Host and its Types in Parasitology

We find in nature that different organisms are linked to each other through various ways, such as through the food chain or food web, through sharing of the common resources like food, shelter and mate. Organisms interact with each other in many ways and these interactions have been categorised as inter-specific and intra-specific interactions.

 Intra-specific Interactions: These are the interactions which take place between the individuals of same species. 

Inter-specific Interactions: These are the interactions which take place between the organisms of different species. Inter-specific interactions may involve such relations where both the interacting individuals may get benefited and the association is non-obligatory or both the interacting individuals may get benefited and the association is obligatory, or one may get benefited and other remains unaffected or one may get benefited and other is harmed as such, such inter-specific interactions are called as Proto-cooperation, Mutualism, Commensalism or Parasitism respectively. All such inter-specific interactions where two individuals of different species interact have been broadly termed as Symbiosis which simply means “living together”. The organism which proves beneficial for other individual involved in symbiotic relationship is called as Host. In commensalism the organism which gets benefited is called as Commensal and the organism from which the benefit is derived is called as Host. In parasitism the organism which gets benefitted is called as Parasite and the organism from which the benefit is derived is called as Host. “Thus Host is an organism that harbours a parasite or a commensal on or in its body and provides shelter, food or both”. 

Types of hosts

Hosts in parasitology have been categorised into different types which have been explained as under:

 1. Definitive Host. The hosts which harbour a mature and adult parasite are called as definitive hosts. The parasite reproduces sexually in definitive hosts. E.g. Sheep is the definitive host for Fasciola hepatica whereas cattle and man are definitive hosts for Fasciola gigantica and Taenia solium respectively, similarly female Anopheles is the definitive host for Plasmodium species. The parasites in these hosts harbour mature stage of their life cycle and reproduce sexually in them. Definitive hosts are the final hosts in the life cycles. Definitive host is also known as Primary Host. 

 2. Intermediate Host: The host which harbours sexually immature parasites is called as Intermediate Host. Intermediate host is also known as Secondary Host. The parasites undergo asexual mode of reproduction in intermediate hosts. Intermediate hosts often act as vectors of the parasites and help the parasites to reach the final host i.e. the definitive host. The intermediate hosts, for this reason, are called as delivery boys of parasitic world. E.g. Snails of genus Planorbis and Lymnaea are intermediate hosts of Fasciola hepatica as they harbour larval forms which reproduce asexually in these hosts. Pig is intermediate host of Taenia solium. Man is intermediate host of Plasmodium. Intermediate host may transmit the parasite to definitive host through active mode e.g. a mosquito transmitting malarial parasite, where a mosquito bites the vertebrates and injects the parasites in their body or through the passive mode e.g. the parasite residing in copepods, the intermediate host, is transmitted to fish when copepods are engulfed by fish, snails also act as passive intermediate host for trematode parasites. It is not always easy or even possible to identify which host is definitive and which is intermediate. As the life cycles of many parasites are not well understood, sometimes the subjectively more important organism is arbitrarily labelled as definitive, and this designation may continue even after it is found to be incorrect. E.g. Man is intermediate host of Plasmodium and mosquito is its definitive host, but as man is important organism as compared to mosquito therefore in many books you will find man as definitive and mosquito as intermediate host of the Plasmodium though biologically it is incorrect. 

 3. Paratenic Host: It is also called as Transport host. It is a host which harbours larval stages of parasite but is not necessary for completion of parasite’s life cycle. It is a substitute intermediate host where no further development takes place although its presence may be required as an essential link in the completion of the parasite’s life cycle. It carries the parasite between the successive hosts e.g. the successive fish hosts that carry the developmental stage (plerocercoid) of broad fish tapeworm Diphyllobothrium latum to larger fishes and ultimately to human beings, act as transport host. Under extreme environmental conditions, the transmission of parasites is facilitated mainly by paratenic hosts e.g. larval stages called as mesocercarial stages of Alaria americana , a trematode, reside in tadpoles which are rarely eaten by definitive canine hosts. These tadpoles containing mesocercarial larvae are frequently eaten by snakes where they don’t undergo further development. These snakes act as Paratenic or Transport host which when devoured by canines pass on the larvae to canine definitive hosts.

 4. Accidental Host: An organism that generally does not allow transmission to the definitive host, thereby preventing the parasite from completing its development/life cycle. Accidental hosts are also called as Dead-end or Incidental hosts. Humans and horses are dead-end hosts for West Nile virus, whose life cycle is normally between mosquitoes and birds. Human beings and horses can become infected, but the level of virus in their blood does not become high enough to pass on the infection to mosquitoes that bite them. 

 5. Reservoir host: An organism that harbours a parasite (pathogen) but it does not suffer illness, however, it serves as a continuous source of infection to other species that are susceptible. In other words, these are hosts that harbour a parasite of another species without itself getting affected, but act a source of infection for the original host e.g. Game animals act as reservoir hosts for the protozoan parasite of man, Trypanosoma gambiense. 

 6. Carrier Host: it is an individual which is capable of transmitting a pathogen without displaying symptoms of the disease. There are two types of Carriers; Passive carrier and active carrier. A passive carrier is contaminated by the pathogen and can mechanically transmit it to another host. Passive carrier is not infected e.g. a cockroach or housefly act as mechanical carrier of cholera and dysentery etc. An active carrier is the one who is infected and who can pass the parasite prior to the appearance of the symptoms of the disease or who can transmit the parasite when it itself has recovered from the disease e.g. a person carrying Salmonella typhi although may not be suffering from the typhoid disease, yet he can transmit it to another person. 

 7. Intercalary host: It is a host that liberates the infective stages of a parasite that are trapped in the body of the original host e.g. Cats by eating mice liberate the eggs of Capillaria hepatica which are trapped in the liver parenchyma of rat. 

 8. Host of predilection: It is also called as Preferred Host. These are those hosts which are preferred over others by a particular parasite. Many parasites are motile and exhibit behavioural preferences for certain host species. Because hosts can vary in their susceptibility to infections, parasites might benefit from preferentially detecting and infecting the most susceptible host e.g. larval trematode parasites have been found to prefer species of tadpole hosts in the following order: Anaxyrus ( Bufo), Terrestris (southern toad), Hyla squirella (squirrel tree frog), Lithobates ( Rana) Sphenocephala (southern leopard frog) and Osteopilus septentrionalis (Cuban tree frog).