Epidemiology and fitness effects of wood mouse herpesvirus in a natural host population

Experimental assessment of cross-species transmission in a natural multihost-multivector-multipathogen community

Vector-borne pathogens, many of which cause major suffering worldwide, often circulate in diverse wildlife communities comprising multiple reservoir host and/or vector species. However, the complexities of these systems make it challenging to determine the contributions these different species make to transmission. We experimentally manipulated transmission within a natural multihost-multipathogen-multivector system, by blocking flea-borne pathogen transmission from either of two co-occurring host species (bank voles and wood mice). Through genetic analysis of the resulting infections in the hosts and vectors, we show that both host species likely act together to maintain the overall flea community, but cross-species pathogen transmission is relatively rare-most pathogens were predominantly found in only one host species, and there were few cases where targeted treatment affected pathogens in the other host species. However, we do provide experimental evidence of some reservoir-spillover dynamics whereby reductions of some infections in one host species are achieved by blocking transmission from the other host species. Overall, despite the apparent complexity of such systems, we show there can be 'covert simplicity', whereby pathogen transmission is primarily dominated by single host species, potentially facilitating the targeting of key hosts for control, even in diverse ecological communities.

Eco-epidemiological screening of multi-host wild rodent communities in the UK reveals pathogen strains of zoonotic interest

Wild rodent communities represent ideal systems to study pathogens and parasites shared among sympatric species. Such studies are useful in the investigation of eco-epidemiological dynamics, improving disease management strategies and reducing zoonotic risk. The aim of this study was to investigate pathogen and parasites shared among rodent species (multi-host community) in West Wales in an area where human/wildlife disease risk was not previously assessed. West Wales is predominantly rural, with human settlements located alongside to grazing areas and semi-natural landscapes, creating a critical human-livestock-wildlife interface. Ground-dwelling wild rodent communities in Wales were live-trapped and biological samples - faeces and ectoparasites - collected and screened for a suite of pathogens and parasites that differ in types of transmission and ecology. Faecal samples were examined to detect Herpesvirus, Escherichia coli, and Mycobacterium microti. Ticks and fleas were collected, identified to species based on morphology and genetic barcodes, and then screened for Anaplasma phagocytophilum, Babesia microti, Borrelia burgdorferi sensu lato, and Bartonella sp. All the pathogens and parasites screened pose a characteristic epidemiological challenge, such as variable level of generalism, unknown zoonotic potential, and lack of data. The results showed that the bank vole Myodes glareolus had the highest prevalence of all pathogens and parasites. Higher flea species diversity was detected than in previous studies, and at least two Bartonella species were found circulating, one of which has not previously been detected in the UK. These key findings offer new insights into the distribution of selected pathogen and parasites and subsequent zoonotic risk, and provide new baselines and perspectives for further eco-epidemiological research.

Longitudinal deep sequencing informs vector selection and future deployment strategies for transmissible vaccines

Vaccination is a powerful tool in combating infectious diseases of humans and companion animals. In most wildlife, including reservoirs of emerging human diseases, achieving sufficient vaccine coverage to mitigate disease burdens remains logistically unattainable. Virally vectored "transmissible" vaccines that deliberately spread among hosts are a potentially transformative, but still theoretical, solution to the challenge of immunising inaccessible wildlife. Progress towards real-world application is frustrated by the absence of frameworks to guide vector selection and vaccine deployment prior to major in vitro and in vivo investments in vaccine engineering and testing. Here, we performed deep sequencing on field-collected samples of Desmodus rotundus betaherpesvirus (DrBHV), a candidate vector for a transmissible vaccine targeting vampire bat-transmitted rabies. We discovered 11 strains of DrBHV that varied in prevalence and geographic distribution across Peru. The phylogeographic structure of DrBHV strains was predictable from both host genetics and landscape topology, informing long-term DrBHV-vectored vaccine deployment strategies and identifying geographic areas for field trials where vaccine spread would be naturally contained. Multistrain infections were observed in 79% of infected bats. Resampling of marked individuals over 4 years showed within-host persistence kinetics characteristic of latency and reactivation, properties that might boost individual immunity and lead to sporadic vaccine transmission over the lifetime of the host. Further, strain acquisitions by already infected individuals implied that preexisting immunity and strain competition are unlikely to inhibit vaccine spread. Our results support the development of a transmissible vaccine targeting a major source of human and animal rabies in Latin America and show how genomics can enlighten vector selection and deployment strategies for transmissible vaccines.

