Effects of parasite pressure on parasite mortality and reproductive output in a rodent-flea system: inferring host defense trade-offs

Ectoparasites enhance survival by suppressing host exploration and limiting dispersal

Parasites enhance their fitness by manipulating host dispersal. However, the strategies used by ectoparasites to influence host movement and the underlying mechanisms remain poorly understood. Here, we show that ectoparasites alter metabolic activity in specific brain regions of mice, with evidence pointing to a potential role for microglial activation in the prefrontal cortex. This activation appears to contribute to synaptic changes and altered neuronal differentiation, particularly in GABAergic neurons. Consequently, exploratory behavior decreases-an effect likely mediated through the skin-brain axis. In both indoor and field experiments with striped hamsters, ectoparasites reduce host exploration and modify their dispersal patterns. This behavioral shift ultimately restricts the host's distribution, enabling parasites to avoid environmental pressures. Our findings reveal that ectoparasites limit host dispersal to improve their own fitness, offering key insights for parasite control strategies that promote health and preserve ecological stability within the One Health framework.

Effects of Different Levels of Flea Infestation on Gut Microbiota of Brandt's Voles (Lasiopodomys brandtii) in China

Brandt's vole is a common small rodent, and its gut microbiota is critical to host health and immune function. The parasitic fleas commonly found in Brandt's voles cause an immune response, but their impact on the gut microbiota remains unclear. According to the level of flea infestation, Brandt's voles were divided into the control group, low-infestation group, and high-infestation group. The changes in the microbial community composition, abundance, and diversity of the gut microbiota were evaluated using 16S rRNA sequencing. Flea infestation significantly affected body weight, food intake, and gut microbiota structure. The low-infestation group exhibited the most pronounced changes in weight and food intake, while the high-infestation group showed the least. In the 4th week, 16S rRNA sequencing revealed an increase in alpha diversity and alterations in microbial composition. Beta-diversity analysis indicated significant differences in the intestinal microbiota between the experimental groups and the control group. By the 8th week, these differences had diminished, suggesting that the microbiota had stabilized or recovered over time. Overall, parasitic flea infestation significantly alters the diversity, structure, and characteristic microbial enrichment of the gut microbiota in Brandt's voles, potentially impacting host metabolism, immunity, and growth. While this study lasted 8 weeks, the long-term health effects of flea infestation may persist. Future research should elucidate the interaction mechanisms between parasites and hosts, define the time frames and mechanisms of these long-term impacts, and provide theoretical support for animal health management and disease control.

The dynamics between limited-term and lifelong coinfecting bacterial parasites in wild rodent hosts

Interactions between coinfecting parasites may take various forms, either direct or indirect, facilitative or competitive, and may be mediated by either bottom-up or top-down mechanisms. Although each form of interaction leads to different evolutionary and ecological outcomes, it is challenging to tease them apart throughout the infection period. To establish the first step towards a mechanistic understanding of the interactions between coinfecting limited-term bacterial parasites and lifelong bacterial parasites, we studied the coinfection of Bartonella sp. (limited-term) and Mycoplasma sp. (lifelong), which commonly co-occur in wild rodents. We infected Bartonella- and Mycoplasma-free rodents with each species, and simultaneously with both, and quantified the infection dynamics and host responses. Bartonella benefited from the interaction; its infection load decreased more slowly in coinfected rodents than in rodents infected with Bartonella alone. There were no indications for bottom-up effects, but coinfected rodents experienced various changes, depending on the infection stage, in their body mass, stress levels and activity pattern, which may further affect bacterial replication and transmission. Interestingly, the infection dynamics and changes in the average coinfected rodent traits were more similar to the chronic effects of Mycoplasma infection, whereas coinfection uniquely impaired the host's physiological and behavioral stability. These results suggest that parasites with distinct life history strategies may interact, and their interaction may be asymmetric, non-additive, multifaceted and dynamic through time. Because multiple, sometimes contrasting, forms of interactions are simultaneously at play and their relative importance alternates throughout the course of infection, the overall outcome may change under different ecological conditions.

Subtle short-term physiological costs of an experimental augmentation of fleas in wild Columbian ground squirrels

Parasites affect many aspects of host physiology and behavior, and thus are generally thought to negatively impact host fitness. However, changes in form of short-term parasite effects on host physiological markers have generally been overlooked in favor of fitness measures. Here, we studied flea (Oropsylla idahoensis and Oropsylla opisocroistis tuberculata) parasitism on a natural population of Columbian ground squirrels (Urocitellus columbianus) in Sheep River Provincial Park, AB, Canada. Fleas were experimentally added to adult female U. columbianus at physiologically demanding times, including birth, lactation and weaning of their young. The body mass of adult females, as well as their oxidative stress and immunity were recorded multiple times over the active season under flea-augmented and control conditions. We also measured the prevalence of an internal parasite (Trypanosoma otospermophili). Doubly labeled water (DLW) was intra-peritoneally injected at peak lactation to examine energy expenditure. Effects of parasites on oxidative stress were only observed after offspring were weaned. There was no direct effect of experimentally heightened flea prevalence on energy use. A short-term 24 h mass loss (-17 g) was detected briefly after parasite addition, likely due to U. columbianus preferentially allocating time for grooming. Our parasite augmentation did not strongly affect hosts and suggested that short-term physiological effects were unlikely to culminate in long-term fitness consequences. Columbian ground squirrels appear to rapidly manage parasite costs, probably through grooming.