The monsters inside you
Previously in Exponential Investor, we’ve looked at the role of your natural bacteria on your health. We saw how nurturing our existing “good bacteria”, or adding new ones, can potentially help with everything from obesity to autism.
But what of our other freeloaders, the parasites? There’s a world of exciting news and profit opportunities surrounding these critters.
Parasites fall into two kinds: endoparasites, which live in us; and ectoparasites, which live on us. An intestinal tapeworm is an endoparasite, and a flea is an ectoparasite. Parasites are far more diverse than bacteria and viruses, at least when it comes to size and shape. They range from tiny, single-celled organisms, right up to coiled intestinal worms many metres long.
You might think it’s bad news that things are crawling on you or squirming inside you. However, it’s not as simple as that. You’re probably familiar with “probiotics” – topping up your gut bugs, to obtain health benefits. Surprisingly, there is also evidence that this approach works with parasites, too. Deliberate parasitic infection can potentially help with allergies – and a range of other health issues.
Our immune system has evolved to work with a heavy load of gut parasites, and a lack of exposure in modern times is believed to be behind various health conditions. That’s the reason some people are using the icky-sounding pig whipworm to treat a variety of different diseases. Pig parasites are an attractive therapy precisely because they’re not parasites of humans. They can live in us, but they can’t reproduce – so the infestation naturally clears in a few weeks. This lets us train up the immune system, restoring it to a more natural condition. These worms have already been used to treat hay fever, Crohn’s disease and ulcerative colitis. My guess is that we’ll soon see far more widespread use of therapies like this – so watch this space. Sadly, this is one piece of body hacking that I’ve not tried yet.
In future, we could even use gut parasites to control our baby-making. A Science study of Amazonian women showed that roundworm infestations tend to be associated with improved fertility, and hookworms with fertility reductions. One possible mechanism is that hookworms are known to increase the number of T1 immune cells, which may cause pregnancies to fail.
Staying with the subject of fertility, let’s look at one of the most widespread parasitic infections in the developed world: Toxoplasmosis gondii (T. gondii). This is carried by between a third and a half of the world’s population, and it’s a truly fascinating creature. It’s famous for making rats more reckless and also attracted to the smell of cat urine – so they’re far more likely to get eaten. This is essential to the breeding phase of the parasite’s life cycle – but it’s bad news for the rat.
Unsurprisingly for a parasite that manipulates rats so cleverly, T. gondii also does many strange things to people. The parasite needs to hide from the body’s defences, which it does by modifying the immune system. What’s more, the parasite has a vast range of effects on human behaviour and physiology. It’s been linked to a wide range of diseases, including schizophrenia, obsessive compulsive disorder (OCD), Parkinson’s and Alzheimer’s. This is an unsurprising cluster of conditions, for a disease that survives by modifying animal behaviour.
Furthermore, prior T. gondii infection is suspected of causing a range of more subtle changes. It increases the risk of traffic accidents, by slowing reaction times or by increasing risk-taking. That’s higher than the risks associated with drink-driving (at around 1g/dl blood) – so it’s conceivable that we could see T. gondii tests being used to screen people working in hazardous occupations. Why would those changes in carelessness and mental illness arise? To answer that question, we need to consider the effect on the parasite’s life cycle. The more socially isolated you are, and the less careful, the more likely you are to end up as dinner for a large predator. This completes the parasite’s life cycle. Even today, in India, tiger attacks are a real risk.
T. gondii also makes men’s faces more “manly” by increasing testosterone. Highly-masculine faces have been associated with infidelity (reported in Biology Letters). That change is not surprising, when you find out that T. gondii can be sexually transmitted by men. It’s not the parasite’s sole way of getting around, but an infection is an infection – regardless of how it’s picked up. From the parasite’s point of view, it makes perfect sense to add more ways to spread itself. Making its hosts more promiscuous will do the job nicely.
And just like the parasite-addled rats, which seek out the smell of cat urine, this change in smell impacts human behaviour. If you’re a committed cat lover, perhaps you have a parasitically-controlled preference for the smell of cat wee?
So whether you’re accident prone, a cat lover or a serial cheat – you may have the perfect parasitic excuse.
Not all our parasites live in our guts. For example, you might be familiar with the bulbous red noses many people ascribe to alcoholism – but the story is not so simple. Certainly, alcoholism can cause reddening of the face. However, another common cause is acne rosacea. This is caused by Demodex mites that live in your skin pores and crawl out at night to mate. Lacking an anus, the mites are little bags of poop – which eventually burst or rot away inside your skin. This gives a burst of bacteria, causing the immune system flare-up which is responsible for the reddened, scarred faces of sufferers. One treatment that targets both the mites, and the resulting inflammation, is Soolantra (from Galderma).
In the West, we’ve largely rid ourselves of intestinal parasites. That has both good and bad effects, as we saw earlier. However, the developing world is still gravely affected by parasitic diseases. Indeed, the symbol of medical care, the snake and staff, is believed to come from a treatment for the parasitic disease caused by the Guinea worm. This parasite grows in the body, breaking through the skin. Very careful removal is necessary – such as by winding the animal round the stick. That’s possibly what gave rise to the iconic image.
Fortunately, as we’ve covered in Exponential Investor before, there’s been a recent breakthrough against a range of parasitic diseases. This work was done by Novartis, funded by the Wellcome Trust (thanks to the late Sir Henry Wellcome). Its new drug shows promise against Chagas disease, leishmaniasis and sleeping sickness. Collectively, these diseases cause 50,000 deaths a year across the developing world. That’s comparable to the day-one deaths from each of the WWII atom bomb drops.
Even parasites can have parasites. Bed bugs, fortunately rare in the UK, are a hot topic elsewhere. They can transmit the parasite responsible for Chagas disease. This can induce heart failure and is one of the diseases whose treatment we discussed above. Growing reports of bedbug infestations are a nightmare for New York hotel owners. However, these blood suckers can fortunately be killed by heat or cold – but their resistance to common pesticides is growing. As with bacteria and fungi, we’ve seen resistance emerging as a consequence of over-reliance on common treatments. One possible alternative is biological control. Infection with Beauveria bassiana fungus is effective against a wide range of insects – and bedbugs are one potential target. But don’t think that a bug-free bed will save you from Chagas disease – as the similarly-bloodthirsty kissing bugs (AKA assassin bugs) will happily transmit it, too.
No discussion of parasites would be complete without mention of the mosquito. Malaria, caused by parasites in the Plasmodium genus, is one of the world’s deadliest diseases. In fact, it’s driven human evolution – with the illness sickle-cell anaemia a by-product of a gene for malaria resistance. The death toll from mosquitoes is not limited to malaria, and they can also transmit other pathogens. These include the roundworm responsible for elephantiasis, which causes gross enlargement of the limbs.
Eliminating mosquitos may seem attractive, but they’re important pollinators – as well as a food source for insectivorous animals, such as bats. One technique that’s recently been considered to effect eradication is the “gene drive”. This involves hijacking the molecular machinery of reproduction, causing the preferential transmission of a harmful gene to the next generation. This is an exciting new weapon in our biotechnology arsenal – but using it to eliminate species poses important practical and ethical questions.