Types of Parasitic Helminths host

INTRODUCTION

Parasitic helminths may have either complicated or straightforward lifecycles. The terms used to describe the hosts harboring different stages in these lifecycles are however the same.

The degree of damage done to the hosts is however varied. For example, in the definitive host, the greatest harm is seen being the one the adult stage of the parasite is found. Sometimes, a host might assume dual functions, and therefore could be challenging to classify strictly into one type. Human host during infection by the malarial parasite is one of such. A human could be classified as the
definitive host is the one in which the greatest harm is seen. Also, intermediate host because of human harbors the asexual stages of the parasite (merozoites and trophozoites). A clear understanding of the relationship between host and parasite and function of the host in survival and transmission of the parasite is, therefore, necessary for a better classification.

Definitive Host

The adult parasites are found in the definitive host. This is where the parasite's the sexual cycle usually takes place, with either cross or self-fertilization with hermaphroditic parasites, or sexual reproduction if the parasites have separate sexes, followed by the production of eggs, or more rarely with viviparous helminths, Larvae. The greatest harm is usually seen in this host.

Intermediate Host

  In many cases the larvae of the parasite are found in different hosts, these are called the Intermediate Hosts. Parasitic helminth larvae may have one, two or more intermediate hosts in their lifecycles, or they may have no intermediate hosts.

 Often asexual stages of reproduction occur in these intermediate hosts, (for example with Platyhelminth parasites). Note that when describing hosts of parasitic protozoa, these terms are slightly different owing to the sexual characteristics of many of these organisms.

With parasitic protozoa, the vertebrate host is generally referred to as the definitive host, while the invertebrate is the intermediate hosts. Some parasitic nematodes (e.g., Strongyloides stercoralis) are
Facultative parasites, having completely free-living lifecycles in addition to parasitic ones. The two terms definitive and intermediate host are the most important in Parasitology when referring to the type of host. A vector, however, should not be mistaken for an intermediate host. A vector actively transmits the infection to a host without necessarily harboring the asexual stage of the parasite, e.g., the vector of African trypanosomiasis Glossina spp.

  These groups of vectors pick up the parasite (the infective stage) from the reservoir hosts during the blood meal and transmit it to the susceptible definitive host. However, some vectors can still serve as intermediate hosts harboring the asexual or the larval forms of the parasite e.g.individual Anopheles mosquitoes that harbor the microfilariae of filarial worms.

Accidental Host

  Accidental hosts are those in which the parasite does not usually develop (due, for example to lack of exposure to infective forms of the parasite), but when occasionally chance infections occur, the parasite can complete its lifecycle.

Hosts, where the parasite can complete its lifecycle, are called Permissive hosts and include true definitive and intermediate hosts as well as many accidental hosts. Examples here include such parasites as Fasciola hepatica, where the normal definitive hosts are ruminants, but humans and other animals may also be infected, and viable adult parasites develop. Another example is a human infection
with the nematode Angiostrongylus cantonensis in the far East.

   In comparison another form of the accidental host is the Non-Permissive host where the parasite, although it may develop to some extent, reaches a dead-end effectively, the parasite not being able to complete its lifecycle and eventually dying within the host. These forms of infection often occur where the parasite has intermediate hosts which may be accidentally ingested by animals other than the true definitive host. For example, with various marine ascarids of the family Anisakidae such as Anisakis sp., which give rise to the condition of 'Anisakiasis.' on ingestion of raw infected fish.

Paratenic Host

  Paratenic hosts may also be included in parasitic helminth lifecycles. In these forms of infection, the parasites undergo an arrested development on contamination, larval forms accumulating in these hosts until they have a chance of infecting the definitive host (e.g., in the Pseudophyllidean tapeworms). These hosts are therefore not essential to the completion of the parasite's lifecycle. This is in contrast to the case with true intermediate hosts whose ingestion is necessary to the lifecycle, for
example, Echinococcus sp.

Reservoir Host

  These are accidental hosts and hosts of parasites which have zoonotic patterns of infection (i.e., usually infect a wide range of hosts), may act as Reservoir Hosts for the parasite. These are also a form of permissive hosts as fully viable infections develop, and a more accurate term would be definitive alternative hosts (though this is not in fact used). The term reservoir host is usually only used when describing the epidemiology of human infections. An example of parasites with zoonotic diseases is Schistosoma japonicum.

This parasite, as well as infected man, can also affect other mammals as definitive hosts, including rodents, cats, dogs, domesticated ruminants such as water buffalo and a wide range of other mammals. In Human African Trypanosomiasis (HAT), the reservoir host is cattle which serve as sources of active infection to man. The presence of these Zoonoses has implications for the control of the parasite in the field.

SUMMARY

  The host of a parasite is the organism that harbors the parasite. The intermediate host is the one in which part of the developmental stages in the life cycle of the parasite takes place, usually the larval or the asexual stage. A vector actively transmits an infective stage of parasite harbored by the reservoir host which serves as a source of active infection to the definitive host. The definitive host
harbors the adult or the sexual stage of the parasite and often experiences the most enormous harm during the life cycle of the parasite.