International Journal of Molecular Zoology 2024, Vol.14, No.4, 233-243 http://animalscipublisher.com/index.php/ijmz 234 2 Genetic Factors Influencing Susceptibility and Resistance 2.1 Genetic variation among species and populations Genetic variation in susceptibility to infections is a well-documented phenomenon across various species and populations. Studies have shown that natural selection by pathogens can increase genetic variation in host populations, particularly in those that have coevolved with their pathogens. For instance, research involving Drosophila species and their host-specific viruses demonstrated greater genetic variation in susceptibility to coevolved viruses compared to novel pathogens, suggesting that major-effect resistance polymorphisms play a significant role in this variation (Duxbury et al., 2018). Similarly, in dipteran insects like Drosophila melanogaster and vector mosquitoes, genetic and microbiota-dependent variations significantly influence antiviral immunity and virus susceptibility (Palmer et al., 2018). These findings underscore the importance of genetic diversity in shaping the susceptibility and resistance profiles of different species and populations. 2.2 Role of host genetics in immune response Host genetics play a crucial role in modulating immune responses to infections. Genetic variants in immune response genes, such as cytokines, have been linked to differences in susceptibility to parasitic and microbial infections. For example, polymorphisms in cytokine genes like TNF, LTα, and IFNβ1 have been associated with varying susceptibility to nematodes and microbial pathogens in bank voles, highlighting the role of non-coding variants in immune regulation (Figure 1) (Kloch et al., 2021). Additionally, studies on human populations have identified numerous genes associated with severe viral infections, including those involved in TLR pathways and inflammasome activation, which are critical for immune response modulation (Elhabyan et al., 2020). These genetic determinants are essential for understanding the mechanisms underlying differential immune responses and susceptibility to infections. The study of Kloch et al. (2021) shows the impact of different single nucleotide polymorphisms (SNPs) in LTα and IFNβ genes on the risk of infection and parasite burden in voles. The graphs suggest that specific genotypes at these SNP loci significantly influence the likelihood of infection and the intensity of parasitism. For instance, certain alleles appear to increase the risk of infection, as indicated by the higher percentage of infected individuals with those genotypes. Additionally, the intensity of infection varies among different genotypes, indicating that genetic variation plays a crucial role in host susceptibility to parasites. 2.3 Evolutionary adaptations to pathogens Evolutionary adaptations to pathogens are evident in the genetic architecture of resistance traits. Host-pathogen coevolution often results in the selection of major-effect genes that confer resistance to infections. In Drosophila, for example, resistance to viruses like the sigma virus and DCV is largely controlled by a few major-effect loci, supporting the idea that pathogen-driven selection can simplify the genetic architecture of resistance traits (Cogni et al., 2016). Furthermore, gene expression studies in threespine stickleback populations have revealed population-specific immune responses to parasite infections, indicating that recent evolutionary divergence can lead to distinct genetic adaptations that enhance resistance (Lohman et al., 2017). These adaptations are crucial for the survival and fitness of host populations in the face of ongoing pathogen pressures. In summary, genetic factors, including variation among species and populations, host genetics in immune response, and evolutionary adaptations, play pivotal roles in influencing susceptibility and resistance to infections in wildlife. Understanding these mechanisms provides valuable insights into the complex interplay between hosts and pathogens, which is essential for developing effective strategies for managing infectious diseases in natural populations. 3 Environmental Factors Affecting Infection Rates 3.1 Influence of habitat quality and availability Habitat quality and availability play a crucial role in determining the infection rates among wildlife populations. Changes in habitat, often driven by anthropogenic activities, can lead to shifts in species diversity and population densities, which in turn influence the prevalence of pathogens. For instance, the modification of continuous habitats into isolated patches has been linked to changes in species assemblages and increased prevalence of
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