hi dr.

see this link about parasitism 

https://www.cdc.gov/parasites/ 

Hi colleague

Many factors can interact some were determine while the other  are no, so these factors like adaptation, immune, genetic...

I hope this below article is benefit

Parasites are generally not evenly or normally distributed in a host population. Rather, a few animals normally have numerous parasites while most others have very few.  A number of host factors that differ among the members of the population that contribute to this pattern, including age, size, other infections, environmental, physiological, and social stressors, alleles that affect immune status, etc.

Hello Dear

The following file may be important for you in this field

INTRODUCTION TO MEDICAL PARASITOLOGY

While some parasites are generalists, many others are specialists,

able to infect only one or a few host species. However, specialization may

be more finely bound than this: it also occurs within species,

as evidenced by local adaptation of parasites to their sympatric host populations, and within populations, as shown by parasite strains that are specialised to particular local host genotypes. The cause of specialization, or conversely, what prevents a parasite from expanding its host range, is of considerable interest. For example, parasitism

may be the principle determinant of rates of genetic recombination, but the impact of parasitism on reproductive systems crucially depends on the nature or degree

of specialization. see this pdf :

You might want to look at parasite-host coevolution and the influence of geographic location on this. 

P. Wenk , A. Renz: The symbiontic  insurance model of the  parasitism 03.06.2017

Introduction: In human and veterinary parasitology, the parasite-host interaction is viewed like the Cold War. In biology, however, it should be considered as a cooperative partnership.

Definition: Parasites withdraw energy from their hosts for their survival, recognized by their pathogenicity/lethality and/or by the reduction of their host’s fitness. The propagation of any parasite is linked to its transmission, i.e. to the change of the host, the frequency of which will be modulated by the contact rate.

Area of application: The partners are taxonomically interspecific, the parasite’s participation is obligatory, and the host’s participation is facultative. A predator-prey relationship is excluded.

Hypothesis: Parasites withdraw energy from their hosts without having to provide any compensation. For permanent coexistence during evolution, a balance between attack and defence is postulated, leading to a dynamic stalemate in an arms race.

Critique: An arms race is economically wasteful and ecologically unstable, two disadvantages threatening the existence of both partners. Evolution supports advances only. The hypothesis of attack and defence should therefore be abandoned. The interpretation of the relevant causal processes contradicts Darwinian evolution.

Antithesis: The contact rate at transmission keeps the population density of the hosts away from their fatality thresholds from which there is no return (Seilacher et al, 2007 [3]). By reason of the facultative occurrence of parasitism, the host population density is levelled, and thereby also that of the parasite’s density, to the actual living conditions and thus the ecosystem will be dynamically balanced within economic ranges (ecostasis) (Hudson et al ).

Mechanism: The coincidental passing of the genetically determined infectiosity of a parasite together with the host’s likewise genetically determined susceptibility constitutes the compatibility of their partnership.

Conclusion: The contact rate modulates the amplitude of the population density and keeps the host and also the involved parasite away from the fatality thresholds (Seilacher et al. 2007). Thus, the compatible partners will be assured against the risk of extinction in the case of extremely changed living conditions, e.g. between geological epochs (Wenk und Renz 2014, 2017).

The metaphor is perfect: The distance to the fatality thresholds, simulating a precaution, resembles an insurance contract. The tolerable costs (pathology, lethality and fitness reduction) are the premium, the infectiosity and susceptibility are the content of the contract, and the compatibility of both factors is its closure. A fundamental change in living conditions corresponds to the insurance case. For the biological interpretation of the causal processes, we propose the term symbiotic insurance model (Wenk und Renz 2014). 

Commentary: All recently existing organisms have facultative parasites. Because they do not occur obligatorily, they appear as vermin and sickness in Medicine. In fact, symbiosis including parasitism  constitutes the fourth column of evolution after mutation, selection and abiotic events. The evolution of parasites has been overlooked for so long, because they almost always do not form fossils.

However, in host animals capable of an immune response linked with memory, different strategies of survival have to be distinguished. Prokaryotic parasites coexist in their host population according to the alternative strategy based on death or acquired protective immunity, as a rule. The population density is regulated over the course of generations (Wenk 1977). Eukaryotic parasites coexist according to a simultaneous strategy, which is also called balancing strategy. Thus, parasite and host propagate together, physiologically interacting as far as the withdrawal of energy reduces the host’s fitness tolerably. Accordingly, the parasite controls its multiplication and propagation inside the host by reason of its own genetic determination. Pathogenicity and lethality are not relevant. The population density of the host is regulated during the individual life time of the host (Wenk und Renz 2003, 2012, 2014).

Any invertebrate that is involved in the parasite’s life cycle as an intermediate host or vector responds with an internal defence system without memory [7]. This demands an almost frightening loss of the invading parasite by a kind of hurdle-run, as a rule. Although the rate of the parasite’s success is small in the case of a few surviving invaders, the parasite has reached its compatible partner. The hurdle-run simulates a recognition survey. Thereafter, the losses are compensated by the multiplication of the parasite. The turnover of the main natural population of the invertebrate host guarantees its adapted density. The section involved as a vector or intermediate host is negligible and, in respect of the population dynamics, has no relevance. The natural population density of invertebrates will be regulated, if necessary, in a sequence of epidemic and endemic periods. Ignoring the principles explained here means committing a systematic mistake. Nothing in biology makes sense except in the light of evolution (Theodosius Dobzhansky 1973).

References may be ordered by E-mail:  Dr,peter,[email protected]