Dose relationships can exacerbate, mute, or reverse the impact of heterospecific host density on infection prevalence

Dose-, stage- and sex- difference of prenatal prednisone exposure on placental morphological and functional development

Prednisone, a synthetic glucocorticoid, is commonly used to treat autoimmune diseases in pregnant women. However, some studies suggest that the use of prednisone during pregnancy may lead to adverse pregnancy outcomes. In this study, we established PPE mouse models at different doses (0.25, 0.5, 1.0 mg/kg·d) and different stages (whole pregnancy, early pregnancy and middle-late pregnancy) and determined outcomes on the placenta and fetus. The results of our study indicated that at the highest dose of 1 mg/kg PPE using a GD 0-18 dosing regime, PPE caused placental morphological changes measured as a decrease in placental weight relative to controls and a decrease in the placenta junctional zone (JZ)/labyrinth zone (LZ) ratio. No changes were observed on the fetuses for number of live, stillborn, and absorbed fetuses between the experimental groups and the control group. In the placentas at some doses, there were decreases in cell proliferation markers measured at the RNA and protein level by Western blot and increased apoptosis. Measures of gene expression at the mRNA level showed altered nutrients (including glucose, amino acid, and cholesterol) transport gene expressions with the most significant change associated with the male placentas at high-dose and whole pregnancy PPE group. It was further found that PPE led to the inhibition of the insulin-like growth factor 2 (IGF2)/insulin-like growth factor 1 receptor (IGF1R) signaling pathway, which was well correlated with the indicators of cell proliferation, syncytialization and nutrient (glucose and amino acid) transport indices. In conclusion, PPE can alter placental morphology and nutrient transport function, with differences in effect related to dose, stage and gender. Differential gene expressions measured for genes of the IGF2/IGF1R signaling pathway suggested this pathway may be involved in the effects seen with PPE. This study provides a theoretical and experimental basis for enhancing the understanding of the effects of prednisone use on placenta during human pregnancy but does not currently raise concerns for human use as effects were not seen on the fetuses and while the effects on cell proliferation are informative they were inconsistent and the differential effects on female and male placentas unexplained suggesting that further work is required to elucidate if these findings have relevance for human use of PPE during pregnancy.

Virulence and transmission biology of the widespread, ecologically important pathogen of zooplankton, Spirobacillus cienkowskii

Spirobacillus cienkowskii (Spirobacillus, hereafter) is a widely distributed bacterial pathogen that has significant impacts on the population dynamics of zooplankton (Daphnia spp.), particularly in months when Daphnia are asexually reproducing. However, little is known about Spirobacillus' virulence, transmission mode, and dynamics. As a result, we cannot explain the dynamics of Spirobacillus epidemics in nature or use Spirobacillus as a model pathogen, despite Daphnia's tractability as a model host. Here, we work to fill these knowledge gaps experimentally. We found that Spirobacillus is among the most virulent of Daphnia pathogens, killing its host within a week and reducing host fecundity. We further found that Spirobacillus did not transmit horizontally among hosts unless the host died or was destroyed (i.e., it is an "obligate killer"). In experiments aimed at quantifying the dynamics of horizontal transmission among asexually reproducing Daphnia, we demonstrated that Spirobacillus transmits poorly in the laboratory. In mesocosms, Spirobacillus failed to generate epidemics; in experiments wherein individual Daphnia were exposed, Spirobacillus' transmission success was low. In the (limited) set of conditions we considered, Spirobacillus' transmission success did not change with host density or pathogen dose and declined following environmental incubation. Finally, we conducted a field survey of Spirobacillus' prevalence within egg cases (ephippia) made by sexually reproducing Daphnia. We found Spirobacillus DNA in ~40% of ephippia, suggesting that, in addition to transmitting horizontally among asexually reproducing Daphnia, Spirobacillus may transmit vertically from sexually reproducing Daphnia. Our work fills critical gaps in the biology of Spirobacillus and illuminates new hypotheses vis-à-vis its life history.

