An enzootic vector-borne virus is amplified at epizootic levels by an invasive avian host

Mixed-species assemblages and disease: the importance of differential vector and parasite attraction in transmission dynamics

Individuals from multiple species often aggregate at resources, group to facilitate defense and foraging, or are brought together by human activity. While it is well-documented that host-seeking disease vectors and parasites show biases in their responses to cues from different hosts, the influence of mixed-species assemblages on disease dynamics has received limited attention. Here, we synthesize relevant research in host-specific vector and parasite bias. To better understand how vector and parasite biases influence infection, we provide a conceptual framework describing cue-oriented vector and parasite host-seeking behaviour as a two-stage process that encompasses attraction of these enemies to the assemblage and their choice of hosts once at the assemblage. We illustrate this framework, developing a case study of mixed-species frog assemblages, where frog-biting midges transmit trypanosomes. Finally, we present a mathematical model that investigates how host species composition and asymmetries in vector attraction modulate transmission dynamics in mixed-species assemblages. We argue that differential attraction of vectors by hosts can have important consequences for disease transmission within mixed-species assemblages, with implications for wildlife conservation and zoonotic disease. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Differential immune gene expression associated with contemporary range expansion in two invasive rodents in Senegal

Biological invasions are major anthropogenic changes associated with threats to biodiversity and health. However, what determines the successful establishment and spread of introduced populations remains unclear. Here, we explore several hypotheses linking invasion success and immune phenotype traits, including those based on the evolution of increased competitive ability concept. We compared gene expression profiles between anciently and recently established populations of two major invading species, the house mouse Mus musculus domesticus and the black rat Rattus rattus, in Senegal (West Africa). Transcriptome analyses identified differential expression between anciently and recently established populations for 364 mouse genes and 83 rat genes. All immune-related genes displaying differential expression along the mouse invasion route were overexpressed at three of the four recently invaded sites studied. Complement activation pathway genes were overrepresented among these genes. By contrast, no particular immunological process was found to be overrepresented among the differentially expressed genes of black rat. Changes in transcriptome profiles were thus observed along invasion routes, but with different specific patterns between the two invasive species. These changes may be driven by increases in infection risks at sites recently invaded by the house mouse, and by stochastic events associated with colonization history for the black rat. These results constitute a first step toward the identification of immune eco-evolutionary processes potentially involved in the invasion success of these two rodent species.

Endemic Infection of Batrachochytrium dendrobatidis in Costa Rica: Implications for Amphibian Conservation at Regional and Species Level

Batrachochytrium dendrobatidis (Bd) has been associated with the severe declines and extinctions of amphibians in Costa Rica that primarily occurred during the 1980s and 1990s. However, the current impact of Bd infection on amphibian species in Costa Rica is unknown. We aimed to update the list of amphibian species in Costa Rica and evaluate the prevalence and infection intensity of Bd infection across the country to aid in the development of effective conservation strategies for amphibians. We reviewed taxonomic lists and included new species descriptions and records for a total of 215 amphibian species in Costa Rica. We also sampled for Bd at nine localities from 2015–2018 and combined these data with additional Bd occurrence data from multiple studies conducted in amphibian communities across Costa Rica from 2005–2018. With this combined dataset, we found that Bd was common (overall infection rate of 23%) across regions and elevations, but infection intensity was below theoretical thresholds associated with mortality. Bd was also more prevalent in Caribbean lowlands and in terrestrial amphibians with an aquatic larval stage; meanwhile, infection load was the highest in direct-developing species (forest and stream-dwellers). Our findings can be used to prioritize regions and taxonomic groups for conservation strategies.

Parasites favour intermediate nestling mass and brood size in cliff swallows

A challenge of life-history theory is to explain why animal body size does not continue to increase, given various advantages of larger size. In birds, body size of nestlings and the number of nestlings produced (brood size) have occasionally been shown to be constrained by higher predation on larger nestlings and those from larger broods. Parasites also are known to have strong effects on life-history traits in birds, but whether parasitism can be a driver for stabilizing selection on nestling body size or brood size is unknown. We studied patterns of first-year survival in cliff swallows (Petrochelidon pyrrhonota) in western Nebraska in relation to brood size and nestling body mass in nests under natural conditions and in those in which hematophagous ectoparasites had been removed by fumigation. Birds from parasitized nests showed highest first-year survival at the most common, intermediate brood-size and nestling-mass categories, but cliff swallows from nonparasitized nests had highest survival at the heaviest nestling masses and no relationship with brood size. A survival analysis suggested stabilizing selection on brood size and nestling mass in the presence (but not in the absence) of parasites. Parasites apparently favour intermediate offspring size and number in cliff swallows and produce the observed distributions of these traits, although the mechanisms are unclear. Our results emphasize the importance of parasites in life-history evolution.

