Host-Parasite Interdependence

 

 Host parasite relationship is a strange inter-specific association where one organism called as parasite is benefitted and the other organism called as host is harmed, where host always wants to get rid of the parasite whereas the parasite never wants to leave the host.

 There are various degrees of interdependence of parasites on the host, which depend on the nature of  parasite and the degrees of parasitism i.e. an obligatory parasite depends on the host all throughout life  where as facultative parasites depend on host for a particular stage of life cycle. 

Among obligate parasites the degree of interdependence on the host also varies for example Staphylococci and Streptococci are obligatory parasites but they can reproduce in ex-situ environment i.e. outside the body of the host in nutrient media whereas Rickettsia, which have structure akin to bacteria show a small degree of metabolic activities outside the body of host but they can’t grow whereas the viruses which consists of  RNA/DNA and protein covering are dependent on host for their reproduction so much that they use the host’s replication machinery while outside the host they are non living entities.  

 Host-parasite relationship is subjected to continuous evolutionary changes and so does the interdependence of the parasite on the host. Adaptations to the parasitic existence include the progressive loss in metabolic capability with eventually complete physiological dependence of parasite on the host. 

 The parasites depend on the hosts for food, shelter, protection, reproduction and dispersal whereas the factors which govern the host-parasite relationship include, host specificity, host susceptibility, virulence of parasite and host resistance, apparent and inapparent infection. Most of the parasites have fixed hosts and they don’t infect other organisms for examples all the parasites of plant don’t infect animals and vice versa, similarly all the parasites of mammals don’t infect aves. Parasite which have wide specificity are more successful and widely spread as compared to the parasites which have got narrow range of specificity. 

Host susceptibility is a feature, which describes the degree of proneness of host to the attack of the parasites and hence the proneness to the infection. The more susceptible a host to a parasite, more is the likelihood of its getting infected by parasite. Every organism has some immunity against the parasites and susceptibility is basically a measure of the immune system of the host which depends on the nutrition, health condition and the age of the host. Parasite flourishes well on the more susceptible host i.e. whose immune system is weak. 

Résistance of the host to the parasite is what determines the existence of parasite, both too high and too low the resistance of the host are detrimental to the existence of the parasite as in case of too high resistance the parasite can’t establish hence dies and in case of too low resistance, the host cannot bear the parasite and hence dies, thereby leading to the death of the parasite. 

Similarly, the virulence which is the degree of the severity of harmfulness of parasite, also plays a crucial role in host parasite relationship, the more virulent a parasite, the more harmful it is for the host and its own existence as it kills the host and hence itself too.

 Therefore, a successful parasite is one that is less virulent, has wide specificity and a successful host is one which is less susceptible and more resistant to the parasite and thus for a parasitic association to exist all these parameters must be on middle path. 

 No successful parasite wants to kill its host and thus highly evolved parasites are those which establish  a cordial relation with its host, it must infect the host without causing the disease i.e. the infection should be inapparent rather than apparent as the inapparent infection serves to transmit the parasite to the susceptible host without affecting the host too much.    

Endoparasites have become so much dependent on the hosts that they have lost various organs and organ systems or various organs have become rudimentary owing to parasitic mode of life. However certain organs as are important for parasitic existance have undergone excessive developmen in these species.

 Parasites are usually regarded in terms of their detrimental effects on host but they can also have positive impacts on their hosts. 

Parasites act as ecosystem engineers as they alter energy budgets and nutrient cycling, and influence biodiversity. 

Scientists have challenged the conventional wisdom that parasites have only negative or inconsequential impacts on hosts but they have established the fact that there exists a potential bidirectional interdependence.