The relationship between parasite fitness and host condition in an insect--virus system

How do parasites and predators choose their victim? A trade-off between quality and vulnerability across antagonistic interactions

From blood-sucking lice and food-stealing gulls to pandemic-inducing viruses and egg-eating snakes: parasites and predators are ubiquitous in shaping ecology and evolution. Fundamental to these interactions is the way in which parasites and predators choose their victim. Here, I argue that a trade-off between host quality and vulnerability can be generalised across systems to understand parasites' choice of hosts. This principle defines quality as the value of resources a host has, and vulnerability as the ease with which a parasite can obtain those resources. A parasite can choose a low-quality host, which is easier to attack but offers limited resources, or a high-quality host, which is more challenging to attack but offers more resources if the parasite is successful. The optimal decision for a parasite will depend on its ecology and the shape of the trade-off in a given system. The trade-off applies equally to predator-prey systems. Many studies of different types of parasitism and predation across taxa have investigated traits pertaining to quality or vulnerability, but their findings have not previously been integrated. Doing so makes it possible to draw out broad principles that determine whether quality or vulnerability has the greater impact on victim choice. It can also help explain contradictory findings, such as why the same antagonists choose high-quality victims in some studies, and low-quality victims in others. Further applications include predicting the effects of global change on host-parasite and predator-prey dynamics, and providing an integrated perspective on coevolutionary adaptations.

Nutrition influences immunity: Diet and host-parasite interactions

Nutrition plays a major role in host immune responses and in pathogen resistance. Understanding the network that modulates the relationship between nutrition and immunity remains a challenge. Several pathways govern the direct effects of nutrition on host immunity and the indirect effects mediated by pathogen populations. We note host microbiota also influence the intricate relationships between nutrition and immunity. The purpose of this review is to discuss recent findings from nutritional research in relation to insect immunology. We outline the relationship between diet, immunity, disease, and microbiota in insects and emphasize the significance of utilizing an integrative, multifaceted approach to grasping the influence of nutrition on immunity.

Marine transmissible cancer navigates urbanized waters, threatening spillover

Inter-individual transmission of cancer cells represents a unique form of microparasites increasingly reported in marine bivalves. In this study, we sought to understand the ecology of the propagation of Mytilus trossulus Bivalve Transmissible Neoplasia 2 (MtrBTN2), a transmissible cancer affecting four Mytilus mussel species worldwide. We investigated the prevalence of MtrBTN2 in the mosaic hybrid zone of M. edulis and M. galloprovincialis along the French Atlantic coast, sampling contrasting natural and anthropogenic habitats. We observed a similar prevalence in both species, probably due to the spatial proximity of the two species in this region. Our results showed that ports had higher prevalence of MtrBTN2, with a possible hotspot observed at a shuttle landing dock. No cancer was found in natural beds except for two sites close to the hotspot, suggesting spillover. Ports may provide favourable conditions for the transmission of MtrBTN2, such as high mussel density, stressful conditions, sheltered and confined shores or buffered temperatures. Ships may also spread the disease through biofouling. Our results suggest ports may serve as epidemiological hubs, with maritime routes providing artificial gateways for MtrBTN2 propagation. This highlights the importance of preventing biofouling on docks and ship hulls to limit the spread of marine pathogens hosted by fouling species.

Efficient assays to quantify the life history traits of algal viruses

IMPORTANCE

Viruses play a crucial role in microbial ecosystems by liberating nutrients and regulating the growth of their hosts. These effects are governed by viral life history traits, i.e., by the traits determining viral reproduction and survival. Understanding these traits is essential to predicting viral effects, but measuring them is generally labor intensive. In this study, we present efficient methods to quantify the full life cycle of lytic viruses. We developed these methods for viruses infecting unicellular Chlorella algae but expect them to be applicable to other lytic viruses that can be quantified by flow cytometry. By making viral phenotypes accessible, our methods will support research into the diversity and ecological effects of microbial viruses.

