Ecology: Biology of Interaction. IV-17. (Supplement) How Parasites “Set Up” Their Hosts

IV-17. (addendum) How Parasites "Set Up" Their Hosts The life cycle of parasites is built "with consideration" for the feeding preferences and behavior of their hosts. Most often, parasites interfere with the life of an intermediate host, increasing the probability of its being eaten by the definitive host. Let's consider three examples, two of which are well-studied, and the third is still a subject of debate. On the surface of shallow, slow-flowing water bodies, one can sometimes see live fish with swollen bellies floating on the surface. Fishermen call such fish "solitary": in the body cavity of these fish lie tangles of white ribbons. This is the plerocercoid stage (intermediate stage) of development of cestodes, tapeworms (relatives of the liver fluke), Ligula and Digramma (Ligula intestinalis, Digramma interrupta). 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 (the second intermediate host), plerocercoids develop in its body (Fig. IV-17.1). After 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 definitive host of the parasite – the bird. By the way, for humans, "solitary" fish and even the plerocercoids themselves are quite edible and pose no danger (some people consider them a delicacy). Fig. IV-17.1. Plerocercoids of Ligula intestinalis and fish infected by them The fluke Leucochloridium paradoxum belongs to the same group as the liver fluke. Its definitive host is songbird insectivores. The eggs of these parasites enter the grass with the birds' droppings, where they are eaten by herbivorous land snails – amber snails (Succinea putris). The life cycle of Leucochloridium 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 emerge into the snail's tentacles. Through the thin coverings of the tentacles, the bright striped outgrowths of the sporocyst are clearly visible. These outgrowths perform rather jerky movements, relaxing and contracting due to their musculature. Birds notice such amber snails, mistake their wriggling tentacles for insect larvae (Fig. IV-17.2), and peck them, thus becoming infected with the parasite. Fig. IV-17.2. Amber snail (Succinea putris) with Leucochloridium paradoxum sporocyst outgrowths in its tentacles The third example of parasite influence on their intermediate hosts concerns Toxoplasma gondii, a widespread parasite of cats and rats (Fig. IV-17.3). It is a distant relative of the malaria plasmodium, belonging to the phylum Apicomplexa. For example, rats are intermediate hosts of Toxoplasma. In order for the intermediate host to more reliably fall into the jaws of the definitive host (cat), where Toxoplasma will undergo a stage of sexual reproduction, Toxoplasma changes the behavior of rats. An infected rat loses caution; it loses its fear of the smell of cats and cat urine. A rat carrying parasites enters a cat's territory and becomes prey for the predator. Fig. IV-17.3. Life cycle of Toxoplasma gondii The potential range of intermediate hosts for Toxoplasma is quite wide; it also includes humans. In addition to infection routes common to all hosts (through water, soil, or food contaminated with cat feces, and through the meat of other intermediate hosts), there are specific ones – for example, contact with cat litter. The danger of toxoplasmosis for humans was previously associated only with infection during pregnancy. If a woman becomes infected with this parasite for the first time during pregnancy after playing with a cute cat, it is transmitted through the placenta to the fetus and can cause blindness, hydrocephalus (water on the brain), and other brain damage. It was believed that apart from this, toxoplasmosis in humans is practically asymptomatic, although it is known that a significant portion of the world's population (about 40%) is infected with it. However, a number of recently obtained data suggest 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 achieve being eaten by a cat, Toxoplasma can affect the host's behavior by altering its brain biochemistry. For example, American insurance companies have significantly 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 is adapted to some extent to using humans as well. Our ancestors, African primates, were eaten by felines: both the lions and leopards that exist today, and the extinct saber-toothed cats. Due to climate and cultural specifics, toxoplasmosis is unevenly distributed. Only about 7% of residents in England and Japan are affected, while 80% of the population in Brazil (a climate favorable to the parasite) and France (where they like meat with blood) are affected. In the USA, infection rates vary by state from 18 to 29%. A link has been found between the infection rates of representatives of different peoples with Toxoplasma and the cultural characteristics of these peoples. According to available data, the parasite intensifies feelings of 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 the social environment of the infected person. Women, under the influence of Toxoplasma, become more intellectual and conscious, compassionate, and prone to moralizing; they make more friends and do more shopping. Men are characterized by a tendency towards weakened intellect, dogmatism, emotional variability, and jealousy. At the same time, it becomes more difficult for both sexes to concentrate on solving a task. When evaluating the data presented, it should be remembered that, in addition to parasites, many other factors (e.g., the effects of alcohol, nicotine, caffeine, artificial lighting, excessive population density, etc.) also alter the psyche of modern humans.