Consecutive natural influenza a virus infections in sentinel mallards in the evident absence of subtype-specific hemagglutination inhibiting antibodies

Evidence for flock transmission of individual subtypes and strains of avian influenza viruses: A monitoring study of wild birds in Kazakhstan

An active surveillance study of avian influenza viruses (AIVs) in wild birds was carried out in Kazakhstan in 2018-2019. In total, 866 samples were collected from wild birds and analyzed for influenza viruses using molecular and virological tests. Genome segments of Asian, European, and Australian lineages were detected in 25 (4.6%) out of 541 waterfowl samples positive for subtype H3N8, and in two (0.6%) out of 325 H3N8 positive samples from terrestrial birds. No highly pathogenic avian influenza virus (AIV) was detected. The results indicated transmission of closely related strains or identical subtypes of AIVs by a flock-unit of migratory birds or annual cyclical pattern of subtype dominance. The simultaneous circulation of genome segments of the Asian, European and Australian genetic lineages of H3N8 AIVs in wild birds in Kazakhstan indicated the important role of Central Asia as a transmission hub of AI viruses linking the East Asian migratory flyways with European flyways and vice versa.

Adaptation of Two Wild Bird-Origin H3N8 Avian Influenza Viruses to Mammalian Hosts

Wild birds play an important role in the emergence, evolution, and spread of zoonotic avian influenza viruses (AIVs). However, there are few studies on the cross-species transmission of the H3N8 AIV originating from wild birds. In this study, we investigated the transmissibility and pathogenicity of two H3N8 low pathogenic avian influenza viruses (LPAIVs) isolated from wild birds, GZA1 and XJ47, to mammals. The HA genes of both strains belonged to Eurasian isolates, while the other genes were derived from a variety of other subtypes of AIVs. Both strains can infect specific-pathogen-free (SPF) chickens, BALB/c mice, and guinea pigs. The XJ47 strain spread horizontally in SPF chickens and guinea pigs. The GZA1 strain did not spread horizontally but caused higher weight loss and mild lung inflammation in mice. P12-GZA1- and P12-XJ47-adapted strains obtained after 12 passages in the lung of mice showed enhanced pathogenicity in mice, which led to obvious clinical symptoms, lung inflammation, and 100% death. Both adapted strains have the reported mutation T97I in the PA, and the reported mutation D701N in PB2 has been found in the P12-GZA1-adapted strain. This study provides an important scientific basis for the continuous monitoring of wild AIVs and the mechanism underlying AIV cross-species transmission.

Modulation of lethal HPAIV H5N8 clade 2.3.4.4B infection in AIV pre-exposed mallards

In 2016/2017, a severe epidemic of HPAIV H5N8 clade 2.3.4.4 group B (H5N8B) affected Europe. To analyse the role of mallards in the spatiotemporal dynamics of global HPAIV H5N8B dispersal, mallards (Anas platyrhynchos), naturally exposed to various AIV and therefore seropositive, were challenged with H5N8B. All experiments were controlled by infection and co-housing of seronegative juvenile Pekin ducklings. All ducks that survived the first infection were re-challenged 21 dpi with the homologous H5N8B strain. After the first H5N8B infection, seropositive mallards showed only mild clinical symptoms. Moderate to low viral shedding, occurring particularly from the oropharynx and lasting for 7 days maximum, led to severe clinical disease of all contact ducklings. All challenged seronegative Pekin ducks and contact ducklings died or had to be euthanized. H5-specific antibodies were detected in surviving birds within 2 weeks. Virus and viral RNA could be isolated from several water samples until 6 and 9 dpi, respectively. Conversely, upon re-infection with homologous H5N8B neither inoculated nor contact ducklings showed any clinical symptoms, nor was an antibody titer increase of seropositive mallards or any seroconversion of contact ducklings observed. Mallard ducks naturally pre-exposed to LPAIV can play a role as a clinically unsuspicious virus reservoir for H5N8B effective in virus transmission. Mallards with homologous immunity did not contribute to virus transmission.