Prevalence of Angiostrongylus cantonensis in definitive and intermediate hosts collected from agricultural areas in Ampayon, Butuan City, Southern Philippines

Angiostrongyliasis is a parasitic disease and a leading cause of human eosinophilic meningoencephalitis caused by rat lungworm Angiostrongylus cantonensis. This parasite infects a wide range of animal hosts, including snails and rats, which plays a significant role in zoonotic transmission. The study was conducted to determine the prevalence of A. cantonensis infection in freshwater snails and definitive rat hosts in the agricultural area in Ampayon, Butuan City, Philippines. A total of 54 rat samples and 719 snail individuals were collected in June and July 2020. An overall 2.36% prevalence rate of A. cantonensis snail infection was recorded, consisting of Pomacea canaliculata and Melanoides tuberculata, with a prevalence rate of 4.05% and 1.38%, respectively. Results revealed an overall prevalence of 38.9% in rat infection. Rattus tanezumi (48.48%) showed a higher infection than Rattus norvegicus (23.80%). Higher infection rates were found in rice field environments than residential houses, with 44.12% and 30% prevalence rates, respectively. Moreover, male rats showed higher infection rates (50%) than female rats (26.92%). Among age classes, adult rats had significantly higher infection rates (48.57%) than juvenile rats (21.05%). Correlation analysis showed a significant positive correlation between A. cantonensis infection intensity to the body length (r = 0.603; p = 0.001) and body weight (r = 0.715; p = 0.000) of rats. The study exemplifies the critical role of intermediate and definitive hosts for angiostrongyliasis. Infected freshwater snails and rats in rice fields make these agricultural areas a venue for A. cantonensis emergence. Integrated actions, health education campaigns, surveillance, hygiene, and good farming practices will help prevent the potential risk of the transmission of angiostrongyliasis in the area.

Resource limitation has a limited impact on the outcome of virus-fungus co-infection in an insect host

Infection by pathogens is strongly affected by the diet or condition of the prospective host. Studies that examine the impact of diet have mainly focused on single pathogens; however, co-infections within a single host are thought to be common. Different pathogen groups might respond differently to resource availability and diverse infections could increase the costs of host defense, meaning the outcome of mixed infections under varying dietary regimes is likely to be hard to predict. We used the generalist cabbage looper, Trichoplusia ni and two of its pathogens, the DNA virus T. ni nucleopolyhedrovirus (TniSNPV) and the entomopathogenic fungus, Beauveria bassiana to examine how nutrient reduction affected the outcome of mixed pathogen infection. We challenged insects with a low or high effective dose of virus, alone or combined with a single dose of fungus. We manipulated food availability after pathogen challenge by diluting artificial diet with cellulose, a non-nutritious bulking agent, and examined its impact on host and pathogen fitness. Reducing diet quantity did not alter overall or pathogen-specific mortality. In all cases, TniSNPV-induced mortality was negatively affected by fungus challenge. Similarly, B. bassiana-induced mortality was negatively affected by TniSNPV challenge, but only at the higher virus dose. Dietary dilution mainly affected B. bassiana speed of kill when mixed with a high dose of TniSNPV, with an increase in the duration of fungal infection when cellulose was low (high quantity). One pathogen dominated the production of transmission stages in the cadavers and co-infection did not affect the yield of either pathogen. There was no evidence that co-infections were more costly to the survivors of pathogen challenge. In conclusion, dietary dilution did not determine the outcome of mixed pathogen infection, but it had more subtle effects, that differed between the two pathogens and could potentially alter pathogen recycling and host-pathogen dynamics.

Small-mammal characteristics affect tick communities in southwestern Tennessee (USA)

Life histories can influence the degree of parasite infestations on a host. Pressures exerted on hosts based on age and sex convey varying degrees of parasite prevalence due to differences in host lifestyles, but it is not known how interactions between different host traits affect tick numbers. The objective of this study was to determine if host characteristics (e.g., age, sex, weight, and their interactions) affect the mean number of ticks found on small mammals regardless of host species or habitat. Sherman live traps were placed in forest and grass/forb habitats representative of the southeastern United States. After capture, host characteristics were recorded, and hosts were then searched for ticks. A total of 281 small mammals (148 Peromyscus leucopus, 34 P. maniculatus, 76 Sigmodon hispidus, 16 Microtus pinetorum, and 7 Ochrotomys nuttalli) and 610 ticks (488 Dermacentor variabilis, 114 Ixodes scapularis, 1 Amblyomma americanum, and 7 A. maculatum) were collected in this study. Host's age, sex, and weight affected the number of ticks collected from small mammals and significant interaction effects between host traits occurred (weight by sex, weight by age, and sex by age). For instance, female subadult rodents had significantly more ticks compared to female adults, male subadults had significantly fewer ticks compared to male adults, and the number of ticks on a host increased as host body mass increased. These results support the hypothesis that the number of ticks vary on rodent hosts based on life histories and trait interactions. Therefore, understanding the behavioral mechanisms of a host can aid in the management of parasites in the environment.

