi. It was only after my trip there that I discovered that the leading centre for the study of Schistosoma is found on Lake Malawi, not far from where we camped. It’s found there because the Lake has the highest incidence of the parasite in the world. There’s almost no way to avoid it if you swim in the lake. That’s because the parasite’s other host is a water snail. After it completes the part of its life cycle in the snail and leaves it, the parasite swims through the water, using its forked tail to propel it, until it finds a human. It attaches to your skin while it begins to search for a hair. As soon as it finds a hair, it then burrows through the hair follicle and into your bloodstream. Of course, it’s so tiny you won’t feel a thing, although you may develop a mild rash at the point of entry. You initially develop normal flu symptoms, but after those symptoms disappear you are left feeling extremely fatigued – a bit like Post Viral Fatigue Syndrome. That’s because the parasite is feeding on your blood, so you don’t have enough red blood cells to meet your own oxygen demands.
But let’s get back to the recent research on parasites.
The Hygiene Hypothesis
This new knowledge about parasites is part of what is becoming known as the “Hygiene Hypothesis”. This hypothesis has been in development since the late 1980s. The term was coined by the media following a short article entitled ‘Hayfever, hygiene and household size’ by David Strachan, in which he commented that:
Over the past century declining family size, improvements in household amenities, and higher standards of personal cleanliness have reduced the opportunity for cross infection in young families. This may have resulted in more widespread clinical expression of atopic disease, emerging earlier in wealthier people….
Atopic disease is an allergic hypersensitivity in parts of the body that are not in direct contact with the allergen. E.g. developing eczema on the elbows.
In the late 1990s, Dr. Erika Von Mutius compared the health of children in East and West Germany before the Wall came down. Her hypothesis was that the children from East Germany, living in much poorer and dirtier environments, would suffer more illnesses related to allergies and auto-immune diseases. When the Wall came down, she was able to really compare the two groups of children. To her surprise, the children from East Germany were far healthier than their counterparts from West Germany, despite having a more restricted diet and living in less healthy environments. This surprising result caused her to investigate further, whereupon she discovered that children in East Germany spent more time in crèches and nurseries where they were exposed to more germs. This piece of research leant further credibility to Strachan’s research.
In a nutshell, the current explanation of this hypothesis says that until sanitation became commonplace, exposure to germs and parasites was a regular threat that our immune systems faced. Our immune systems have two types of immune responses, caused by two different T Helper cells – TH1 and TH2. TH1 cells react to germs (viruses and bacteria), while TH2 cells react to allergens. Exposure to germs stimulates the TH1 cells, which in turn suppresses the TH2 cells. Thus, if we are not exposed to germs, the TH1 cells are not produced, so the TH2 cells overproduce, and we experience allergies.
This theory has been refined to include our reaction to parasites. In pre-sanitation times, if the immune system reacted to a parasitic infestation with a TH2 response strong enough to kill the parasites, the body’s own tissues would be seriously damaged in the process. Thus, our immune systems evolved to be able to tolerate parasites to some extent, by producing fewer TH2 cells, rather than kill them outright and damage our own bodies in the process. Since the body’s immune system has adapted to tolerate parasites, its responses to the mild allergens like pollen have also effectively been toned down, since these also require a TH2 response.
However, since the advent of sanitation, we are no longer routinely exposed to these parasites. The consequence of that is that our immune systems are no longer maturing in the way they should, and with fewer anti-inflammatory chemicals being produced, with the result that there has been a dramatic increase in the number of allergies and auto-immune diseases.
This research has been backed up by tests done comparing lab mice and rats to those found in the wild. The lab mice and rats were fed a healthy, nutritionally rich diet as well as being given various vaccines and kept in conditions similar to what people in the First World experience. Researchers then compared the production of antibodies called immunoglobins between the two groups of organisms. They found that those in the wild had significantly higher concentrations of the antibodies that prevented parasitic, bacterial and viral infections and also had significantly fewer allergies or auto-immune diseases.
