Within-host priority effects and epidemic timing determine outbreak severity in co-infected populations

Order of arrival and nutrient supply alter outcomes of co-infection with two fungal pathogens

A pathogen arriving on a host typically encounters a diverse community of microbes that can shape priority effects, other within-host interactions and infection outcomes. In plants, environmental nutrients can drive trade-offs between host growth and defence and can mediate interactions between co-infecting pathogens. Nutrients may thus alter the outcome of pathogen priority effects for the host, but this possibility has received little experimental investigation. To disentangle the relationship between nutrient availability and co-infection dynamics, we factorially manipulated the nutrient availability and order of arrival of two foliar fungal pathogens (Rhizoctonia solani and Colletotrichum cereale) on the grass tall fescue (Lolium arundinaceum) and tracked disease outcomes. Nutrient addition did not influence infection rates, infection severity or plant biomass. Colletotrichum cereale facilitated R. solani, increasing its infection rate regardless of their order of inoculation. Additionally, simultaneous and C. cereale-first inoculations decreased plant growth and-in plants that did not receive nutrient addition-increased leaf nitrogen concentrations compared to uninoculated plants. These effects were partially, but not completely, explained by the duration and severity of pathogen infections. This study highlights the importance of understanding the intricate associations between the order of pathogen arrival, host nutrient availability and host defence to better predict infection outcomes.

Compatibility of the fungus Beauveria bassiana and Trichoplusia ni SNPV against the cabbage looper Trichoplusia ni: crop plant matters

BACKGROUND

Microbial insecticides are an important weapon in insect pest management, but their use is still relatively limited. One approach for increasing their efficacy and use could be to combine different pathogens to increase pest mortality. However, little is known about whether increasing pathogen diversity will improve pest management. Here, we investigated the compatibility of two pathogens for the management of the cabbage looper, Trichoplusia ni, T. ni nucleopolyhedrovirus (TniSNPV) and the entomopathogenic fungus Beauveria bassiana, on two crops, tomato and broccoli. The pathogens were applied to individual plants using ultra low volume sprays, alone or in combination, either synchronously or asynchronously. Healthy third-instar T. ni larvae were introduced to the plants before application and collected by destructive sampling 24 h after the last pathogen application.

RESULTS

Combined applications did not result in an increase in larval mortality compared to TniSNPV alone, although mortality was generally high. B. bassiana was considerably less effective on broccoli compared to tomato. In both the combined treatments, virus-induced mortality was approximately 50% lower when applied together with the fungus, while fungus-induced mortality was not affected by the virus, even when the virus was introduced 24 h before the fungus.

CONCLUSION

While our results suggest that applying this combination of entomopathogens would not be beneficial for pest management, this study illustrates the need to consider the target crop as an important driver of the efficacy of both single and mixed pathogen applications in the field. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Inconsistent dilution: experimental but not field evidence for a dilution effect in Daphnia-bacteria interactions

The dilution effect hypothesis, which suggests greater host biodiversity can reduce infectious disease transmission, occurs in many systems but is not universal. Most studies only investigate the dilution of a single parasite in a community, but many host communities have multiple parasites circulating. We studied a zooplankton host community with prior support for a dilution effect in laboratory- and field-based studies of a fungal parasite, Metschnikowia bicuspidata. We used paired experiments and field studies to ask whether dilution also occurred for a bacterial parasite, Pasteuria ramosa. We hypothesized that the similarities between the parasites might mean the dilution pattern seen in Metschnikowia would also be seen in Pasteuria. However, because Daphnia-Pasteuria interactions have strong host-parasite genotype specificity, dilution may be less likely if diluter host genotypes vary in their capacity to dilute Pasteuria. In a lab experiment, Pasteuria prevalence in susceptible Daphnia dentifera was reduced strongly by higher densities of D. pulicaria and marginally by higher densities of D. retrocurva. In a second experiment, different D. pulicaria genotypes had a similar capacity to dilute both Metschnikowia and Pasteuria, suggesting that Pasteuria's strong host-parasite genotype specificity should not prevent dilution. However, we found no evidence of an impact of the dilution effect on the size of Pasteuria epidemics in D. dentifera in Midwestern U.S. lakes. Our finding that a second parasite infecting the same host community does not show a similar dilution effect in the field suggests the impact of biodiversity can differ even among parasites in the same host community.