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Host life histories predict the number of ticks present on a rodent.

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Male and subadult rodents are infested with an overall greater number of ticks than females and adults.

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Rodents have more ticks as their weight increases.

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Interactions between sex and age predict the number of ticks present on a rodent.

Host food quality and quantity differentially affect Ascogregarina barretti parasite burden, development and within-host competition in the mosquito Aedes triseriatus

Host condition depends in large part on the quality and quantity of available food and heavily influences the outcome of parasite infection. Although parasite fitness traits such as growth rate and size may depend on host condition, whether host food quality or quantity is more important to parasite fitness and within-host interactions is poorly understood. We provided individual mosquito hosts with a standard dose of a gregarine parasite and reared mosquitoes on two food types of different quality and two quantities. We measured host size, total parasite count and area, and average size of parasites within each treatment. Food quality significantly influenced the number of parasites in a host; hosts fed a low-quality diet were infected with more parasites than those provided a high-quality diet. In addition, we found evidence of within-host competition; there was a negative relationship between parasite size and count though this relationship was dependent on host food quality. Host food quantity significantly affected total parasite area and parasite size; lower food quantity resulted in smaller parasites and reduced overall parasite area inside the host. Thus both food quality and quantity have the potential to influence parasite fitness and population dynamics.

On the diverse and opposing effects of nutrition on pathogen virulence

Climate change and anthropogenic activity are currently driving large changes in nutritional availability across ecosystems, with consequences for infectious disease. An increase in host nutrition could lead to more resources for hosts to expend on the immune system or for pathogens to exploit. In this paper, we report a meta-analysis of studies on host-pathogen systems across the tree of life, to examine the impact of host nutritional quality and quantity on pathogen virulence. We did not find broad support across studies for a one-way effect of nutrient availability on pathogen virulence. We thus discuss a hypothesis that there is a balance between the effect of host nutrition on the immune system and on pathogen resources, with the pivot point of the balance differing for vertebrate and invertebrate hosts. Our results suggest that variation in nutrition, caused by natural or anthropogenic factors, can have diverse effects on infectious disease outcomes across species.

Fish hosts, glochidia features and life cycle of the endemic freshwater pearl mussel Margaritifera dahurica from the Amur Basin

Margaritiferidae is a small freshwater bivalve family with 16 species. In spite of a small number of taxa and long-term history of research, several gaps in our knowledge on the freshwater pearl mussels still exist. Here we present the discovery of host fishes for Margaritifera dahurica, i.e. Lower Amur grayling, sharp-snouted lenok, and blunt-snouted lenok. The host fishes were studied in rivers of the Ussuri Basin. The identification of glochidia and fish hosts was confirmed by DNA analysis. The life cycle of M. dahurica and its glochidia are described for the first time. The SEM study of glochidia revealed that the rounded, unhooked Margaritifera dahurica larvae are similar to those of the other Margaritiferidae. Margaritifera dahurica is a tachytictic breeder, the larvae of which attach to fish gills during the Late August – September and finish the metamorphosis in June. Ancestral host reconstruction and a review of the salmonid - pearl mussel coevolution suggest that the ancestral host of the Margaritiferidae was a non-salmonid fish, while that of the genus Margaritifera most likely was an early salmonid species or their stem lineage. The overfishing of lenoks and graylings appears to be the most significant threat for this rare mussel species.

