Intermediate Hosts

INTERMEDIATE HOSTS, animals in which parasites, passing through a developmental cycle not in one but in two or three hosts, live in a larval state or in a form capable of asexual reproduction; in the final, or definitive, host the parasite reaches sexual maturity and reproduces sexually. The phenomenon of intermediate hosts is more sharply expressed in internal parasites, but some external parasites also require several hosts to complete their life cycle. For example, the tick Rhipicephalus sanguineus is a three-host tick, since for each of its transformation stages a host is required for feeding on blood. Each stage—larva, nymph, and adult tick—feeds once on the host, after which it falls off and molts in the external environment to the next transformation stage or begins laying eggs (female). Ixodes ricinus can feed on hosts of the same species in all stages, for example on a hedgehog, or it can use hosts of different species. In other ticks, a change of hosts of different species is the rule, for example Dermacentor venustus lives on ground squirrels Citellus grammurus in the larval and nymphal stages, and in the adult state attacks humans or large cattle. In this case, the phenomenon of host change is carried out over the course of each generation of the parasite. In many endoparasites the matter is more complicated, since host change proceeds simultaneously with the change of generations of the parasite itself in the form of metagenesis, heterogony, or other peculiar biological relationships. The simplest example of host change can serve as the life cycle of the unarmed tapeworm (Taenia saginata). The tapeworm stage, reproducing sexually, lives only in the human intestine, while the larval form in the form of a bladder worm or cysticercus lives in the muscles of large cattle. The cow is thus an intermediate host, and the human is the definitive host of the unarmed tapeworm. The bladder worm does not reproduce in the body of the cow, but, entering the human intestine upon eating meat, begins to reproduce asexually (bud), with all the separated parts (proglottids) remaining connected to each other and forming the strobila of the tapeworm. The definitive and intermediate hosts serve as sources of mutual infection by the corresponding parasites: a person becomes infected by eating meat with bladder worms, and a cow becomes infected by swallowing the tapeworm eggs, scattered by the parasite's proglottids with human feces. In some cases the same individual of a host can be both an intermediate and a definitive host with respect to a given parasite. The normal life cycle of the armed tapeworm proceeds in the human intestine (tapeworm stage) and in the muscles of the pig (bladder worms). If, however, the eggs of the armed tapeworm fall into the human stomach (from outside or during strong antiperistalsis), then their further fate will be the same as in the pig's body, i.e., bladder worms will develop in different organs of the human. It is characteristic that in such cases the definitive host must be infected twice and each time in a special way: 1) infection through a bladder worm and 2) infection through the parasite's eggs (even if in the form of autoinfection). In a number of cases the intermediate hosts of a given parasite can be animals of different species and orders; for example, bladder worms of the dwarf tapeworm (Hymenolepis diminuta) parasitize various fleas, beetles, orthopterans, and butterflies. Intermediate and main hosts can belong to the same class of animals or relate to different types. Man and pig (parasitism of Taenia solium) are members of the class of mammals, man and the rat flea (parasitism of Hymenolepis nana) belong to the types of chordates and arthropods. An intermediate host can be two in the cycle of a given parasite. In such cases the life cycle of the parasite infecting hosts belonging to different systematic groups of animals is complicated. For the broad fish tapeworm (Diphyllobothrium latum) the first intermediate hosts are copepods Cyclops or Diaptomus (for the procercoid stage), the second intermediate hosts serve as various fish—perch, pike, trout, etc. (for the plerocercoid stage); the definitive hosts are humans, dogs, cats, foxes, bears, and many other mammals. The life cycle of various flukes captures as first intermediate hosts some mollusks; the second intermediate hosts serve, for example, as fish; the definitive hosts are humans and various vertebrates (for example, the Chinese liver fluke—Clonorchis sinensis). In an intermediate host the parasite is in various states: a) the parasite reaches a larval stage which directly transforms into a sexual mature form in the definitive host (larvae of guinea worm—Dracunculus medinensis—in Cyclops); b) the larval stage of the parasite develops in an intermediate host, but reproduces in the definitive host (bladder worms of many cestodes); c) in the intermediate host live alternating generations of the parasite, reproducing asexually in it itself (schizogony of the malaria plasmodium in human blood, reproduction of piroplasms in the blood of cattle, horses, dogs, etc.); d) in the intermediate host lives one generation of the parasite, but with multiple formation of future adult forms (echinococcus). The ways of infection of intermediate hosts by parasites are different. In a number of cases intermediate hosts are actively infected by parasites: the dog louse swallows the egg of the dog tapeworm (Dipylidium caninum), Cyclops swallows the embryo of the guinea worm, the pig eats the egg of the armed tapeworm, etc. In other cases the parasite itself actively infects the intermediate host: miracidia of the liver fluke burrow into the body of the small pond snail (Limnaeus minutus). The second intermediate hosts are infected similarly—actively (the perch eats Cyclops with procercoids) or passively (metacercariae of flukes burrow into the skin scales of fish). The further fate of the parasites in their intermediate hosts is different. In some cases the intermediate host is the final stage for a given stage of the parasite, which itself cannot either leave or transfer to the definitive host. Thus, bladder worms and larvae of trichinae are walled up in the muscles or other organs of their hosts. When the latter die a natural death, the parasites will also die in the corpse, if they have not lost their activity earlier (calcification of old trichina cysts in mammals). The chances for further development of the life cycle of such "walled up" parasites are obtained in the case where the intermediate host is eaten by the definitive host. Another category of intermediate hosts can be characterized as a transient stage in the life of some stages or generations of parasites. This is the case with some flukes and nematodes. Miracidia burrow into the body of mollusks, reproduce in them in sporocyst and redia stages, and in the final form of cercaria leave their intermediate host, returning to the aquatic environment. Microfilariae get into the organism of mosquitoes, horseflies, and gadflies with absorbed blood and here turn into larvae, which over time leave their host during its bloodsucking and burrow into the integuments of the definitive host. In both cases the parasite passes through the body of the intermediate host; however, it is necessary to note a significant peculiarity: in the first case the intermediate host has no relation to the definitive host, whereas in the second the intermediate host is a carrier of the parasite with respect to the definitive host (see Carriers), on whose account the parasite itself also parasitizes. The influence of parasites on their intermediate hosts is different: some of the intermediate hosts do not undergo any noticeable diseases, others suffer greatly from the parasites that have entered them (human and avian malaria, piroplasmosis, theileriosis, nuttalliosis of domestic animals, human echinococcus, etc.). In relation to the paths of phylogenetic development of the phenomenon of intermediate hosts, one can assume that parasitic worms first adapted to inhabiting their present definitive hosts; the eggs or larvae of these parasites, falling into the external environment, were swallowed by various animals, among which there were forms predominantly used by present definitive hosts for food. This circumstance improved the chances of the parasite for further existence precisely in these animals and became the object of natural selection; finally it was fixed as permanent relationships, in which the parasite reaches a larval stage of development in the intermediate host; the latter transforms into a sexual mature form in the definitive host, devouring the intermediate host. According to this theory, intermediate hosts are phylogenetically younger than the definitive hosts. The study of intermediate hosts and their biological-ecological peculiarities is extremely important for the purposes of the prevention of parasitic diseases. (Intermediate hosts for various parasites see in the corresponding articles.)