Since the corona crisis, we all know what a zoonosis is: a disease caused by a pathogen, usually a virus or bacterium, that can spread from an animal to humans. The term comes from the Greek words zōon = animal, and nosos = disease. Malaria, plague, yellow fever and West Nile virus are examples of zoonoses; So are Sars and COVID-19.
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But there are also anthroponoses, ailments that can only be transmitted from person to person (from anthrópos = human). Scarlet fever and gonorrhea (‘scarlet fever’) are examples. A very well-known anthroponosis is head lice. This animal, Pediculus humanis capitis, only knows humans as a host and shares the space on the human body with the pubic louse. To avoid conflict, the two species each have their own biotope; the head lice are in the hair on the head, the pubic lice in pubic, chest and armpit hair.
Head lice feed on blood that they take from the scalp. Their eggs, called nits, are attached to the hairs and if they are not removed the population can get out of control. For thousands of years, humanity has complained about troublesome parasites. The best remedy for head lice is to completely shave the head because this destroys their habitat, but that is not everyone’s favorite choice; An alternative is to comb thoroughly with a nit comb.
Because head lice are only transmissible from person to person, they are a good indicator of human migrations over the centuries. Just as the original inhabitants of Africa, Europe and America differ genetically by a tiny but nevertheless noticeable amount from each other, the lice that live on the heads of various population groups also differ from each other. This fact offers starting points for studying historical migration routes.
A head louse. Photo: TR image
A study published in the online journal Plos One showed that lice can serve to reconstruct the separation or coming together of peoples. Conclusions can even be drawn that are not apparent from archaeological research. The researchers collected 274 head lice from haircuts from all over the world, including specimens that they personally combed from the hair of Mexican and Argentinian schoolchildren.
It would take us too far to explain the techniques used in detail here, but chromosomes contain short pieces of repetitive DNA that are not used to produce proteins and whose function is often unclear. They are called microsatellites. One cluster of such microsatellites showed a similarity between head lice from Asia and Central America. It reflects the original migration of people crossing the Bering Strait to the Americas from Siberia, an event that occurred near the end of the last Ice Age.
Another cluster of microsatellites linked head lice from America to their European counterparts. By relating changes in the lice DNA to the animals’ reproductive rate, it was possible to calculate that the common ancestor of American and European lice lived about 500 years ago – exactly when the first Europeans crossed the Atlantic Ocean. Reconstructing human migration routes: what such a nasty itchy insect is not good for.