Age Structure Eliminates the Impact of Coinfection on Epidemic Dynamics in a Freshwater Zooplankton System

AbstractParasites often coinfect host populations and, by interacting within hosts, might change the trajectory of multiparasite epidemics. However, host-parasite interactions often change with host age, raising the possibility that within-host interactions between parasites might also change, influencing the spread of disease. We measured how heterospecific parasites interacted within zooplankton hosts and how host age changed these interactions. We then parameterized an epidemiological model to explore how age effects altered the impact of coinfection on epidemic dynamics. In our model, we found that in populations where epidemiologically relevant parameters did not change with age, the presence of a second parasite altered epidemic dynamics. In contrast, when parameters varied with host age (based on our empirical measures), there was no longer a difference in epidemic dynamics between singly infected and coinfected populations, indicating that variable age structure within a population eliminates the impact of coinfection on epidemic dynamics. Moreover, infection prevalence of both parasites was lower in populations where epidemiologically relevant parameters changed with age. Given that host population age structure changes over time and space, these results indicate that age effects are important for understanding epidemiological processes in coinfected systems and that studies focused on a single age group could yield inaccurate insights.

Coinfection with chytrid genotypes drives divergent infection dynamics reflecting regional distribution patterns

By altering the abundance, diversity, and distribution of species-and their pathogens-globalization may inadvertently select for more virulent pathogens. In Brazil's Atlantic Forest, a hotspot of amphibian biodiversity, the global amphibian trade has facilitated the co-occurrence of previously isolated enzootic and panzootic lineages of the pathogenic amphibian-chytrid (Batrachochytrium dendrobatidis, 'Bd') and generated new virulent recombinant genotypes ('hybrids'). Epidemiological data indicate that amphibian declines are most severe in hybrid zones, suggesting that coinfections are causing more severe infections or selecting for higher virulence. We investigated how coinfections involving these genotypes shapes virulence and transmission. Overall, coinfection favored the more virulent and competitively superior panzootic genotype, despite dampening its transmission potential and overall virulence. However, for the least virulent and least competitive genotype, coinfection increased both overall virulence and transmission. Thus, by integrating experimental and epidemiological data, our results provide mechanistic insight into how globalization can select for, and propel, the emergence of introduced hypervirulent lineages, such as the globally distributed panzootic lineage of Bd.

Marburg virus disease: the paradox of Nigeria's preparedness and priority effects in co-epidemics

BACKGROUND

The recent outbreaks of Marburg virus disease (MVD) in Guinea and Ghana have become a major public health concern not only to the West African sub-region but a threat to global health.

MAIN BODY OF THE ABSTRACT

Given the poorly elucidated ecological and epidemiological dynamics of the Marburg virus, it would be imprudent to preclude the possibility of another pandemic if urgent efforts are not put in place. However, the prior emergence and impact of COVID-19 and other co-occurring epidemics may add 'noise' to the epidemiological dynamics and public health interventions that may be required in the advent of a MVD outbreak in Nigeria.

SHORT CONCLUSION

Paying attention to the lessons learned from previous (and current) multiple epidemics including Avian Influenza, Yellow fever, Ebola virus disease, Monkeypox, Lassa fever, and COVID-19 could help avoid a potentially devastating public health catastrophe in Nigeria.

Specific sequence of arrival promotes coexistence via spatial niche pre-emption by the weak competitor

Historical contingency, such as the order of species arrival, can modify competitive outcomes via niche modification or pre-emption. However, how these mechanisms ultimately modify stabilising niche and average fitness differences remains largely unknown. By experimentally assembling two congeneric spider mite species feeding on tomato plants during two generations, we show that order of arrival affects species' competitive ability and changes the outcome of competition. Contrary to expectations, order of arrival did not cause positive frequency dependent priority effects. Instead, coexistence was predicted when the inferior competitor (Tetranychus urticae) arrived first. In that case, T. urticae colonised the preferred feeding stratum (leaves) of T. evansi leading to spatial niche pre-emption, which equalised fitness and reduced niche differences, driving community assembly to a close-to-neutrality scenario. Our study demonstrates how the order of species arrival and the spatial context of competitive interactions may jointly determine whether species can coexist.

Altered within- and between-host transmission under coinfection underpin parasite co-occurrence patterns in the wild

Interactions among parasite species coinfecting the same host individual can have far reaching consequences for parasite ecology and evolution. How these within-host interactions affect epidemics may depend on two non-exclusive mechanisms: parasite growth and reproduction within hosts, and parasite transmission between hosts. Yet, how these two mechanisms operate under coinfection, and how sensitive they are to the composition of the coinfecting parasite community, remains poorly understood. Here, we test the hypothesis that the relationship between within- and between-host transmission of the fungal pathogen, Phomopsis subordinaria, is affected by co-occurring parasites infecting the host plant, Plantago lanceolata. We conducted a field experiment manipulating the parasite community of transmission source plants, then tracked P. subordinaria within-host transmission, as well as between-host transmission to naïve recipient plants. We find that coinfection with the powdery mildew pathogen, Podosphaera plantaginis, causes increased between-host transmission of P. subordinaria by affecting the number of infected flower stalks in the source plants, resulting from altered auto-infection. In contrast, coinfection with viruses did not have an effect on either within- or between-host transmission. We then analyzed data on the occurrence of P. subordinaria in 2018 and the powdery mildew in a multi-year survey data set from natural host populations to test whether the positive association predicted by our experimental results is evident in field epidemiological data. Consistent with our experimental findings, we observed a positive association in the occurrence of P. subordinaria and historical powdery mildew persistence. Jointly, our experimental and epidemiological results suggest that within- and between-host transmission of P. subordinaria depends on the identity of coinfecting parasites, with potentially far-reaching effects on disease dynamics and parasite co-occurrence patterns in wild populations.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10682-022-10182-9.

