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1. “Resistance Is Futile”: Weaker Selection for Resistance by Abundant Parasites Increases Prevalence and Depresses Host Density

   https://doi.org/10.1086/724426  

   Walsman, Jason C.; Duffy, Meghan A.; Cáceres, Carla E.; Hall, Spencer R. (June 2023, The American Naturalist)

   Theory often predicts that host populations should evolve greater resistance when parasites become abundant. Furthermore, that evolutionary response could ameliorate declines in host populations during epidemics. Here, we argue for an update: when all host genotypes become sufficiently infected, higher parasite abundance can select for lower resistance because its cost exceeds its benefit. We illustrate such a “resistance is futile” outcome with mathematical and empirical approaches. First, we analyzed an eco-evolutionary model of parasites, hosts, and hosts’ resources. We determined eco-evolutionary outcomes for prevalence, host density, and resistance (mathematically, “transmission rate”) along ecological and trait gradients that alter parasite abundance. With high enough parasite abundance, hosts evolve lower resistance, amplifying infection prevalence and decreasing host density. In support of these results, a higher supply of nutrients drove larger epidemics of survival-reducing fungal parasites in a mesocosm experiment. In two-genotype treatments, zooplankton hosts evolved less resistance under high-nutrient conditions than under low-nutrient conditions. Less resistance, in turn, was associated with higher infection prevalence and lower host density. Finally, in an analysis of naturally occurring epidemics, we found a broad, bimodal distribution of epidemic sizes consistent with the resistance is futile prediction of the eco-evolutionary model. Together, the model and experiment, supplemented by the field pattern, support predictions that drivers of high parasite abundance can lead to the evolution of lower resistance. Hence, under certain conditions, the most fit strategy for individual hosts exacerbates prevalence and depresses host populations. 

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2. Spatio-temporal patterns of Perkinsus marinus infections are driven by a changing environment in the Chesapeake Bay

   https://doi.org/10.3354/dao03876  

   Kachmar, Mariah L; Bergman, Chelsea; Schreier, Harold J; Feild, Gemma; Pagenkopp_Lohan, Katrina M; Carnegie, Ryan B; Burge, Colleen A; Gignoux-Wolfsohn, Sarah (November 2025, Diseases of Aquatic Organisms)

   Shellfish fisheries and aquaculture within the Chesapeake Bay (hereafter 'the Bay') and its tributaries have been historically impacted by disease and climate events. Climate-driven shifts in temperature and salinity can alter host-parasite dynamics, influencing outbreaks. Here, we explore the relationship between temperature, salinity and parasite distribution and abundance in the eastern oysterCrassostrea virginica-Perkinsus marinussystem. We use long-term (30 yr) environmental data andP. marinussurveys in the Bay to identify (1) how climate affectsP. marinusprevalence and intensity, (2) seasonal and climate-driven infection patterns, and (3) regional environmental influences on disease. We found significant relationships betweenP. marinusinfection intensity, prevalence, increasing temperature and decreasing salinity. Our results indicated that there is an overall decreased abundance ofP. marinusprevalence and intensity throughout the Bay driven by decreases in salinity over time, most prominently from 2003-2020. However, these temporal trends in prevalence and intensity vary largely by region, with some regions still experiencing high disease burden. Examining monthly environmental parameters reinforced the dominant role of salinity in driving disease patterns. Salinity had significant relationships with prevalence and intensity year-round, with the largest effects in late spring/early summer. Monthly temperatures had fewer significant relationships to prevalence and intensity, but the largest significant effects were seen in late winter/early spring. Notably, this study is the first to document that winter salinity influences fall parasite prevalence, sometimes exerting a greater effect than temperature. Continued and expanded monitoring of marine disease is crucial to understand how the changing climate is impacting disease. 

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3. Causation without correlation: parasite-mediated frequency-dependent selection and infection prevalence

   https://doi.org/10.1098/rsbl.2021.0321  

   Lively, Curtis M.; Xu, Julie; Ben-Ami, Frida (December 2021, Biology Letters)

   Parasite-mediated selection is thought to maintain host genetic diversity for resistance. We might thus expect to find a strong positive correlation between host genetic diversity and infection prevalence across natural populations. Here, we used computer simulations to examine host–parasite coevolution in 20 simi-isolated clonal populations across a broad range of values for both parasite virulence and parasite fecundity. We found that the correlation between host genetic diversity and infection prevalence can be significantly positive for intermediate values of parasite virulence and fecundity. But the correlation can also be weak and statistically non-significant, even when parasite-mediated frequency-dependent selection is the sole force maintaining host diversity. Hence correlational analyses of field populations, while useful, might underestimate the role of parasites in maintaining host diversity. 

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4. Multiple transmission routes sustain high prevalence of a virulent parasite in a butterfly host

   https://doi.org/10.1098/rspb.2019.1630  

   Majewska, Ania A.; Sims, Stuart; Schneider, Anna; Altizer, Sonia; Hall, Richard J. (September 2019, Proceedings of the Royal Society B: Biological Sciences)

   Understanding factors that allow highly virulent parasites to reach high infection prevalence in host populations is important for managing infection risks to human and wildlife health. Multiple transmission routes have been proposed as one mechanism by which virulent pathogens can achieve high prevalence, underscoring the need to investigate this hypothesis through an integrated modelling-empirical framework. Here, we examine a harmful specialist protozoan infecting monarch butterflies that commonly reaches high prevalence (50–100%) in resident populations. We integrate field and modelling work to show that a combination of three empirically-supported transmission routes (vertical, adult transfer and environmental transmission) can produce and sustain high infection prevalence in this system. Although horizontal transmission is necessary for parasite invasion, most new infections post-establishment arise from vertical transmission. Our study predicts that multiple transmission routes, coupled with high parasite virulence, can reduce resident host abundance by up to 50%, suggesting that the protozoan could contribute to declines of North American monarchs. 

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5. Large-herbivore nemabiomes: patterns of parasite diversity and sharing

   https://doi.org/10.1098/rspb.2021.2702  

   Titcomb, Georgia C.; Pansu, Johan; Hutchinson, Matthew C.; Tombak, Kaia J.; Hansen, Christina B.; Baker, Christopher C.; Kartzinel, Tyler R.; Young, Hillary S.; Pringle, Robert M. (May 2022, Proceedings of the Royal Society B: Biological Sciences)

   Amidst global shifts in the distribution and abundance of wildlife and livestock, we have only a rudimentary understanding of ungulate parasite communities and parasite-sharing patterns. We used qPCR and DNA metabarcoding of fecal samples to characterize gastrointestinal nematode (Strongylida) community composition and sharing among 17 sympatric species of wild and domestic large mammalian herbivore in central Kenya. We tested a suite of hypothesis-driven predictions about the role of host traits and phylogenetic relatedness in describing parasite infections. Host species identity explained 27–53% of individual variation in parasite prevalence, richness, community composition and phylogenetic diversity. Host and parasite phylogenies were congruent, host gut morphology predicted parasite community composition and prevalence, and hosts with low evolutionary distinctiveness were centrally positioned in the parasite-sharing network. We found no evidence that host body size, social-group size or feeding height were correlated with parasite composition. Our results highlight the interwoven evolutionary and ecological histories of large herbivores and their gastrointestinal nematodes and suggest that host identity, phylogeny and gut architecture—a phylogenetically conserved trait related to parasite habitat—are the overriding influences on parasite communities. These findings have implications for wildlife management and conservation as wild herbivores are increasingly replaced by livestock. 

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