Viral exposure effects on life-history, flight-related traits, and wing melanisation in the Glanville fritillary butterfly

Infections represent a constant threat for organisms and can lead to substantial fitness losses. Understanding how individuals, especially from natural populations, respond towards infections is thus of great importance. Little is known about immunity in the Glanville fritillary butterfly (Melitaea cinxia). As the larvae live gregariously in family groups, vertical and horizontal transmission of infections could have tremendous effects on individuals and consequently impact population dynamics in nature. We used the Alphabaculovirus type strain Autographa californica multiple nucleopolyhedrovirus (AcMNPV) and demonstrated that positive concentration-dependent baculovirus exposure leads to prolonged developmental time and decreased survival during larval and pupal development, with no sex specific differences. Viral exposure did not influence relative thorax mass or wing morphometric traits often related to flight ability, yet melanisation of the wings increased with viral exposure, potentially influencing disease resistance or flight capacity via thermal regulation. Further research is needed to explore effects under sub-optimal conditions, determine effects on fitness-related traits, and investigate a potential adaptive response of increased melanisation in the wings due to baculovirus exposure.

Context-dependent medicinal effects of anabasine and infection-dependent toxicity in bumble bees

BACKGROUND

Floral phytochemicals are ubiquitous in nature, and can function both as antimicrobials and as insecticides. Although many phytochemicals act as toxins and deterrents to consumers, the same chemicals may counteract disease and be preferred by infected individuals. The roles of nectar and pollen phytochemicals in pollinator ecology and conservation are complex, with evidence for both toxicity and medicinal effects against parasites. However, it remains unclear how consistent the effects of phytochemicals are across different parasite lineages and environmental conditions, and whether pollinators actively self-medicate with these compounds when infected.

APPROACH

Here, we test effects of the nectar alkaloid anabasine, found in Nicotiana, on infection intensity, dietary preference, and survival and performance of bumble bees (Bombus impatiens). We examined variation in the effects of anabasine on infection with different lineages of the intestinal parasite Crithidia under pollen-fed and pollen-starved conditions.

RESULTS

We found that anabasine did not reduce infection intensity in individual bees infected with any of four Crithidia lineages that were tested in parallel, nor did anabasine reduce infection intensity in microcolonies of queenless workers. In addition, neither anabasine nor its isomer, nicotine, was preferred by infected bees in choice experiments, and infected bees consumed less anabasine than did uninfected bees under no-choice conditions. Furthermore, anabasine exacerbated the negative effects of infection on bee survival and microcolony performance. Anabasine reduced infection in only one experiment, in which bees were deprived of pollen and post-pupal contact with nestmates. In this experiment, anabasine had antiparasitic effects in bees from only two of four colonies, and infected bees exhibited reduced-rather than increased-phytochemical consumption relative to uninfected bees.

CONCLUSIONS

Variation in the effect of anabasine on infection suggests potential modulation of tritrophic interactions by both host genotype and environmental variables. Overall, our results demonstrate that Bombus impatiens prefer diets without nicotine and anabasine, and suggest that the medicinal effects and toxicity of anabasine may be context dependent. Future research should identify the specific environmental and genotypic factors that determine whether nectar phytochemicals have medicinal or deleterious effects on pollinators.

A Population Biology Perspective on the Stepwise Infection Process of the Bacterial Pathogen Pasteuria ramosa in Daphnia

The infection process of many diseases can be divided into series of steps, each one required to successfully complete the parasite's life and transmission cycle. This approach often reveals that the complex phenomenon of infection is composed of a series of more simple mechanisms. Here we demonstrate that a population biology approach, which takes into consideration the natural genetic and environmental variation at each step, can greatly aid our understanding of the evolutionary processes shaping disease traits. We focus in this review on the biology of the bacterial parasite Pasteuria ramosa and its aquatic crustacean host Daphnia, a model system for the evolutionary ecology of infectious disease. Our analysis reveals tremendous differences in the degree to which the environment, host genetics, parasite genetics and their interactions contribute to the expression of disease traits at each of seven different steps. This allows us to predict which steps may respond most readily to selection and which steps are evolutionarily constrained by an absence of variation. We show that the ability of Pasteuria to attach to the host's cuticle (attachment step) stands out as being strongly influenced by the interaction of host and parasite genotypes, but not by environmental factors, making it the prime candidate for coevolutionary interactions. Furthermore, the stepwise approach helps us understanding the evolution of resistance, virulence and host ranges. The population biological approach introduced here is a versatile tool that can be easily transferred to other systems of infectious disease.