CHARACTERIZING THE EPIDEMIOLOGY OF HISTORIC AND NOVEL PATHOGENS IN BLANDING'S TURTLES (EMYDOIDEA BLANDINGII)

Pathogens such as herpesviruses, Mycoplasma spp., and frog virus 3-like ranavirus have contributed to morbidity and mortality in many species of free-living and zoo-maintained chelonians. However, their prevalence is understudied in Blanding's turtles (Emydoidea blandingii) across North America. To assess the presence of these pathogens, Blanding's turtles were sampled in Lake County, Illinois, in 2017 (N = 213) and 2018 (N = 160). DNA from cloacal-oral swabs was assayed for four ranaviruses, three Mycoplasma spp., two Salmonella spp., Emydoidea herpesvirus 1 (EBHV1), and tortoise intranuclear coccidiosis (TINC) using a multiplex quantitative polymerase chain reaction (qPCR). Pathogens were most frequently detected in adult turtles (n = 25) and rarely in subadults (n = 2) or juveniles (n = 1). EBHV1 was detected in 22 individuals with no clinical signs of illness, most (n = 20) occurring in the month of May (P < 0.0001). EBHV1 cases at one study site significantly clustered within the same 0.64-km area from 17 to 22 May 2017 (P < 0.0001) and 14 to 15 May 2018 (P = 0.0006). Individuals were rarely positive for Salmonella typhimurium (n = 6). A novel Mycoplasma sp. sharing high homology with other emydid Mycoplasma spp. was detected in one turtle with nasal discharge. Neither TINC nor any ranaviruses were detected. Continued monitoring of this population and habitat may facilitate identification of risk factors for pathogen occurrence and clarify the impact of infectious diseases on Blanding's turtle conservation outcomes.

Who acquires infection from whom? Estimating herpesvirus transmission rates between wild rodent host groups

To date, few studies of parasite epidemiology have investigated 'who acquires infection from whom' in wildlife populations. Nonetheless, identifying routes of disease transmission within a population, and determining the key groups of individuals that drive parasite transmission and maintenance, are fundamental to understanding disease dynamics. Gammaherpesviruses are a widespread group of DNA viruses that infect many vertebrate species, and murine gammaherpesviruses (i.e. MuHV-4) are a standard lab model for studying human herpesviruses, for which much about the pathology and immune response elicited to infection is well understood. However, despite this extensive research effort, primarily in the lab, the transmission route of murine gammaherpesviruses within their natural host populations is not well understood. Here, we aimed to understand wood mouse herpesvirus (WMHV) transmission, by fitting a series of population dynamic models to field data on wood mice naturally infected with WMHV and then estimating transmission parameters within and between demographic groups of the host population. Different models accounted for different combinations of host sex (male/female), age (subadult/adult) and transmission functions (density/frequency-dependent). We found that a density-dependent transmission model incorporating explicit sex groups fitted the data better than all other proposed models. Male-to-male transmission was the highest among all possible combinations of between- and within-sex transmission classes, suggesting that male behaviour is a key factor driving WMHV transmission. Our models also suggest that transmission between sexes, although important, wasn't symmetrical, with infected males playing a significant role in infecting naïve females but not vice versa. Overall this work shows the power of coupling population dynamic models with long-term field data to elucidate otherwise unobservable transmission processes in wild disease systems.