IMPORTANCE

Spirobacillus cienkowskii is a bacterial pathogen of zooplankton, first described in the 19th century and recently placed in a new family of bacteria, the Silvanigrellaceae. Spirobacillus causes large epidemics in lake zooplankton populations and increases the probability that zooplankton will be eaten by predators. However, little is known about how Spirobacillus transmits among hosts, to what extent it reduces host survival and reproduction (i.e., how virulent it is), and what role virulence plays in Spirobacillus' life cycle. Here, we experimentally quantified Spirobacillus' virulence and showed that Spirobacillus must kill its host to transmit horizontally. We also found evidence that Spirobacillus may transmit vertically via Daphnia's seed-like egg sacks. Our work will help scientists to (i) understand Spirobacillus epidemics, (ii) use Spirobacillus as a model pathogen for the study of host-parasite interactions, and (iii) better understand the unusual group of bacteria to which Spirobacillus belongs.

Is there a sicker sex? Dose relationships modify male-female differences in infection prevalence

Throughout the animal kingdom, there are striking differences in the propensity of one sex or the other to become infected. However, precisely when we should expect males or females to be the sicker sex remains unclear. A major barrier to answering this question is that very few studies have considered how the susceptibility of males and females changes across the full range of pathogen doses encountered in nature. Without quantifying this 'dose-susceptibility' relationship, we have likely underestimated the scope for sex differences to arise. Here, we use the Daphnia magnia-Pasteuria ramosa system to reveal that sex differences in susceptibility are entirely dose-dependent, with pathogens having a higher probability of successfully establishing an infection in mature males at low doses, but mature females at high doses. The scope for male-female differences to emerge is therefore much greater than previously appreciated-extending to sex differences in the upper limits to infection success, per-propagule infectivity risks and density-dependent pathogen behaviour. Applying this expanded scope across the animal kingdom will help us understand when and why a sicker sex emerges, and the implications for diseases in nature-where sex ratios, age structure and pathogen densities vary drastically.

Effects of azithromycin exposure during pregnancy at different stages, doses and courses on testicular development in fetal mice

Azithromycin is a commonly used antibiotic during pregnancy, but some studies have suggested its potential developmental toxicity. Currently, the effects and mechanisms of prenatal azithromycin exposure (PAzE) on fetal testicular development are still unclear. The effects of prenatal exposure to the same drug on fetal testicular development could vary depending on different stages, doses, and courses. Hence, in this study, based on clinical medication characteristics, Kunming mice was administered intragastrically with azithromycin at different stages (mid-/late-pregnancy), doses (50, 100, 200 mg/kg·d), and courses (single-/multi-course). Fetal blood and testicular samples were collected on GD18 for relevant assessments. The results indicated that PAzE led to changes in fetal testicular morphology, reduced cell proliferation, increased apoptosis, and decreased expression of markers related to Leydig cells (Star), Sertoli cells (Wt1), and spermatogonia (Plzf). Further investigation revealed that the effects of PAzE on fetal testicular development were characterized by mid-pregnancy, high dose (clinical dose), and single course having more pronounced effects. Additionally, the TGFβ/Smad and Nrf2 signaling pathways may be involved in the changes in fetal testicular development induced by PAzE. In summary, this study confirmed that PAzE influences fetal testicular morphological development and multicellular function. It provided theoretical and experimental evidence for guiding the rational use of azithromycin during pregnancy and further exploring the mechanisms underlying its developmental toxicity on fetal testicles.

Capturing complex interactions in disease ecology with simplicial sets

Network approaches have revolutionized the study of ecological interactions. Social, movement and ecological networks have all been integral to studying infectious disease ecology. However, conventional (dyadic) network approaches are limited in their ability to capture higher-order interactions. We present simplicial sets as a tool that addresses this limitation. First, we explain what simplicial sets are. Second, we explain why their use would be beneficial in different subject areas. Third, we detail where these areas are: social, transmission, movement/spatial and ecological networks and when using them would help most in each context. To demonstrate their application, we develop a novel approach to identify how pathogens persist within a host population. Fourth, we provide an overview of how to use simplicial sets, highlighting specific metrics, generative models and software. Finally, we synthesize key research questions simplicial sets will help us answer and draw attention to methodological developments that will facilitate this.

