Parasitic helminths might have either simple or complex life-cycles. The word utilized to explain the various phases of hosts harboring in these lifecycles are nevertheless the same. The degree of damage done to the hosts is though differed. For illustration in definitive host, the greatest harm is observed being the one the adult phase of the parasite is found. At times, a host might suppose dual functions and thus could be hard to categorize strictly into one kind. Human host during infection through malaria parasite is one of such. Human could be categorized as the definitive host being the one in which maximum harm is observed. As well, intermediate host because of human harbors the asexual phases of the parasite (that is, merozoites and trophozoites). A clear comprehending of the relationship among host and parasite and function of host in survival and transmission of parasite is thus essential for a better categorization.
Types of Host:
1) Definitive Host:
The adult parasites are obtained in the definitive host. This is where the parasite's sexual cycle generally occurs, with either cross or self fertilization having hermaphroditic parasites or sexual reproduction when the parasites have separate sexes, followed through production of eggs or hardly ever by viviparous helminths, larvae. The greatest harm is generally seen in this host.
2) Intermediate Host:
In most of the cases the parasites larvae are found in various hosts, these are termed as the Intermediate Hosts. Parasitic helminths larvae might have one, two or more intermediate hosts in their life-cycles or they might have no intermediate hosts.
Often asexual phases of reproduction take place in these intermediate hosts, (for illustration by Platyhelminthes parasites). Note that if explaining hosts of parasitic protozoa, these words are slightly dissimilar owing to the asexual features of most of these organisms. By means of parasitic protozoa, the vertebrate host is usually termed to as the definitive host, whereas the invertebrate is the intermediate host. A few parasitic nematodes (example: Strongyloides stercoralis) are Facultative parasites, having fully free living life-cycles in addition to the parasitic ones. The two terms definitive and intermediate host are the most significant in Parasitology if referring to the kind of host. A vector though, must not be mistaken for intermediate host. A vector actively transmits infection to a host devoid of necessarily harboring the asexual phase of the parasite.
3) Accidental Host:
Accidental hosts are such in which the parasite does not normally build up (due, for illustration to lack of exposure to infective forms of the parasite), however when occasionally chance infections take place, the parasite is capable to complete the life-cycle.
Hosts, where the parasite can complete its life-cycle are termed as Permissive hosts and comprise true definitive and intermediate hosts and also numerous accidental hosts. Illustrations here comprise such parasites like Fasciola hepatica, where the normal definitive hosts are ruminants, however humans and other animals might as well be infected and viable adult parasites grow. The other illustration is human infection having the nematode Angiostrongylus cantonensis in the distant East.
In comparison, the other form of the accidental host is the Non-Permissive host in which the parasite, however it might build up to some extent, reaches efficiently a dead end, the parasite not being capable to complete its life-cycle and ultimately dying within the host. Such forms of infection often take place where the parasite consists of intermediate hosts which might be accidentally ingested through animals other than the true definitive host.
4) Paratenic Host:
Paratenic hosts might also be comprised in parasitic helminths lifecycles. In such forms of infection the parasites experience an arrested development on infection, larval forms accumulating in such hosts till they encompass a chance of infecting the definitive host (example in the Pseudophyllidean tapeworms). These hosts are thus not necessary for the completion of the parasites life-cycle. This is in contrast to the case having true intermediate hosts whose ingestion is necessary to the lifecycle, for instance Echinococcus sp.
5) Reservoir Host:
These are the accidental hosts and hosts of parasites which encompass zoonotic patterns of infection (that is, generally infect a broad range of hosts), might act as Reservoir Hosts for the parasite. These are as well a form of permissive hosts as completely viable infections develop, and a more accurate word would be alternative definitive hosts (although this is not in fact employed). The word reservoir host is generally used if explaining the epidemiology of human infections. An illustration of parasites having zoonotic infections is Schistosoma japonicum. This parasite and infected man can as well infect other mammals as definitive hosts, comprising rodents, cats, dogs, domesticated ruminants like water buffalo and a broad range of other mammals. In Human African Trypanosomiasis (HAT), the reservoir host is cattle that serve as sources of active infection to man. The presence of such Zoonoses consists of implications for the control of the parasite in the field.
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