Infection Load and Prevalence of Novel Viruses Identified from the Bank Vole Do Not Associate with Exposure to Environmental Radioactivity

Bank voles (Myodes glareolus) are host to many zoonotic viruses. As bank voles inhabiting areas contaminated by radionuclides show signs of immunosuppression, resistance to apoptosis, and elevated DNA repair activity, we predicted an association between virome composition and exposure to radionuclides. To test this hypothesis, we studied the bank vole virome in samples of plasma derived from animals inhabiting areas of Ukraine (contaminated areas surrounding the former nuclear power plant at Chernobyl, and uncontaminated areas close to Kyiv) that differed in level of environmental radiation contamination. We discovered four strains of hepacivirus and four new virus sequences: two adeno-associated viruses, an arterivirus, and a mosavirus. However, viral prevalence and viral load, and the ability to cause a systemic infection, was not dependent on the level of environmental radiation.

Linking community assembly and structure across scales in a wild mouse parasite community

Understanding what processes drive community structure is fundamental to ecology. Many wild animals are simultaneously infected by multiple parasite species, so host-parasite communities can be valuable tools for investigating connections between community structures at multiple scales, as each host can be considered a replicate parasite community. Like free-living communities, within-host-parasite communities are hierarchical; ecological interactions between hosts and parasites can occur at multiple scales (e.g., host community, host population, parasite community within the host), therefore, both extrinsic and intrinsic processes can determine parasite community structure. We combine analyses of community structure and assembly at both the host population and individual scales using extensive datasets on wild wood mice (Apodemus sylvaticus) and their parasite community. An analysis of parasite community nestedness at the host population scale provided predictions about the order of infection at the individual scale, which were then tested using parasite community assembly data from individual hosts from the same populations. Nestedness analyses revealed parasite communities were significantly more structured than random. However, observed nestedness did not differ from null models in which parasite species abundance was kept constant. We did not find consistency between observed community structure at the host population scale and within-host order of infection. Multi-state Markov models of parasite community assembly showed that a host's likelihood of infection with one parasite did not consistently follow previous infection by a different parasite species, suggesting there is not a deterministic order of infection among the species we investigated in wild wood mice. Our results demonstrate that patterns at one scale (i.e., host population) do not reliably predict processes at another scale (i.e., individual host), and that neutral or stochastic processes may be driving the patterns of nestedness observed in these communities. We suggest that experimental approaches that manipulate parasite communities are needed to better link processes at multiple ecological scales.

Age affects antibody levels and anthelmintic treatment efficacy in a wild rodent

The role of the host immune system in determining parasite burdens and mediating within-host parasite interactions has traditionally been studied in highly controlled laboratory conditions. This does, however, not reflect the diversity of individuals living in nature, which is often characterised by significant variation in host demography, such as host age, sex, and infection history. Whilst studies using wild hosts and parasites are beginning to give insights into the complex relationships between immunity, parasites and host demography, the cause-and-effect relationships often remain unknown due to a lack of high resolution, longitudinal data. We investigated the infection dynamics of two interacting gastrointestinal parasites of wild wood mice (Apodemus sylvaticus), the nematode Heligmosomoides polygyrus and the coccidian Eimeria hungaryensis, in order to assess the links between infection, coinfection, and the immunological dynamics of two antibodies (IgG1 and IgA). In an anthelmintic treatment experiment, mice were given a single oral dose of an anthelmintic treatment, or control dose, and then subsequently followed longitudinally over a period of 7–15 days to measure parasite burdens and antibody levels. Anthelmintic treatment successfully reduced burdens of H. polygyrus, but had no significant impact on E. hungaryensis. Treatment efficacy was driven by host age, with adult mice showing stronger reductions in burdens compared to younger mice. We also found that the relationship between H. polygyrus-specific IgG1 and nematode burden changed from positive in young mice to negative in adult mice. Our results highlight that a key host demographic factor like age could account for large parts of the variation in nematode burden and nematode-specific antibody levels observed in a naturally infected host population, possibly due to different immune responses in young vs. old animals. Given the variable success in community-wide de-worming programmes in animals and humans, accounting for the age-structure of a population could increase overall efficacy.