Pathogen transport amplifies or dilutes disease transmission depending on the host dose-response relationship

Pathogen transport by biotic or abiotic processes (e.g. mechanical vectors, wind, rain) can increase disease transmission by creating more opportunities for host exposure. But transport without replication has an inherent trade-off, that creating new venues for exposure decreases the average pathogen abundance at each venue. The host dose-response relationship is therefore required to correctly assess infection risk. We model and analyse two examples-biotic mechanical vectors in plant-pollinator networks, and abiotic-facilitated long-distance pathogen dispersal-to illustrate how oversimplifying the dose-response relationship can lead to incorrect epidemiological predictions. When the minimum infective dose is high, mechanical vectors amplify disease transmission less than suggested by simple compartment models, and may even dilute transmission. When long-distance dispersal leads to infrequent large exposures, models that assume a linear force of infection can substantially under-predict the speed of epidemic spread. Our work highlights an important general interplay between dose-response relationships and pathogen transport.

Mechanisms by which predators mediate host-parasite interactions in aquatic systems

It is often assumed that predators reduce disease prevalence and transmission by lowering prey population density and/or by selectively feeding on infected individuals. However, recent studies, many of which come from aquatic systems, suggest numerous alternative mechanisms by which predators can influence disease dynamics in their prey. Here, we review the mechanisms by which predators can mediate host-parasite interactions in aquatic prey. We highlight how life histories of aquatic hosts and parasites influence transmission pathways and describe how such pathways intersect with predation to shape disease dynamics. We also provide recommendations for future studies; experiments that account for multiple effects of predators on host-parasite interactions, and that examine how predator-host-parasite interactions shift under changing environmental conditions, are particularly needed.

Dose relationships can exacerbate, mute, or reverse the impact of heterospecific host density on infection prevalence

The likelihood an individual becomes infected depends on the community in which it is embedded. For environmentally transmitted parasites, host community composition can alter host density, the density of parasites that hosts encounter in the environment, and the dose to which hosts are subsequently exposed. While some multi-host theory incorporates some of these factors (e.g., competition among hosts), it does not currently consider the nonlinear relationships between parasite exposure dose and per-propagule infectivity (dose-infectivity relationships), between exposure dose and infected host mortality (dose-mortality relationships), and between exposure dose and parasite propagule excretion (dose-excretion relationships). This makes it difficult to predict the impact of host species on one another's likelihood of infection. To understand the implications of these nonlinear dose relationships for multi-host communities, we first performed a meta-analysis on published dose-infectivity experiments to quantify the proportion of accelerating, linear, or decelerating dose-infectivity relationships; we found that most experiments demonstrated decelerating dose-infectivity relationships. We then explored how dose-infectivity, dose-mortality, and dose-excretion relationships might alter the impact of heterospecific host density on infectious propagule density, infection prevalence, and density of a focal host using two-host, one-parasite models. We found that dose relationships either decreased the magnitude of the impact of heterospecific host density on propagule density and infection prevalence via negative feedback loops (decelerating dose-infectivity relationships, positive dose-mortality relationships, and negative dose-excretion relationships), or increased the magnitude of the impact of heterospecific host density on infection prevalence via positive feedback loops (accelerating dose-infectivity relationships and positive dose-excretion relationships). Further, positive dose-mortality relationships resulted in hosts that traditionally decrease disease (e.g., low competence, strong competitors) increasing infection prevalence, and vice versa. Finally, we found that dose relationships can create positive feedback loops that facilitate friendly competition (i.e., increased heterospecific density has a positive effect on focal host density because the reduction in disease outweighs the negative effects of interspecific competition). This suggests that without taking dose relationships into account, we may incorrectly predict the effect of heterospecific host interactions, and thus host community composition, on environmentally transmitted parasites.