When worms crawl on mice's brains
The canine roundworm (Toxocara canis) is a nematode parasitic usually in the intestines of canids. It grows to a maximum size of 20 cm and has a typical worm-like appearance. Canids serve as definitive hosts in which roundworms sexually reproduce. However, other animals - not excluding humans - may become occasionally infected by the eggs with developed larvae that leave the dog with its feces. In humans, infection is most often caused by ingestion of contaminated soil, so typically young children become infected during outdoor activities. However, roundworm larvae recognize that they are not in the correct host (e.g. a dog) and do not complete their sexual development. Instead, they wait until their current host becomes prey to a canine carnivore in which to continue their development. By waiting in the wrong host (e.g. a human or mouse), the larvae migrate to the liver, lungs, or brain (see image) and cause tissue damage. If the larvae reach the eye, they can even cause blindness.
Our scientists, in collaboration with colleagues from the Institute of Immunology and Microbiology at the First Faculty of Medicine, UK, investigated how infection with this common parasite affects the course of a mouse model of multiple sclerosis. They based their study on the idea that many parasitic worms (or helminths) actively influence host immunity and thus improve the course of allergies or other autoimmune diseases. To test this hypothesis, they looked at four different groups: healthy mice, mice infected with T. canis, mice with induced encephalomyelitis, and finally infected mice with induced encephalomyelitis. Contrary to expectations, the highest (i.e. worst) clinical score, which assesses overall health, was observed in the group of infected mice with induced encephalomyelitis. This was mainly due to weight loss and increased concentrations of inflammatory and anti-inflammatory cytokines - molecules responsible for regulating the immune response. This group also had the lowest proportion of regulatory T lymphocytes, which are involved in the suppression of inflammatory processes, compared to the others in the nervous tissue. The results clearly show that mice infected with roundworm larvae have a significantly worse course of encephalomyelitis.
Of course, the results of this pilot study are not directly transferable to the human situation. They do not give a clear answer to the question of whether humans infected with dog roundworms will be more susceptible to a worse course of MS. However, they do open up a new way for research and make it clear that the effect of parasitic helminths on the course of inflammatory diseases is not necessarily all positive, which is what experimental helminth therapy is based on. The result of the study motivates further research on similar issues and has a high potential to discover new links or treatments for diseases associated with the parasite Toxocara canis.
Novák, J., Macháček, T., Majer, M., Kostelanská, M., Skulinová, K., Černý, V., . . . Horák, P. (2022). Toxocara canis infection worsens the course of experimental autoimmune encephalomyelitis in mice. Parasitology, 1-9.