The Hygiene Hypothesis Unraveled
Worms that live in our intestines, such as blood-sucking 'hookworms', are currently being tested as medical treatments for diseases such as hayfever and certain bowel conditions. But how do parasitic worms, technically known as 'helminths', protect against disease, are they a safe and effective therapy, and why have they evolved to help us in the first place?
This group of inquiries falls collectively under the territory of the so-called 'hygiene hypothesis'. This fairly loose hypothesis proposes that there is a connection between improved hygiene in the developed world and an increase in the incidence of certain diseases in these countries.
What is the hygiene hypothesis?
The hygiene hypothesis was first proposed in 1989 when a doctor from the United Kingdom noticed that children from larger families tended to suffer fewer allergies than those from smaller ones. His explanation was that larger families had greater exposure to bacteria and viruses, and this programmed the childhood immune system in such a way that allergic reactions became less likely.
The hypothesis has evolved considerably over the past twenty years, becoming much broader in scope. At its core is the observation that a group of related conditions which can be called 'immunological diseases' are much more common in the developed world (Europe, the United States etc) than in developing countries.
Immunological diseases occur when the immune system, which evolved to fight off infections and other harmful agents, like bacteria and viruses, 'accidentally' attacks the wrong targets. Two main categories are 'autoimmunity', where the immune system attacks the body's own tissues, and 'allergy', where harmless particles from our environment are targeted with an excessively vigorous response.
The second core feature of the hygiene hypothesis is the claim that reduced exposure to certain infectious organisms in more developed, and hence more hygienic countries, has caused this rise in immunological diseases.
The infections thought to play a role in the hygiene hypothesis include several species of bacteria and a variety of parasitic worms, or helminths.
The evidence for this comes firstly from the fact that globally, exposure to parasitic infections and the occurrence of immunological diseases are inversely correlated, i.e. countries with more infectious diseases tend to have fewer immunological diseases (Figure 1, right).
But correlation does not prove causation. It could be that the greater risk of immunological disease and reduced exposure to infections in the developed world is mere coincidence. Or the two factors could both be caused by something else that we don't yet understand.
Several lines of evidence support the claim that reduced exposure to infectious agents has a causative role in immunological disease. These can broadly be divided into two types: firstly, animal models of immunological diseases have shown that parasitic worms inhibit the development of disease. And secondly, human immunological diseases have been demonstrated as being influenced by real-life infectious organisms.
Evidence for the hygiene hypothesis: Animal models
An animal model generally refers to when scientists study a disease process in a non-human animal to learn about the equivalent disease in humans.
Various animal models have been used to study immunological diseases. For example, 'Non Obese Diabetic mice', or NOD mice, are very susceptible to developing type 1 diabetes, which is an autoimmune condition that also affects humans. By studying what goes wrong with NOD mice, scientists hope to learn about the diabetes disease process and ultimately discover potential treatments that work on humans.
In type 1 diabetes, the immune system destroys the cells in the pancreas responsible for producing insulin, a hormone which regulates blood sugar. The resulting drop in insulin production causes uncontrollably high blood sugar, and prior to the development of insulin injections, it was invariably fatal.
Professor Anne Cooke and colleagues from the University of Cambridge have tested whether infection with helminths has any effect on the development of type 1 diabetes in NOD mice.
"We looked to see whether or not we could actually test out the hygiene hypothesis using a mouse model of type 1 diabetes," says Cooke.
In 1999, Cooke showed that infecting NOD mice with a particular type of helminth called Schistosoma mansoni blocked the development of type 1 diabetes (Figure 2, left). 73% of the 'control' NOD mice, which had no helminths, went on to develop diabetes, compared with just 28% in the group given an injection of 30 Schistosoma mansoni parasites at 5 weeks of age.
"It inhibited them from getting diabetes quite markedly."
Schistosoma mansoni, also known as Bilharzias, is a parasitic fluke that inhabits the blood vessels around our intestines (Figure 3, right). It can live for up to 40 years, fully exposed to the host immune system, apparently without suffering any ill-effects. This extraordinary ability to survive the onslaught of our immune system is testament to how well-adapted the parasite has become to its host.
"Humans have coevolved with their parasites over a very, very long period of time," Cooke points out. "We're talking millions of years."
The immune system deploys a particular attack strategy to combat helminths, known as the 'TH2 response'. This response uses a type of antibody known as IgE, which binds to specific molecules on the parasites and activates a range of worm-killing mechanisms, including cells called 'eosinophils' that release toxic granules to damage the worm's surface. Parasites like S. mansoni (right) have had to adapt to survive such a hostile environment.
"Worms have evolved a whole pile of strategies to evade the host immune system," says Cooke.
One immune evasion mechanism that worms have evolved is to manipulate the TH2 response and bias it in favour of 'immune regulation'. This is where the immune system limits its 'offensive', or inflammatory activity, and instead dampens down the inflammation and switches to a 'restore and repair' setting.