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Anthelmintic treatment reveals strong force of infection for H. polygyrus in wild wood mice.

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Anthelmintic treatment is more successful in younger compared to older mice.

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Relationship between IgG1 and H. polygyrus burden reverts with host age.

Diverse Beta- and Gammaherpesviruses in Neotropical Rodents from Costa Rica

Neotropical wild rodents from Costa Rica were analyzed for the presence of herpesviruses (order Herpesvirales, family Herpesviridae). Using a broadly generic PCR, herpesvirus sequences were detected in 5% (8/160) of liver and heart samples: seven putative gammaherpesviruses in samples from Talamancan oryzomys (Nephelomys devius), sprightly colilargo (Oligoryzomys vegetus), Mexican deer mouse (Peromyscus nudipes), and Chiriqui harvest mouse (Reithrodontomys creper) and one putative betaherpesvirus in long-tailed singing mouse (Scotinomys xerampelinus). Results from this study could guide ecological investigations targeting the prevalence and host associations of herpesviruses in wild rodents from Costa Rica.

Expansion of the metazoan virosphere: progress, pitfalls, and prospects

Metagenomic sequencing has led to a recent and rapid expansion of the animal virome. It has uncovered a multitude of new virus lineages from under-sampled host groups, including many that break up long branches in the virus tree, and many that display unexpected genome sizes and structures. Although there are challenges to inferring the existence of a virus from a `virus-like sequence', in the absence of an isolate the analysis of nucleic acid (including small RNAs) and sequence data can provide considerable confidence. As a consequence, this period of molecular natural history is helping to reshape our views of deep virus evolution.

Tick-Borne Transmission of Murine Gammaherpesvirus 68

Herpesviruses are a large group of DNA viruses infecting mainly vertebrates. Murine gammaherpesvirus 68 (MHV68) is often used as a model in studies of the pathogenesis of clinically important human gammaherpesviruses such as Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus. This rodent virus appears to be geographically widespread; however, its natural transmission cycle is unknown. Following detection of MHV68 in field-collected ticks, including isolation of the virus from tick salivary glands and ovaries, we investigated whether MHV68 is a tick-borne virus. Uninfected Ixodes ricinus ticks were shown to acquire the virus by feeding on experimentally infected laboratory mice. The virus survived tick molting, and the molted ticks transmitted the virus to uninfected laboratory mice on which they subsequently fed. MHV68 was isolated from the tick salivary glands, consistent with transmission via tick saliva. The virus survived in ticks without loss of infectivity for at least 120 days, and subsequently was transmitted vertically from one tick generation to the next, surviving more than 500 days. Furthermore, the F1 generation (derived from F0 infected females) transmitted MHV68 to uninfected mice on which they fed, with MHV68 M3 gene transcripts detected in blood, lung, and spleen tissue of mice on which F1 nymphs and F1 adults engorged. These experimental data fulfill the transmission criteria that define an arthropod-borne virus (arbovirus), the largest biological group of viruses. Currently, African swine fever virus (ASFV) is the only DNA virus recognized as an arbovirus. Like ASFV, MHV68 showed evidence of pathogenesis in ticks. Previous studies have reported MHV68 in free-living ticks and in mammals commonly infested with I. ricinus, and neutralizing antibodies to MHV68 have been detected in large mammals (e.g., deer) including humans. Further studies are needed to determine if these reports are the result of tick-borne transmission of MHV68 in nature, and whether humans are at risk of infection.