No Detectable Insecticide Resistance in Swallow Bugs (Hemiptera: Cimicidae) Following Long-Term Exposure to Naled (Dibrom 8)

The swallow bug, Oeciacus vicarius Horvath, is a hematophagous ectoparasite of the cliff swallow, Petrochelidon pyrrhonota Vieillot, and is closely related to bed bugs (Cimex spp.). Evolution of insecticide resistance has been documented for bed bugs but not studied in Oeciacus. For periods of 17 and 32 yr, two cliff swallow colonies in western Nebraska were treated during the summer breeding season using the organophosphate insecticide Dibrom. Despite continual treatments, O. vicarius has been observed frequently within these colonies. We evaluated the efficacy of Dibrom 8 on O. vicarius during the 2016 season at two treated colonies and four that had never experienced treatment. Dibrom 8 was found to be effective in 100% of trials, with immobilization within minutes and death within 72 h, for individuals from all colonies. In control treatments (water), individuals collected from treated colonies exhibited greater survival than individuals from untreated colonies, and those from active colonies (bugs fed) had greater survival than those from inactive colonies (bugs unfed). A residual effect was observed in both lab and field trials: 100% mortality occurred in the lab after exposure to filter paper substrates treated both 5 and 10 d earlier, and in the field, nests treated once early in the season had O. vicarius counts 43 d later that were <1% of those from untreated nests within the same colony. We hypothesize that the lack of resistance results from the limited potential for resistance allele fixation due to outbreeding and frequent immigration of insecticide-naïve individuals.

Overlooking the smallest matter: viruses impact biological invasions

Parasites and pathogens have recently received considerable attention for their ability to affect biological invasions, however, researchers have largely overlooked the distinct role of viruses afforded by their unique ability to rapidly mutate and adapt to new hosts. With high mutation and genomic substitution rates, RNA and single-stranded DNA (ssDNA) viruses may be important constituents of invaded ecosystems, and could potentially behave quite differently from other pathogens. We review evidence suggesting that rapidly evolving viruses impact invasion dynamics in three key ways: (1) Rapidly evolving viruses may prevent exotic species from establishing self-sustaining populations. (2) Viruses can cause population collapses of exotic species in the introduced range. (3) Viruses can alter the consequences of biological invasions by causing population collapses and extinctions of native species. The ubiquity and frequent host shifting of viruses make their ability to influence invasion events likely. Eludicating the viral ecology of biological invasions will lead to an improved understanding of the causes and consequences of invasions, particularly as regards establishment success and changes to community structure that cannot be explained by direct interspecific interactions among native and exotic species.

Eco-immunology and bioinvasion: revisiting the evolution of increased competitive ability hypotheses

Immunity is at the core of major theories related to invasion biology. Among them, the evolution of increased competitive ability (EICA) and EICA‐refined hypotheses have been used as a reference work. They postulate that the release from pathogens often experienced during invasion should favour a reallocation of resources from (costly) immune defences to beneficial life‐history traits associated with invasive potential. We review studies documenting immune changes during animal invasions. We describe the designs and approaches that have been applied and discuss some reasons that prevent drawing generalized conclusions regarding EICA hypotheses. We detail why a better assessment of invasion history and immune costs, including immunopathologies and parasite communities, could improve our understanding of the relationships between immunity and invasion success. Finally, we propose new perspectives to revisit the EICA hypotheses. We first emphasize the neutral and adaptive mechanisms involved in immune changes, as well as timing of the later. Such investigation will help decipher whether immune changes are a consequence of pre‐adaptation, or the result of postintroduction adaptations to invasion front conditions. We next bring attention to new avenues of research that remain unexplored, namely age‐dependent immunity and gut microbiota, potential key factors underlying adaptation to invasion front environment and modulating invasion success.

Predation by ants controls swallow bug (Hemiptera: Cimicidae: Oeciacus vicarius) infestations

The swallow bug (Oeciacus vicarius) is the only known vector for Buggy Creek virus (BCRV), an alphavirus that circulates in cliff swallows (Petrochelidon pyrrhonota) and house sparrows (Passer domesticus) in North America. We discovered ants (Crematogaster lineolata and Formica spp.) preying on swallow bugs at cliff swallow colonies in western Nebraska, U.S.A. Ants reduced the numbers of visible bugs on active swallow nests by 74-90%, relative to nests in the same colony without ants. Ant predation on bugs had no effect on the reproductive success of cliff swallows inhabiting the nests where ants foraged. Ants represent an effective and presumably benign way of controlling swallow bugs at nests in some colonies. They may constitute an alternative to insecticide use at sites where ecologists wish to remove the effects of swallow bugs on cliff swallows or house sparrows. By reducing bug numbers, ant presence may also lessen BCRV transmission at the spatial foci (bird colony sites) where epizootics occur. The effect of ants on swallow bugs should be accounted for in studying variation among sites in vector abundance.