The balance between immune attack and immune regulation is a fine one- too much of the former and you get excessive damage of your own tissues (e.g. with autoimmune disease or allergy), too much of the latter and you don't kill off the wards of invading pathogens that bombard our body on a daily basis (Figure 4, left).
Worms like Schistosoma mansoni seem to work by producing what is called a 'modified TH2 response', pushing the see-saw in favour of inhibitory immune regulation and so partially shielding itself from immune-mediated destruction.
Cooke went on to show that live infection was not necessary to have a protective effect. The presence of certain molecules on the surface of Schistosoma mansoni eggs, known as 'Soluble Egg Antigens' or SEAs, was sufficient to inhibit the development of diabetes in NOD mice (Figure 5, below right).
"You don't need to give a live infection with this parasitic worm, or helminth, in fact you can use extracts of the worm and that also stops diabetes," she says.
Cooke hopes that studying the molecules produced by worms like Schistosoma mansoni
"Our focus has been mainly on trying to identify the molecules which are produced and encoded by these infectious agents, that themselves can actually modulate the immune system," she said. "So rather than having a live infection, maybe we would be able to identify these molecules and use these to prevent disease, a bit like having a vitamin pill."
And this is not just a one-off idiosyncrasy of S. mansoni and type 1 diabetes, either.
A similar strategy has been employed by Margaret Harnett and colleagues at the University of Glasgow, who have shown that a molecule produced by filarial nematode worms, called ES-62, inhibits the development of autoimmunity in mice.
ES-62 is produced by a worm called Acanthocheilonema viteae, which is related to the worms that cause 'River Blindness' and elephantiasis in the developing world. When given to mice that have been primed to develop 'Collagen-Induced Arthritis', an animal model for Rheumatoid Arthritis, disease progression was significantly inhibited.
Rheumatoid arthritis is an autoimmune condition mainly affecting the small bones and joints of the body; it affects about 1% of the world's global population.
Importantly, ES-62 still limited the progression of Collagen-Induced Arthritis in mice even if it was administered after the onset of clinical symptoms, suggesting it could potentially work as a treatment for rheumatoid arthritis in humans.
Evidence like this supports the notion that certain infectious organisms inhibit the development of immunological diseases. Anne Cooke has also shown that a range of other helminths, such as Heligmosomoides polygyrus and Trichinella spiralis, have the same protective effect in preventing type 1 diabetes in NOD mice as Schitsosoma mansoni does (Figure 6).
In fact, a whole range of infectious organisms or their soluble products have inhibited the development of immunological disorders in animal models. This further generalizes the principle that parasitic worms activate immune regulatory mechanisms which can inhibit the development of autoimmunity and allergy.
The hygiene hypothesis at work on humans
Similar effects to those seen with animal models have been convincingly demonstrated in humans in a wealth of studies. Back in 1993, Dr Neil Lynch showed that treating children from a Venezuelan slum with helminth-killing drugs (known as 'antihelminthics') precipitated the development of allergy in those children, while individuals who retained their parasite burden remained relatively allergy-free.
More recently, Jorge Correale and colleagues at the Raúl Carrea Institute for Neurological research in Argentina have shown that infection with helminths protects against relapses of symptoms in human patients suffering from Multiple Sclerosis (MS).
Multiple Sclerosis is a complex autoimmune disease in which the immune system destroys the outer layer of nerve fibers in the brain and spinal cord, causing a range of neurological symptoms. In the 'relapse-remitting' form of the illness, the symptoms wax and wane in an unpredictable manner over time.
In Correale's original study, published in 2007, twelve relapse-remitting MS patients were identified as having high levels of eosinophils in their blood, the cell type that is associated with anti-parasite immunity. By examining the patient's stools, the researchers confirmed the presence of various species of helminth, including the whipworm Trichuris trichiura, and the roundworms Ascaris lumbricoides and Strongyloides stercolaris.
These twelve individuals were then followed-up over a 4 - 5 year period and compared with twelve uninfected relapse-remitting MS patients that were matched for age, sex and so on.
The helminth-infected patients faired considerably better than their uninfected, 'hygienic' counterparts, suffering fewer episodes of 'relapse' (worsening symptoms, or new symptoms appearing).
In a later study, Correale and colleagues showed that administering antihelminthic drugs to MS patients infected with helminths precipitated worsening of their symptoms and increased their risk of relapse, a finding that echoes the study performed by Dr Neil Lynch in the Venezuelan slums.
Correale's studies also hint at a mechanism for how the helminths actually work to inhibit the progression of MS disease. Infected patients had higher levels of immune signaling molecules, called 'cytokines', that dampen down immune reactions. These so-called anti-inflammatory cytokines include the memorably named IL-10 and Transforming Growth Factor beta, or TGF-β.
Infected patients also had raised levels of a special type of immune cell known as a 'regulatory T-cell', or 'Treg' for short. Tregs are known to play a key role in suppressing the immune system, preventing excessive damage. This lends yet more support to the hypothesis that helminths promote a dominant anti-inflammatory, immune regulatory environment, tipping the delicate immunological balance away from aggression and towards passive appeasement.