Development of a quantitative PCR assay for measurement of trichechid herpesvirus 1 load in the Florida manatee ( Trichechus manatus latirostris)

Trichechid herpesvirus 1 (TrHV-1) is currently the only known herpesvirus in any sirenian. We hypothesized that stress may lead to recrudescence of TrHV-1 in manatees, thus making TrHV-1 a potential biomarker of stress. We optimized and validated a TrHV-1 real-time quantitative probe hybridization PCR (qPCR) assay that was used to quantify TrHV-1 in manatee peripheral blood mononuclear cells (PBMCs). Average baseline TrHV-1 loads in a clinically healthy wild Florida manatee ( Trichechus manatus latirostris) population ( n = 42) were 40.9 ± SD 21.2 copies/100 ng DNA; 19 of 42 manatees were positive. TrHV-1 loads were significantly different between the 2 field seasons ( p < 0.025). This optimized and validated qPCR assay may be used as a tool for further research into TrHV-1 in Florida manatees.

PREVALENCE OF TERRAPENE HERPESVIRUS 1 IN FREE-RANGING EASTERN BOX TURTLES (TERRAPENE CAROLINA CAROLINA) IN TENNESSEE AND ILLINOIS, USA

Diseases affecting the upper respiratory tract, such as herpesviruses, are well described in captive chelonians worldwide, but their importance in free-ranging populations is less well known. To characterize the disease epidemiology of terrapene herpesvirus 1 (TerHV1), 409 free-ranging eastern box turtles ( Terrapene carolina carolina) in Tennessee and Illinois, US were tested for TerHV1 in 2013 and 2014 using TaqMan quantitative PCR. Whole blood and swabs of the oral mucosa were collected from 365 adults (154 females, 195 males, 16 unknown sex) and 44 juveniles. The prevalence of detection was 31.3% (n=128). Turtles were more likely to be positive for TerHV1 in July (50%; n=67) compared to September (38%; n=44) and May (11%; n=17). Turtles sampled in 2014 had a significantly higher prevalence (50%; n=98) than in 2013 (14%; n=30). In a multivariate model, only season, year, and the interaction between season and year were maintained; turtles were most likely to be positive in July (odds ratio: 30.5) and September (odds ratio: 41.8) 2014 compared to May 2013. The prevalence was not statistically different by state of collection, sex, or age class. Packed cell volume (25.5%) and total solids (4.8 mg/dL) in positive turtles were significantly higher than in negative turtles (23.0%; 4.3 mg/dL). Positive turtles had increased eosinophil concentrations, fewer lymphocytes, and fewer monocytes. No clinical sign was associated with detection of herpesvirus. Widespread DNA evidence of TerHV1 infection was detected in eastern box turtles, and knowledge of the epidemiology of this virus may aid in management of free-ranging and captive individuals.

Hampered performance of migratory swans: intra- and inter-seasonal effects of avian influenza virus

The extent to which animal migrations shape parasite transmission networks is critically dependent on a migrant's ability to tolerate infection and migrate successfully. Yet, sub-lethal effects of parasites can be intensified through periods of increased physiological stress. Long-distance migrants may, therefore, be especially susceptible to negative effects of parasitic infection. Although a handful of studies have investigated the short-term, transmission-relevant behaviors of wild birds infected with low-pathogenic avian influenza viruses (LPAIV), the ecological consequences of LPAIV for the hosts themselves remain largely unknown. Here, we assessed the potential effects of naturally-acquired LPAIV infections in Bewick's swans, a long-distance migratory species that experiences relatively low incidence of LPAIV infection during early winter. We monitored both foraging and movement behavior in the winter of infection, as well as subsequent breeding behavior and inter-annual resighting probability over 3 years. Incorporating data on infection history we hypothesized that any effects would be most apparent in naïve individuals experiencing their first LPAIV infection. Indeed, significant effects of infection were only seen in birds that were infected but lacked antibodies indicative of prior infection. Swans that were infected but had survived a previous infection were indistinguishable from uninfected birds in each of the ecological performance metrics. Despite showing reduced foraging rates, individuals in the naïve-infected category had similar accumulated body stores to re-infected and uninfected individuals prior to departure on spring migration, possibly as a result of having higher scaled mass at the time of infection. And yet individuals in the naïve-infected category were unlikely to be resighted 1 year after infection, with 6 out of 7 individuals that never resighted again compared to 20 out of 63 uninfected individuals and 5 out of 12 individuals in the re-infected category. Collectively, our findings indicate that acute and superficially harmless infection with LPAIV may have indirect effects on individual performance and recruitment in migratory Bewick's swans. Our results also highlight the potential for infection history to play an important role in shaping ecological constraints throughout the annual cycle.