Evolutionary genetics and vector adaptation of recombinant viruses of the western equine encephalitis antigenic complex provides new insights into alphavirus diversity and host switching

Western equine encephalitis virus (WEEV), Highlands J virus (HJV), and Fort Morgan virus (FMV) are the sole representatives of the WEE antigenic complex of the genus Alphavirus, family Togaviridae, that are endemic to North America. All three viruses have their ancestry in a recombination event involving eastern equine encephalitis virus (EEEV) and a Sindbis (SIN)-like virus that gave rise to a chimeric alphavirus that subsequently diversified into the present-day WEEV, HJV, and FMV. Here, we present a comparative analysis of the genetic, ecological, and evolutionary relationships among these recombinant-origin viruses, including the description of a nsP4 polymerase mutation in FMV that allows it to circumvent the host range barrier to Asian tiger mosquito cells, a vector species that is normally refractory to infection. Notably, we also provide evidence that the recombination event that gave rise to these three WEEV antigenic complex viruses may have occurred in North America.

Dispersing hemipteran vectors have reduced arbovirus prevalence

A challenge in managing vector-borne zoonotic diseases in human and wildlife populations is predicting where epidemics or epizootics are likely to occur, and this requires knowing in part the likelihood of infected insect vectors dispersing pathogens from existing infection foci to novel areas. We measured prevalence of an arbovirus, Buggy Creek virus, in dispersing and resident individuals of its exclusive vector, the ectoparasitic swallow bug (Oeciacus vicarius), that occupies cliff swallow (Petrochelidon pyrrhonota) colonies in western Nebraska. Bugs colonizing new colony sites and immigrating into established colonies by clinging to the swallows' legs and feet had significantly lower virus prevalence than bugs in established colonies and those that were clustering in established colonies before dispersing. The reduced likelihood of infected bugs dispersing to new colony sites indicates that even heavily infected sites may not always export virus to nearby foci at a high rate. Infected arthropods should not be assumed to exhibit the same dispersal or movement behaviour as uninfected individuals, and these differences in dispersal should perhaps be considered in the epidemiology of vector-borne pathogens such as arboviruses.

Chaos in a seasonally perturbed SIR model: avian influenza in a seabird colony as a paradigm

Seasonality is a complex force in nature that affects multiple processes in wild animal populations. In particular, seasonal variations in demographic processes may considerably affect the persistence of a pathogen in these populations. Furthermore, it has been long observed in computer simulations that under seasonal perturbations, a host-pathogen system can exhibit complex dynamics, including the transition to chaos, as the magnitude of the seasonal perturbation increases. In this paper, we develop a seasonally perturbed Susceptible-Infected-Recovered model of avian influenza in a seabird colony. Numerical simulations of the model give rise to chaotic recurrent epidemics for parameters that reflect the ecology of avian influenza in a seabird population, thereby providing a case study for chaos in a host- pathogen system. We give a computer-assisted exposition of the existence of chaos in the model using methods that are based on the concept of topological hyperbolicity. Our approach elucidates the geometry of the chaos in the phase space of the model, thereby offering a mechanism for the persistence of the infection. Finally, the methods described in this paper may be immediately extended to other infections and hosts, including humans.

Immune responses of a native and an invasive bird to Buggy Creek Virus (Togaviridae: Alphavirus) and its arthropod vector, the swallow bug (Oeciacus vicarius)

Invasive species often display different patterns of parasite burden and virulence compared to their native counterparts. These differences may be the result of variability in host-parasite co-evolutionary relationships, the occurrence of novel host-parasite encounters, or possibly innate differences in physiological responses to infection between invasive and native hosts. Here we examine the adaptive, humoral immune responses of a resistant, native bird and a susceptible, invasive bird to an arbovirus (Buggy Creek virus; Togaviridae: Alphavirus) and its ectoparasitic arthropod vector (the swallow bug; Oeciacus vicarius). Swallow bugs parasitize the native, colonially nesting cliff swallow (Petrochelidon pyrrhonota) and the introduced house sparrow (Passer domesticus) that occupies nests in cliff swallow colonies. We measured levels of BCRV-specific and swallow bug-specific IgY levels before nesting (prior to swallow bug exposure) and after nesting (after swallow bug exposure) in house sparrows and cliff swallows in western Nebraska. Levels of BCRV-specific IgY increased significantly following nesting in the house sparrow but not in the cliff swallow. Additionally, house sparrows displayed consistently higher levels of swallow bug-specific antibodies both before and after nesting compared to cliff swallows. The higher levels of BCRV and swallow bug specific antibodies detected in house sparrows may be reflective of significant differences in both antiviral and anti-ectoparasite immune responses that exist between these two avian species. To our knowledge, this is the first study to compare the macro- and microparasite-specific immune responses of an invasive and a native avian host exposed to the same parasites.

