Host (in the context of infectious disease) is defined as an organism or animal which harbors the parasite (another organism or animal) and provides nourishment and shelter to it. The host is always larger than the parasite. Example: Lice is a parasite dwelling in the human head or animal body.
Types of host
There are five major types of hosts depending upon their role in the life cycle of the parasites.
Definitive or Primary Host
The host which harbors the adult parasites or where the parasite replicates sexually is called the definitive host. The definitive host can be a mammalian host or other living hosts. Examples include sheep for Fasciola gigantica, a dog for Echinococcus granulosus, and a female anopheles mosquito for Plasmodium spp.
In the majority of human parasitic infections, man is definitive host; in malaria and hydatid disease, however, man acts as the intermediate host.
Intermediate or Secondary Host
Refers to the host which harbors the larval stages of a parasite or in which the parasite undergoes asexual multiplication. For example, humans are the intermediate hosts for Plasmodium (malarial parasites).
Intermediate hosts are mandatory for the completion of the life cycle for some parasites. Some parasites require two intermediate hosts to complete their different larval stages. These are known as the first and second intermediate hosts respectively. For e.g. Amphibian snails are the first intermediate host and aquatic plants are the second intermediate host for Fasciola hepatica.
It is a host, which harbors the parasites, possibly grow, and multiply and serves as an important source of infection to other susceptible hosts. For e.g. a dog is the reservoir host for cystic echinococcosis. Reservoir hosts do not get the disease carried by the pathogen or it is asymptomatic and non-lethal.
Paratenic or Storage Host
A paratenic host serves as a temporary refuge and vehicle for reaching an obligatory host, usually the definitive host. A paratenic host harbors the sexually immature parasite, but it cannot develop further in this host. If a suitable definitive host ingests the paratenic host or a part of it containing the infective stage, the parasite can grow to maturity otherwise it remains stored in the host itself.
For example, lizards act as paratenic hosts for Spirocera lupi in dogs. The role of such a host is to fill up an ecological gap between the intermediate host and the definitive host.
Incidental or Accidental host
A host organism that shelters the parasite, but since it can’t progress the life cycle development, it is dead-end for it. For example, humans are dead-end hosts for the Japanese encephalitis virus (JEV), whose life cycle is normally between culicine mosquitoes and birds. People 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.
Host-parasite relationships or symbiotic relationships are those in which the organisms (host and parasite) live in close proximity to each other and are dependent on each other in one or another way for their survival. The nature and extent of the association will determine the type of relationship existing between the co-habiting organisms (also called symbionts).
Symbiotic relationships are usually of four main types namely parasitism, mutualism, commensalism, and phoresis.
Mutualism is a symbiotic relationship that is defined as an association between two living beings in such a way that both benefit from each other’s existence. This relationship can either be within the species or between the two different species.
For example, humans have a mutualistic relationship with the bacterium Escherichia coli, which is a normal flora of the large intestine. E.coli produces vitamins (K and B) and bacteriocins (a chemical that wards off harmful bacteria) and the large intestine provides shelter and nutrients for its growth and multiplication.
Mutualism can further be classified as
Obligate mutualism: This is the type of relationship where both symbionts entirely depend on each other for survival. A famous example of obligate mutualism is lichen. Green algae and a colorless fungus form obligatory symbiotic organisms called lichens. The alga supplies carbohydrates formed during photosynthesis to the fungus. In turn, the fungus gives water, mineral salts, and protection to the alga. Thus, both are benefited from symbiosis.
Facultative mutualism: In this relationship, mutualism benefits an organism, but the organism is not so dependent on mutualism that it cannot survive without it.
Parasitism is defined as a non-mutual symbiotic relationship in which one of the symbionts (the parasite), benefits at the expense of the host, while the host is harmed. The parasite lives on or in the body of the host.
Examples of parasitism include interactions between vertebrate hosts and diverse animals such as tapeworm, flukes, the plasmodium species, and fleas. Worms attach themselves to the insides of the intestines of animals such as cows, pigs, and humans. They get food by eating the host’s partly digested food, depriving the host of nutrients, affecting the absorption capacity of the host intestine. Some worms may also cause a loss of blood and iron, which could lead to anemia.
Commensalism is a type of symbiotic relationship where one partner benefits whereas the second partner (the host) are neither helped nor harmed. The organism that receives the refuge and nourishment is called the ‘commensal’. Most of the normal floras of the human body can be considered as commensals.
For example, Humans harbor several species of commensal protistans such as Entamoeba gingivalis which lives in the mouth where it feeds on bacteria, food particles, and dead epithelial cells but never harms healthy tissues.
When organisms live on the external surface of the body of their hosts, they are called ‘ectocommensals’. Such an association is called ‘ectocommensalism’ but if the commensal is living inside the tissues or cavities of animals, they are called ‘endocommensals’, and the association is called ‘endocommensalism’.
Amensalism is the type of relationship in which one species is inhibited or completely harmed and the other is not affected. For example, a sapling growing under the shadow of a mature tree. The mature tree usually robs the sapling of necessary sunlight and another nutrient (eg rainwater). The mature tree remains unaffected while the sapling dwindles and dies. The mature tree will even make use of nutrients arising from the decaying sapling.
Amensalism can be of two types depending upon the species involved and harm possessed
- Competition: A negative interaction between a large and a small species or similar species for same resources, where the stronger one excludes the smaller or weaker one from living space or deprives it of food. In this type of relationship, usually the smaller is harmed whereas the larger remains unaffected.
- Antibiosis: A type of interaction in which one of the species is killed by a chemical secretion while the other remains unaffected. For example: Penicillium (a fungus) secretes penicillin as a secondary metabolite during growth which kills various bacterial species.
- N. Ukibe Solomon, I. Mbanugo James , N. Obi-Okaro Alphonsus and R. Ukibe Nkiruka”A Review of Host-Parasite Relationships” Annual Research & Review in Biology 5(5): 372-384, 2015
- Apurba Sankar Sastry, Sandhya Bhat K “Essentials of Medical Parasitology” First Edition: 2014 ISBN: 978-93-5152-329-1
- Makarenko E.N., Erina N.V., Kopteva T.S., Nikolenko T.S. “Introducation to Medical Parasitology”. Stavropol State Medical University.