We probably did not evolve a requirement for helminth infection
Helminths downregulate inflammation to avoid causing fatal immunopathology in the host and many authors have suggested that we are therefore in a state of evolved dependence on this helminth-mediated immunoregulation. In other words, they suggest that without helminths our immune systems are excessively pro-inflammatory.
Evolution converts the inevitable into a necessity
This was a nice idea which I used to support. Evolution often converts the inevitable into a necessity. For example, most mammals can synthesize vitamin C, but in humans and some other species, a crucial enzyme has become corrupted because the diet of evolving humans ‘inevitably’ contained adequate supplies of the vitamin. As a consequence, vitamin C in the diet is now a necessity.
But evolution does not turn intermittent or temporary exposures into necessities because this leads to gene-environment mismatch when the exposure is absent. Helminth infection, in terms of human evolution, must be regarded as intermittent or temporary.
Helminths vary in site, load and immunoregulatory mechanism
The problem is that different helminth species live in blood, tissues, bladder or gut, and each species downregulates inflammatory responses via a different mechanism. Moreover, the loads of helminths differ wildly between individuals, even when they live in similar geographical locations. There is no constant ‘inevitable’ helminth-associated factor that could drive hard-wired genetic dependence on helminths. Rather than germ-line encoded dependence, intermittent or temporary environmental or infectious stresses are coped with via epigenetic adaptations in the developing immune system that can fade over several generations, or be renewed if required.
Clinical trials with helminths
This argument helps us to understand the conflicting results of trials of helminth therapy in multiple sclerosis (MS). Allowing Argentinian MS patients to become infected with helminths they would have encountered in childhood can stop progression of the disease because their immune systems developed in the presence of the same helminths and require their continuing presence for normal function.
In sharp contrast, trials of helminth therapy for MS or other autoimmune disorders are disappointing in locations where helminths have not been endemic for several generations.
Evolution does indeed turn the inevitable into a necessity, but it turns the intermittent or temporary into an option via epigenetic adjustments, so the need for helminths in Europe and the USA has faded.
Anecdotal evidence of efficacy may be valid
The immunoregulatory molecules produced by helminths are interesting and they probably have therapeutic value in patients with certain genetic backgrounds and a specific immunoregulatory deficit suffering from specific chronic inflammatory conditions. There is a huge quantity of anecdotal evidence for this from people who have self-treated with helminths. This is how we will exploit helminth products in the future. But until we understand how to recognise the right combination of patient, genetics, disease and helminth, I doubt whether clinical trials will work, and there is no justification for trying to reconstitute the human biome with a helminth component. The illustration on this page shows the helminths that have most often been used to treat humans.
[ A more complete and fully referenced version of this text can be found here. ]