If true, then this Hypothesis has potentially major implications for diseases like multiple sclerosis, Alzheimer’s, asthma, hayfever, some cancers, eczema, Parkinson’s, Irritable Bowel Syndrome and a host of other diseases. It might be possible, for example, to harvest the soluble enzyme produced by the Schistosoma eggs, or to synthesise it in a lab, to give to diabetics to stop the development or escalation of the diabetes.
In 2002 it was found that long term infections with tapeworms resulted in increased levels of anti-inflammatory chemicals called cytokines being produced. At the same time it was noted that high levels of these anti-inflammatory chemicals corresponded with fewer allergies and auto-immune diseases. Thus, the spin off from a parasitic infection is that these cytokines are produced. It might be possible to synthesise these chemicals in a lab to assist those who suffer from auto-immune diseases.
In the meantime, though, a branch of medical treatment is being trialled in which people are deliberately infected with parasitic worms, or helminths. The helminths obviously have to fulfil certain criteria, such as not being able to reproduce in humans, in order to protect the patient from potentially life-threatening side effects of a parasitic infection. Currently, there are only 3 species that meet all the criteria necessary: Hookworms, Pig Whipworm and Human Whipworm.
Crohn’s disease is one type of Inflammatory Bowel Disorder, or Irritable Bowel Syndrome. It is an auto-immune disease that is also thought to be inherited at some level. Treating patients who suffer with Crohn’s with Hookworms results in a 100% recovery rate. This kind of medical treatment falls under a new branch of medicine called Darwinian Medicine or Evolutionary Medicine.
Multiple Sclerosis is a brain disease in which the myelin sheath around nerves is slowly lost. As this layer is lost, the nerves cease functioning and progressive disability ensues. In 2007 Correale and Farez conducted a study with Multiple Sclerosis patients. 12 patients were already infected with intestinal parasites, so 12 uninfected patients at a similar stage in the disease were found. The 24 patients were then followed for 4.5 years, monitoring the development of the disease in each of them. It was found that the 12 who were infected with the intestinal parasites had fewer incidences of relapses and a greatly reduced disability accumulation. Being infected certainly did not cure the MS, but it did substantially reduce the impact of the disease.
Of course, nothing is ever quite that simple. Since the Hygiene Hypothesis was first proposed there have been several studies that seemed to contradict it. The incidence of asthma, for example, in poorer urban areas in the USA is increasing. This contradicts the Hygiene Hypothesis, which would expect that the poorer areas have a higher incidence of germs and worms, and thus that children should have fewer incidences of this auto-immune disease.
The problem arises because asthma can be caused by a variety of different things. It might be a reaction to cigarette smoke, vehicle emissions, an allergic reaction, obesity or an inherited condition, to list just a few. If the condition is inherited, then no amount of exposure to helminths during early childhood is going to help. If it is caused by obesity, and the subsequent inflammatory response of the lungs, the only a change in diet and loss of weight will help.
Thus, when considering the Hygiene Hypothesis, one must always bear in mind that chemicals, viruses and hygiene (i.e. exposure to helminths) impact upon the prevalence of a particular disease. When treating such a disease, or trying to prevent it, one must tackle the problem from several different directions at the same time.
One of the reasons that I quite like studying parasites is that they are so different to us. They usually consist of a just a digestive system and a reproduction system, with the more advanced ones possessing a few very basic senses. Yet, for all their relatively primitive development, they have such a massive impact on the world. From their impact on food production, to our health, they are responsible for so much of the suffering in the world. With advances in medical technology, we may yet be able to curb them, but I seriously doubt we will ever be able to eradicate them. If this recent research is to be believed though, then I’m not sure we would want to eradicate them.
So what does all this mean for us? Does it mean we should give up being vaccinated, or that we should stop cooking our food properly? Does it mean we should go around deliberately infecting ourselves with parasites? Quite simply, no. While exposure to germs and parasites is now accepted as having immunological benefits for humans, these things are still pathogens. They still cause tremendous disease and can threaten our very lives. However, there is a case to be made for not using antibacterial soaps, for allowing our young children to spend time with lots of other children so that they can be exposed to germs, and for letting our children play in the dirt. Maybe we should all relax and get a bit filthy, and maybe our grannies were right when they told us that what didn’t kill us would make us stronger! Maybe washing our hands all the time is actually not as good for us as we used to think.
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