Ecology: Biology of Interaction. 4.18. (supplement) How Parasites “Set Up” Their Hosts
A correlation has been detected between the degree of Toxoplasma infection in different peoples and cultural traits of those peoples. Available data suggest the parasite may intensify feelings of anxiety, uncertainty, depression, and guilt in humans, though expression of these effects depends on sociocultural context.
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4.17. (addendum) Strategies of intraspecific interaction
D. Shabanov, M. Kravchenko. Ecology: Biology of Interaction. Chapter 4. Population Ecology
5.01. Environment and ecological environment
4.18. (addendum) How parasites "set up" their hosts The life cycle of parasites is built "taking into account" the food preferences and behavior of their host. Often, parasites interfere with the life activity of the intermediate host, increasing the probability of its being eaten by the definitive host. Here are three examples, two of which are well-studied, and the third is still a subject of discussion. On the surface of small, slow-flowing water bodies, one can sometimes see fish swimming at the surface with swollen bellies. Fishermen call such fish "solitary": in the body cavity, there are bundles of white tapeworms. This is an intermediate stage (plerocercoid) in the development of tapeworms (relatives of the liver fluke) – Ligula and Digramma. The definitive stage of these cestodes parasitizes fish-eating birds – herons, gulls, and others. The parasite's eggs enter the water bodies with the birds' droppings. The larva that hatches from them infects the first intermediate host – a cyclops; when such a cyclops is eaten by a fish, plerocercoids develop in it. Upon reaching maturity, they disrupt the functioning of the fish's swim bladder, forcing it to rise to the surface. This makes the fish an easy prey for the parasite's definitive host – the bird. By the way, for humans, "solitary" fish and even the plerocercoids themselves are quite edible and pose no danger. To the same group as the liver fluke belongs the fluke Leucochloridium paradoxum. Its definitive host is songbirds that eat insects. With the droppings of birds, the eggs of these parasites fall onto grass, where they are eaten by herbivorous terrestrial snails – amber snails. The life cycle of this parasite is shortened, and sporocysts appear in the body of the amber snail, which immediately produce future flukes. The sporocyst penetrates the entire body of the amber snail, and its outgrowths extend into the snail's tentacles. The brightly striped outgrowths of the sporocyst are clearly visible through the thin covers of the tentacles. These outgrowths perform rather sharp movements, relaxing and contracting due to their powerful musculature. Birds notice such amber snails, mistake their winding tentacles for insect larvae, and peck them, thus becoming infected with the parasite. The third, controversial example of parasite influence on their intermediate hosts concerns Toxoplasma (Toxoplasma gondii) – a widespread parasite of cats and rats. This protozoan, a distant relative of the malaria plasmodium, belongs, like it, to the phylum Apicomplexa. The rat is an intermediate host of Toxoplasma. To ensure that the intermediate host more reliably falls into the jaws of the definitive host (the cat), and Toxoplasma can complete the sexual reproduction stage, they alter the rat's behavior. An infected rat loses its caution; its fear of the smell of cats and cat urine disappears. The rat carrying the parasites enters the cat's territory and becomes prey for the predator. The danger of toxoplasmosis to humans was previously associated only with infection during pregnancy. If a woman, after playing with a cute cat, becomes infected with this parasite for the first time during pregnancy, it is transmitted through the placenta to the fetus and can cause blindness, hydrocephalus (water on the brain), or calcification in the brain. It was believed that apart from this, toxoplasmosis in humans is practically asymptomatic, although it was known that a significant portion of the world's population (about 40%) is infected with it. The source of infection is the feces of cats that have eaten rats, or undercooked meat of animals that have come into contact with this feces. However, a number of recent data indicate that the parasite can affect the psyche not only of rats but also of humans. Although it is difficult for a parasite in the human brain to ensure it is eaten by a cat, Toxoplasma can affect the host's behavior by altering its brain biochemistry. For example, American insurance companies have reliably registered that the risk of crashing one's car is almost three times higher in Toxoplasma carriers than in people free of these parasites. Perhaps Toxoplasma are adapted to some extent to using humans as well. Our ancestors, African primates, were eaten by predators of the cat family: both current lions and leopards, and extinct saber-toothed cats. Due to climate and cultural peculiarities, toxoplasmosis is unevenly distributed. Only about 7% of residents in England and Japan are affected, and 80% of the population in Brazil (favorable climate for the parasite) and France (where they like meat with blood). In the USA, infection rates vary by state from 18% to 29%. A link has been found between the infection rate of different peoples with Toxoplasma and the cultural characteristics of these peoples. According to available data, the parasite increases anxiety, insecurity, depression, and guilt in humans. The nature of the anomaly – whether it manifests as a domestic neurosis, a morbid sense of personal sinfulness, or simply increased religiosity – is determined by the level of culture and social environment of the infected person. Under the influence of Toxoplasma, women become more intellectual and conscious, empathetic, and prone to moralizing; they make more friends and do more shopping. For men, a tendency towards weakened intellect, dogmatism, emotional variability, and jealousy is characteristic. At the same time, it becomes more difficult for both sexes to concentrate on solving a task. When evaluating the data presented, one should not forget that, in addition to parasites, processes in the human psyche are influenced by numerous other factors (e.g., the effects of alcohol, nicotine, caffeine, artificial lighting, overpopulation, etc.).
4.17. (addendum) Strategies of intraspecific interaction
D. Shabanov, M. Kravchenko. Ecology: Biology of Interaction. Chapter 4. Population Ecology
5.01. Environment and ecological environment