Felis catus gammaherpesvirus 1; a widely endemic potential pathogen of domestic cats

Felis catus gammaherpesvirus 1 (FcaGHV1), recently discovered in the USA, was detected in domestic cats in Australia (11.4%, 95% confidence interval 5.9-19.1, n=110) and Singapore (9.6%, 95% confidence interval 5.9-14.6, n=176) using qPCR. FcaGHV1 qPCR positive cats were 2.8 times more likely to be sick than healthy. Risk factors for FcaGHV1 detection included being male, increasing age and coinfection with pathogenic retroviruses, feline immunodeficiency virus (FIV) or feline leukaemia virus. FcaGHV1 DNA was detected in multiple tissues from infected cats with consistently high virus loads in the small intestine. FcaGHV1 viral load was significantly higher in FIV-infected cats compared with matched controls, mimicking increased Epstein-Barr virus loads in human immunodeficiency virus-infected humans. FcaGHV1 is endemic in distant geographic regions and is associated with being sick and with coinfections. Horizontal transmission of FcaGHV1 is supported, with biting being a plausible route. A pathogenic role for FcaGHV1 in domestic cats is supported.

Insights from natural host-parasite interactions: the Drosophila model

Immune responses against opportunistic pathogens have been extensively studied in Drosophila, leading to a detailed map of the genetics behind innate immunity networks including the Toll, Imd, Jak-Stat, and JNK pathways. However, immune mechanisms of other organisms, such as plants, have primarily been investigated using natural pathogens. It was the use of natural pathogens in plant research that revealed the plant R-Avr system, a specialized immune response derived from antagonistic coevolution between plant immune proteins and their natural pathogens' virulence proteins. Thus, we recommend that researchers begin to use natural Drosophila pathogens to identify novel immune strategies that may have arisen through antagonistic coevolution with common natural pathogens. In this review, we address the benefits of using natural pathogens in research, describe the known natural pathogens of Drosophila, and discuss the future prospects for research on natural pathogens of Drosophila.

A reliable and effective method of DNA isolation from old human blood paper cards

Blood paper cards provide an effective DNA storage method. In this study, we used three DNA dissolving reagents (Tris-EDTA [TE] buffer, Tris–HCl buffer, and water) and one common commercially available kit (DN131 from MRC Inc) to elute DNA from 105 human blood paper cards collected up to 10 years ago. These DNA samples were used as templates for amplification of a single nucleotide polymorphism (SNP, C125T) region of human caspase-12 by PCR and a specific Taqman genotyping assay using the same amount of DNA. We show that DNA isolated by Tris–HCl buffer has higher yield and quality in comparison to DN131 solution. PCR success rate to amplify caspase-12 C125T SNP using Tris–HCl is comparable to the method using DN131 (89.5% vs 87.6%). The Taqman genotyping success rate using Tris–HCl is higher than using DN131 (81.9% vs 70.5%). Using TE or water, PCR success rates are lower than using DN131 (73.3% [TE]; 72.4% [H₂O]), but Taqman genotyping success rates are comparable to the method using DN131 (70.5% [TE]; 79.1% [H₂O]). We concluded that using Tris–HCl is a reliable and effective method to elute DNA from old human blood paper cards. The crude DNA isolated by Tris–HCl can be used to study genetic polymorphisms in human populations.