A Microbial Mutualist Within Host Individuals Increases Parasite Transmission Between Host Individuals: Evidence From a Field Mesocosm Experiment

The interactions among host-associated microbes and parasites can have clear consequences for disease susceptibility and progression within host individuals. Yet, empirical evidence for how these interactions impact parasite transmission between host individuals remains scarce. We address this scarcity by using a field mesocosm experiment to investigate the interaction between a systemic fungal endophyte, Epichloë coenophiala, and a fungal parasite, Rhizoctonia solani, in leaves of a grass host, tall fescue (Lolium arundinaceum). Specifically, we investigated how this interaction impacted transmission of the parasite under field conditions in replicated experimental host populations. Epichloë-inoculated populations tended to have greater disease prevalence over time, though this difference had weak statistical support. More clearly, Epichloë-inoculated populations experienced higher peak parasite prevalences than Epichloë-free populations. Epichloë conferred a benefit in growth; Epichloë-inoculated populations had greater aboveground biomass than Epichloë-free populations. Using biomass as a proxy, host density was correlated with peak parasite prevalence, but Epichloë still increased peak parasite prevalence after controlling for the effect of biomass. Together, these results suggest that within-host microbial interactions can impact disease at the population level. Further, while Epichloë is clearly a mutualist of tall fescue, it may not be a defensive mutualist in relation to Rhizoctonia solani.

Bacterial and Viral Co-Infection in the Intestine: Competition Scenario and Their Effect on Host Immunity

Bacteria and viruses are both important pathogens causing intestinal infections, and studies on their pathogenic mechanisms tend to focus on one pathogen alone. However, bacterial and viral co-infections occur frequently in clinical settings, and infection by one pathogen can affect the severity of infection by another pathogen, either directly or indirectly. The presence of synergistic or antagonistic effects of two pathogens in co-infection can affect disease progression to varying degrees. The triad of bacterial–viral–gut interactions involves multiple aspects of inflammatory and immune signaling, neuroimmunity, nutritional immunity, and the gut microbiome. In this review, we discussed the different scenarios triggered by different orders of bacterial and viral infections in the gut and summarized the possible mechanisms of synergy or antagonism involved in their co-infection. We also explored the regulatory mechanisms of bacterial–viral co-infection at the host intestinal immune interface from multiple perspectives.

Untangling the complexity of priority effects in multispecies communities

The history of species immigration can dictate how species interact in local communities, thereby causing historical contingency in community assembly. Since immigration history is rarely known, these historical influences, or priority effects, pose a major challenge in predicting community assembly. Here, we provide a graph-based, non-parametric, theoretical framework for understanding the predictability of community assembly as affected by priority effects. To develop this framework, we first show that the diversity of possible priority effects increases super-exponentially with the number of species. We then point out that, despite this diversity, the consequences of priority effects for multispecies communities can be classified into four basic types, each of which reduces community predictability: alternative stable states, alternative transient paths, compositional cycles and the lack of escapes from compositional cycles to stable states. Using a neural network, we show that this classification of priority effects enables accurate explanation of community predictability, particularly when each species immigrates repeatedly. We also demonstrate the empirical utility of our theoretical framework by applying it to two experimentally derived assembly graphs of algal and ciliate communities. Based on these analyses, we discuss how the framework proposed here can help guide experimental investigation of the predictability of history-dependent community assembly.

Co-infecting pathogen lineages have additive effects on host bacterial communities

Amphibian skin bacteria may confer protection against the fungus Batrachochytrium dendrobatidis (Bd), but responses of skin bacteria to different Bd lineages are poorly understood. The global panzootic lineage (Bd-GPL) has caused amphibian declines and extinctions globally. However, other lineages are enzootic (Bd-Asia-2/Brazil). Increased contact rates between Bd-GPL and enzootic lineages via globalization pose unknown consequences for host-microbiome-pathogen dynamics. We conducted a laboratory experiment and used 16S rRNA amplicon-sequencing to assess: (i) whether two lineages (Bd-Asia-2/Brazil and Bd-GPL) and their recombinant, in single and mixed infections, differentially affect amphibian skin bacteria; (ii) and the changes associated with the transition to laboratory conditions. We determined no clear differences in bacterial diversity among Bd treatments, despite differences in infection intensity. However, we observed an additive effect of mixed infections on bacterial alpha diversity and a potentially antagonistic interaction between Bd genotypes. Additionally, observed changes in community composition suggest a higher ability of Bd-GPL to alter skin bacteria. Lastly, we observed a drastic reduction in bacterial diversity and a change in community structure in laboratory conditions. We provide evidence for complex interactions between Bd genotypes and amphibian skin bacteria during coinfections, and expand on the implications of experimental conditions in ecological studies.

