General Parasitology - Parasitic Helminth Lifecycles, and the Nomenclature of the Host Organism
Parasitic helminths may have either simple or complicated lifecycles. The terms used to describe the hosts harboring different stages in these lifecycles are however the same. Firstly the adult parasites are found in the Definitive Host. This is where the parasite's sexual cycle usually takes place, with either cross or self fertilisation with hermaphroditic parasites, or sexual reproduction if the parasites have separate sexes, followed by production of eggs, or more rarely with viviparous helminths, larvae. Secondly, in many cases the parasite larvae 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 asexual characteristics of many of these organisms. With parasitic protozoa the vertebrate host is generally referred to as the definitive host, whilst the invertebrate is the intermediate host. 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. There are however a number of other types of host;
Accidental Hosts are those in which the parasite do not normally develop (due, for example to lack of exposure to infective forms of the parasite), but when occasionally chance infections do occur, the parasite is able to 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 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 effectively a dead end, 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 Anisakissp., which give rise to the condition of 'Anisakiasis' on ingestion of raw infected fish.
Paratenic Hosts may also be included in parasitic helminth lifecycles. In these forms of infection the parasites undergo an arrested development on infection, 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 completion of the parasites lifecycle. This is in contrast to the case with true intermediate hosts whose ingestion is essential to the lifecycle, for example Echinococcus sp.
Finally, these incidental hosts and hosts of parasites which have zoonotic patterns of infection (i.e. normally infect a wide range of hosts), may act as Reservoir Hosts for the parasite. These are also a form of permissive host as fully viable infections develop, and a more accurate term would be alternative definitive 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 infections is Schistosoma japonicum. This parasite, as well as infecting man, can also infect other mammals as definitive hosts, including rodents, cats, dogs, domesticated ruminants such as water buffalo and a wide range of other mammals. The presence of these Zoonoses has implications for the control of the parasite in the field.