Introduced Siberian chipmunks (Tamias sibiricus barberi) contribute more to lyme borreliosis risk than native reservoir rodents

The variation of the composition in species of host communities can modify the risk of disease transmission. In particular, the introduction of a new host species can increase health threats by adding a new reservoir and/or by amplifying the circulation of either exotic or native pathogens. Lyme borreliosis is a multi-host vector-borne disease caused by bacteria belonging to the Borrelia burgdorferi sensu lato complex. It is transmitted by the bite of hard ticks, especially Ixodes ricinus in Europe. Previous studies showed that the Siberian chipmunk, Tamias sibiricus barberi, an introduced ground squirrel in the Forest of Sénart (near Paris, France) was highly infested by I. ricinus, and consequently infected by B. burgdorferi sl. An index of the contribution of chipmunks to the density of infected questing nymphs on the vegetation (i.e., the acarological risk for humans) was compared to that of bank voles (Myodes glareolus) and of wood mice (Apodemus sylvaticus), two known native and sympatric competent reservoir hosts. Chipmunks produced nearly 8.5 times more infected questing nymphs than voles and mice. Furthermore, they contribute to a higher diversity of B. burgdorferi sl genospecies (B. afzelii, B. burgdorferi sensu stricto and B. garinii). The contribution of chipmunks varied between years and seasons, according to tick availability. As T. s. barberi must be a competent reservoir, it should amplify B. burgdorferi sl infection, hence increasing the risk of Lyme borreliosis in humans.

Group size and nest spacing affect Buggy Creek virus (Togaviridae: Alphavirus) infection in nestling house sparrows

The transmission of parasites and pathogens among vertebrates often depends on host population size, host species diversity, and the extent of crowding among potential hosts, but little is known about how these variables apply to most vector-borne pathogens such as the arboviruses (arthropod-borne viruses). Buggy Creek virus (BCRV; Togaviridae: Alphavirus) is an RNA arbovirus transmitted by the swallow bug (Oeciacus vicarius) to the cliff swallow (Petrochelidon pyrrhonota) and the introduced house sparrow (Passer domesticus) that has recently invaded swallow nesting colonies. The virus has little impact on cliff swallows, but house sparrows are seriously affected by BCRV. For house sparrows occupying swallow nesting colonies in western Nebraska, USA, the prevalence of BCRV in nestling sparrows increased with sparrow colony size at a site but decreased with the number of cliff swallows present. If one nestling in a nest was infected with the virus, there was a greater likelihood that one or more of its nest-mates would also be infected than nestlings chosen at random. The closer a nest was to another nest containing infected nestlings, the greater the likelihood that some of the nestlings in the focal nest would be BCRV-positive. These results illustrate that BCRV represents a cost of coloniality for a vertebrate host (the house sparrow), perhaps the first such demonstration for an arbovirus, and that virus infection is spatially clustered within nests and within colonies. The decreased incidence of BCRV in sparrows as cliff swallows at a site increased reflects the "dilution effect," in which virus transmission is reduced when a vector switches to feeding on a less competent vertebrate host.

Prevalence of Buggy Creek virus (Togaviridae: Alphavirus) in insect vectors increases over time in the presence of an invasive avian host

Invasive species can disrupt natural disease dynamics by altering pathogen transmission among native hosts and vectors. The relatively recent occupancy of cliff swallow (Petrochelidon pyrrhonota) nesting colonies in western Nebraska by introduced European house sparrows (Passer domesticus) has led to yearly increases in the prevalence of an endemic arbovirus, Buggy Creek virus (BCRV), in its native swallow bug (Oeciacus vicarius) vector at sites containing both the invasive sparrow host and the native swallow host. At sites without the invasive host, no long-term changes in prevalence have occurred. The percentage of BCRV isolates exhibiting cytopathicity in Vero-cell culture assays increased significantly with year at sites with sparrows but not at swallow-only sites, suggesting that the virus is becoming more virulent to vertebrates in the presence of the invasive host. Increased BCRV prevalence in bug vectors at mixed-species colonies may reflect high virus replication rates in house sparrow hosts, resulting in frequent virus transmission between sparrows and swallow bugs. This case represents a rare empirical example of a pathogen effectively switching to an invasive host, documented in the early phases of the host's arrival in a specialized ecosystem and illustrating how an invasive species can promote long-term changes in host-parasite transmission dynamics.