Intraspecific competition for host resources in a parasite

Intraspecific competition among parasites should, in theory, increase virulence, but we lack clear evidence of this from nature.^1-3 Parasitic plants, which are sessile and acquire carbon-based resources through both autotrophy (photosynthesis) and heterotrophy (obtaining carbon from the host), provide a unique opportunity to experimentally study the role of intraspecific competition for nutrients in shaping the biology of both parasite and host.^4-6 Here, we manipulated the spatial position of naturally occurring individuals of desert mistletoe (Phoradendron californicum), a xylem hemiparasite, by removing parasites from co-infected branches of a common nitrogen-fixing host, velvet mesquite (Prosopsis velutina), in the Sonoran Desert. We measured physiological performance of both host and parasite individuals under differing competitive environments-parasite location along the xylem stream-through time. Performance was determined by measuring resource availability and use, given that resource demand changed with competitor removal and monsoon-driven amelioration of seasonal drought. Our principal finding was that intraspecific competition exists for xylem resources between mistletoe individuals, including host carbon. Host performance and seasonal climate variation altered the strength of competition and virulence. Hemiparasitic desert mistletoes demonstrated high heterotrophy, yet experimental removals revealed density- and location-dependent effects on the host through feedbacks that reduced mistletoe autotrophy and improved resource availability for the remaining mistletoe individual. Trophic flexibility tempered intraspecific competition for resources and reduced virulence. Mistletoe co-infections might therefore attenuate virulence to maintain access to resources in particularly stressful ecological environments. In summary, experimental field manipulations revealed evidence for intraspecific competition in a parasite species.

Facilitative priority effects drive parasite assembly under coinfection

Host individuals are often coinfected with diverse parasite assemblages, resulting in complex interactions among parasites within hosts. Within hosts, priority effects occur when the infection sequence alters the outcome of interactions among parasites. Yet, the role of host immunity in this process remains poorly understood. We hypothesized that the host response to the first infection could generate priority effects among parasites, altering the assembly of later-arriving strains during epidemics. We tested this by infecting sentinel host genotypes of Plantago lanceolata with strains of the fungal parasite Podosphaera plantaginis and measuring susceptibility to subsequent infection during experimental and natural epidemics. In these experiments, prior infection by one strain often increased susceptibility to other strains, and these facilitative priority effects altered the structure of parasite assemblages, but this effect depended on host genotype, host population and parasite genotype. Thus, host genotype, spatial structure and priority effects among strains all independently altered parasite assembly. Using a fine-scale survey and sampling of infections on wild hosts in several populations, we then identified a signal of facilitative priority effects, which altered parasite assembly during natural epidemics. Together, these results provide evidence that within-host priority effects of early-arriving strains can drive parasite assembly, with implications for how strain diversity is spatially and temporally distributed during epidemics.

Within-host priority effects and epidemic timing determine outbreak severity in co-infected populations

Co-infections of hosts by multiple pathogen species are ubiquitous, but predicting their impact on disease remains challenging. Interactions between co-infecting pathogens within hosts can alter pathogen transmission, with the impact on transmission typically dependent on the relative arrival order of pathogens within hosts (within-host priority effects). However, it is unclear how these within-host priority effects influence multi-pathogen epidemics, particularly when the arrival order of pathogens at the host-population scale varies. Here, we combined models and experiments with zooplankton and their naturally co-occurring fungal and bacterial pathogens to examine how within-host priority effects influence multi-pathogen epidemics. Epidemiological models parametrized with within-host priority effects measured at the single-host scale predicted that advancing the start date of bacterial epidemics relative to fungal epidemics would decrease the mean bacterial prevalence in a multi-pathogen setting, while models without within-host priority effects predicted the opposite effect. We tested these predictions with experimental multi-pathogen epidemics. Empirical dynamics matched predictions from the model including within-host priority effects, providing evidence that within-host priority effects influenced epidemic dynamics. Overall, within-host priority effects may be a key element of predicting multi-pathogen epidemic dynamics in the future, particularly as shifting disease phenology alters the order of infection within hosts.