Generality
Helminths are worm-like metazoan parasites. They are therefore multicellular organisms, similar to worms, which live at the expense of a host, drawing sustenance from it and causing damage to it.
Unlike other parasites, such as lice or fleas, helminths live inside the host, which can be man or another animal. However, worms do not always cause considerable damage to the organism that they hosts and generally do not cause death; this event, in fact, would turn into a great damage for the parasite itself. Rather, thanks to the host's defense mechanisms, a sort of equilibrium is created, which prevents excessive increases in the parasitic population. or malnutrition.
The host may differ in the various phases of the worm's life cycle; therefore we can distinguish:
- definitive guests: in which the reproductive phase takes place;
- intermediate hosts: in which a phase is carried out for the formation of infesting forms that are transmitted to the definitive host;
- transport hosts: accidentally infested with infectious forms;
- terminal hosts: in which the infection runs out.
An estimated three billion people worldwide are infested with at least one of these parasites.
Classification
From a taxonomic point of view, helminths can be classified into:
- Flatworms or flat worms: they are characterized by a bilaterally symmetrical body. Mostly hermaphrodites (with the exception of flukes belonging to the genus Schistosoma), They are divided into:
- Trematodes: leaf-shaped or elongated worms that can parasitize the intestinal tract, liver, lung, and blood vessels (eg schistosomiasis);
- Cestodes: ribbon-shaped worms with a segmented body, characterized by the presence of the scholic (a particular adhesion organ); according to the species of cestode, man can represent the host of the adult stage in the digestive tract (eg. Taenia saginata) or the larval stage in the tissues (e.g. cystic hydatidosis).
- Roundworms or cylindrical worms: they are dioecious, elongated, with thinned ends; they can be both intestinal parasites (eg. Enterobius vermicularis), and blood or tissue (e.g. heartworms).
Pathogenic Helminths for Man
The most important helminths of human interest are: Ascaris lumbricoides (ascariasis), Trichuris trichiura (triocephalosis), Ancylostoma duodenale and Necator americanus (hookworm), Enterobius vermicularis (responsible for "oxyuriasis), Strongyloides stercoralis (strongyloidiasis), Capillaria spp. (C. hepatica and C. philippinensis), Trichostrongylus spp., Schistosoma spp. (S. mansoni, S. japonicum, S. intercalatum, S. mekongi, S. haematobium - schistosomiasis), Clonorchis sinensis, Opistorchis spp., Paragonimus westermani, Fasciola hepatica, Metagonimus spp, Heterophyes spp., Taenia spp. (Taenia saginata - cattle - and Taenia solium - pigs - Solitary worm -) Hymenolepis nana, Hymenolepis diminuta, Diphyllobothrium latum.
The underlined helminths are the most common in our country, where Ossiuri infestations (in children) and to a lesser extent those from Ascaris clearly prevail.
Infection
Helminths can be transmitted in different ways.
- Helminthiasis with direct human transmission: the entire biological cycle of the parasite takes place in man and the eggs eliminated with the faeces are immediately infesting. In oxyuriasis, for example, the female migrates to the anal opening at night to lay her eggs; the result is severe itching in the perianal region, with a tendency to scratching. With this gesture the child involuntarily collects the eggs, which will then be dispersed with the hands in the surrounding environment or worse still brought to the mouth.
- Geohelminthiasis: as before, the entire biological cycle takes place in man, but the eggs eliminated with the faeces are not immediately infesting. This ability is acquired in the soil over a period of time which - in optimal conditions of temperature and humidity - varies depending on the species from 7 to 21 days. The lack of sanitary facilities and the use of black water as fertilizer leads to an enormous spread of these pathologies (eg ascaridiasis, trichocephalosis, hookworms, strongyloidosis);
- Zooanthroponotic helminthiasis: humans are generally infected by involuntarily ingesting the larval forms present in raw or undercooked meats (eg teniasis, trichinellosis).