So the evidence that at least some parasitic worms inhibit the development of some immunological diseases, and that they do so via activating immune regulatory mechanisms, is quite conclusive. But could worms really work as a viable medical treatment for humans? And would people actually take them? Professor Joel Weinstock from Tufts University in the United States thinks that the answer to both those questions is a resounding yes, as we'll discuss in
the next article: Will Whipworm Work?
Glossary of terms:
Allergy - Diseases caused by an excessive and unnecessary immune reaction to harmless particles in our environment, like grass pollen, which causes hayfever.
Antihelminthics - Drugs that kill helminths (parasitic worms).
Autoimmunity - Diseases caused by the body's immune system targeting its own tissues 'by mistake', resulting in tissue damage.
Cytokine - A chemical signaling molecule that influences processes concerning the immune system. An 'inflammatory cytokine' promotes immune reactions, activating immune aggression. An 'anti-inflammatory cytokine' has the opposite effect, suppressing and subduing the immune system.
Helminth - A multi-cellular animal that lives in or on humans and which has an 'earthworm-like' shape. These include nematodes, like hookworm and whipworm, flukes, like Schistosoma mansoni, and tapeworms, which can grow to meters in size. These creatures are not actually very closely related to each-other.
Immune system - A complex network of molecules, cells and organs which have evolved to seek out potentially damaging threats, like infectious bacteria and viruses, and destroy them.
Immunological disease - Diseases characterized by a dysfunction of the immune system. See 'Allergy' and 'Autoimmunity'.
Inflammation - This is a strong response mounted by the immune system against potential threats. Causes the inflamed area to become hot, swollen and painful.
Multiple Sclerosis (MS) - An autoimmune disease in which the immune system targets a fatty substance called 'myelin', which surrounds nerve fibres in the brain and spinal cord. Myelin acts as insulation around the nerve and its destruction causes neurological deficits. In the 'relapse-remitting' form, the symptoms wax and wane in an unpredictable manner over time.
Non-Obese Diabetic/ NOD mice - A special breed of mice that spontaneously develop type 1 diabetes.
Parasite - An organism that lives in or on another creature, the 'host', from which it derives benefits at the host's expense. Most medical doctors think of parasites as separate from bacteria and viruses, covering 'all the rest'- from single-celled organisms like amoeba to helminths, like tapeworms.
Rheumatoid arthritis - An autoimmune disease that affects many body systems, including the skeletal joints. Affects 1% of the global population and can cause chronic disability and pain. An animal model for Rheumatoid arthritis is 'Collagen-Induced Arthritis' (CIA) in mice.
Regulatory T cells, Tregs - A special type of immune cell that is activated by specific molecules and functions to inhibit other cells in the immune system, dampening down immune responses and producing anti-inflammatory chemicals.
Type 1 Diabetes - An autoimmune disease that comes on during childhood where the immune system destroys cells in the pancreas that secrete insulin, causing uncontrollably high blood-sugar. Prior to insulin injections this was universally fatal in humans, and is still a debilitating disease. (Distinct from type 2 diabetes, which comes on later in life, is more affected by diet and lifestyle, and is less severe.)
References and further reading:
* General overview of the hygiene hypothesis:
'Review series on helminths, immune modulation and the hygiene hypothesis', in Immunology, January 2009, Volume 126, Issue 1.
* Animal models supporting the hygiene hypothesis:
Cooke, A. et al., 1999. Infection with Schistosoma mansoni prevents insulin dependent diabetes mellitus in non-obese diabetic mice. Parasite Immunology 21: 169-176.
Zaccone, P. et al. 2006. Parasitic worms and inflammatory diseases. Parasite Immunology 28: 515-523.
Dunne, D.W. & Cooke, A. 2005. A worm's eye view of the immune system: consequences for evolution of human autoimmune disease. Nat Rev Immunol. May;5(5):420-6.
Filarial nematode secreted product ES-62 is an anti-inflammatory agent: therapeutic potential of small molecule derivatives and ES-62 peptide mimetics. Harnett W, Harnett MM. Clin Exp Pharmacol Physiol. 2006 May-Jun;33(5-6):511-8.
Harnett, M.M. et al. 2010. The therapeutic potential of the filarial nematode-derived immunodulator, ES-62 in inflammatory disease. Clin Exp Immunol 159(3): 256-267.
* The hygiene hypothesis as applied to humans:
Lynch NR, Hagel I, Perez M, et al. 1993. Effect of anthelmintic treatment on the allergic reactivity of children in a tropical slum. J Allergy Clin Immunol 92:404-411.
Correale J, Farez M. 2007. Association between parasite infection and immune responses in multiple sclerosis. Ann Neurol. 61(2):97-108.
Correale J, Farez MF. 2011. The impact of parasite infections on the course of multiple sclerosis. J Neuroimmunol. 233(1-2):6-11. Epub 2011 Jan 31.