Ashwini Ramesh - Infectious Disease Scientist

Multi-pathogenesis or coinfections poses a serious threat to health both at the individual and population scale. Even with its virulent costs to hosts, coinfections arise pervasively amongst diverse host-parasite systems. If virulent coinfection is so common: what factors facilitate it? Equally importantly, what prevents it?

To tackle this puzzle, I translate classic ideas of species coexistence into a pathogen realm using new mathematical theory and experimental tests to explain parasite diversity. My work advances how a niche framework of pathogen coinfection can create powerful new explanations both within and outside of host.

I. Within-host parasite diversity: On the cause and consequences of coinfection

Despite structural differences between food webs and within-host infection models, the key trait trade-offs determining the “minimum resource requirements" (R* rule), hence coexistence remain the same

Unifying predatory-prey and within host infection theory: Coinfection outbreak poses the largest threat to global health. Yet, despite two decades of empirical focus in biomedical literature, we still have limited insights of within-host mechanisms governing coinfection. By applying ecological theory, our work establishes the foundational theoretical principles of pathogen coinfection. Specifically, predator-prey foodweb theory provided excellent blueprint because they share basic consumer–resource structure with their within-host analogues: where, predator-like immune cells and their prey-like parasites feed on a shared resource. While predator-prey theory and withinhost infection models have been compared in the past, a quantitative comparison had not yet been realized. We used a feedback loop analysis facilitate this comparison for the first time. Surprisingly, despite structural differences between food webs and within-host infection models, the key trait trade-offs determining the “minimum resource requirements" (R* rule), hence coexistence remain the same. Overall, food web models offer powerful analogies to feedbacks underlying the dynamical repertoire of parasites within hosts.

Ramesh & Hall 2023. Ecology Letters: Perspective Niche theory for within-host parasite dynamics: Analogies to food web modules via feedback loops

A niche framework for withinhost coinfection: Using ecological theory, we demonstrate that coinfection, like coexistence, necessitates a competition-resistance tradeoff such that each parasite primarily affects the niche factor to which it is most sensitive. We also synthesized how those forms of competition underlie a comprehensive within-host framework that offers explanations for: Why does nutrient supplementation shift the relative abundances of competing parasites? How does the timing of parasite invasion aka priority effects allow the early invader to dominate, be suppressed, or have no effect on the late invader? Such insights will hopefully reorient the past decades of within-host experimentation and guide future experiments

Ramesh & Hall 2025. Ecology Letters: Synthesis. On the cause and consequences of coinfection: A general mechanistic framework of within-host parasite competition

"Community Assembly Rules of Coinfection" aka
niche theory framework for withinhost coinfection models

II. Between-host parasite diversity: Maintence of parasite diversity beyond the host

The maintenance of parasite diversity has historically taken a host-centric approach. Yet, most parasites spend at least some time outside of their host, exposed to the environment, thus raising a fundamental question: what mechanisms govern pathogen species coexistence outside of the host? To tackle this question, I applied classic and contemporary ecological theory, to test two mechanisms of coexistence in entomopathogenic nematodes (EPN) using year-long field surveys and laboratory experiments. We experimentally demonstrated how pathogen aggregation or spatial clumping facilitates coexistence of multiple EPN species across sites and seasons. By linking patterns to fitness, we showed that hosts with either too few or too many parasites limit their own population growth rate, leading to a stabilizing effect that promotes parasite species coexistence. Interestingly, if pathogens themselves cannot actively spatially aggregate, it’s vectors may facilitate such aggregation. My work has also shown how in the mosquito, Ae aegypti, social information via other breeding females can strongly modify individual reproductive decisions: supressing egg laying when sites are highly competitive to their progeny but facilitate egg laying under predation.

Ramesh et al. 2024, in review, Am. Nat Maintenance of parasite species diversity: Spatiotemporal niche partitioning and aggregation facilitate species coexistence

Ramesh et al 2025, Beh. Eco Soc., Featured Student Paper

Within a site, nematode aggregation occurs when one species occupies few patches, allowing the other to seek refuge in the unoccupied ones, facilitating coexistence. Aggregation results in non-linear consequences for parasite fitness, creating a mechanism for negative feedback on population growth.