Alternate routes of influenza A virus infection in Mallard (Anas platyrhynchos)

The natural reservoir for all influenza A viruses (IAVs) is wild birds, particularly dabbling ducks. During the autumn, viral prevalence can be very high in dabbling ducks (> 30%) in the Northern Hemisphere, and individuals may be repeatedly infected. Transmission and infection is through the fecal-oral route, whereby birds shed viruses in feces and conspecifics are infected though feeding in virus-contaminated water. In this study we wanted to assess two alternative infection routes: cloacal drinking and preening. Using experimental infections, we assessed patterns of infection using a combination of virus shedding, as assessed by real-time PCR from cloacal swabs, and patterns of viral replication using virus-immunohistochemistry of gastrointestinal tissues. The cloacal drinking experiment consisted of two trials using cloacal inoculation at two different time points to account for age differences, as well as a trial whereby ducks were allowed to take up virus-laden water through the cloaca. All ducks became infected, and rather than the bursa of Fabricius being the main site of replication, the colon had the highest intensity of replication, as inferred through immunohistochemistry. In experiments assessing preening, feathers were contaminated with virus-laden water and all ducks became infected, regardless of whether they were kept individually or together. Further, naive contacts were infected by the individuals whose feathers were virus-contaminated. Overall, we reinforce that IAV transmission in dabbling ducks is multifactorial-if exposed to virus-contaminated water ducks may be infected through dabbling, preening of infected feathers, and cloacal drinking.

Avian influenza

Previous introductions of highly pathogenic avian influenza virus (HPAIV) to the EU were most likely via migratory wild birds. A mathematical model has been developed which indicated that virus amplification and spread may take place when wild bird populations of sufficient size within EU become infected. Low pathogenic avian influenza virus (LPAIV) may reach similar maximum prevalence levels in wild bird populations to HPAIV but the risk of LPAIV infection of a poultry holding was estimated to be lower than that of HPAIV. Only few non-wild bird pathways were identified having a non-negligible risk of AI introduction. The transmission rate between animals within a flock is assessed to be higher for HPAIV than LPAIV. In very few cases, it could be proven that HPAI outbreaks were caused by intrinsic mutation of LPAIV to HPAIV but current knowledge does not allow a prediction as to if, and when this could occur. In gallinaceous poultry, passive surveillance through notification of suspicious clinical signs/mortality was identified as the most effective method for early detection of HPAI outbreaks. For effective surveillance in anseriform poultry, passive surveillance through notification of suspicious clinical signs/mortality needs to be accompanied by serological surveillance and/or a virological surveillance programme of birds found dead (bucket sampling). Serosurveillance is unfit for early warning of LPAI outbreaks at the individual holding level but could be effective in tracing clusters of LPAIV-infected holdings. In wild birds, passive surveillance is an appropriate method for HPAIV surveillance if the HPAIV infections are associated with mortality whereas active wild bird surveillance has a very low efficiency for detecting HPAIV. Experts estimated and emphasised the effect of implementing specific biosecurity measures on reducing the probability of AIV entering into a poultry holding. Human diligence is pivotal to select, implement and maintain specific, effective biosecurity measures.

Urbanization and the dynamics of RNA viruses in Mallards (Anas platyrhynchos)

Urbanization is intensifying worldwide, and affects the epidemiology of infectious diseases. However, the effect of urbanization on natural host-pathogen systems remains poorly understood. Urban ducks occupy an interesting niche in that they directly interact with both humans and wild migratory birds, and either directly or indirectly with food production birds. Here we have collected samples from Mallards (Anas platyrhynchos) residing in a pond in central Uppsala, Sweden, from January 2013 to January 2014. This artificial pond is kept ice-free during the winter months, and is a popular location where the ducks are fed, resulting in a resident population of ducks year-round. Nine hundred and seventy seven (977) fecal samples were screened for RNA viruses including: influenza A virus (IAV), avian paramyxovirus 1, avian coronavirus (CoV), and avian astrovirus (AstroV). This intra-annual dataset illustrates that these RNA viruses exhibit similar annual patterns to IAV, suggesting similar ecological factors are at play. Furthermore, in comparison to wild ducks, autumnal prevalence of IAV and CoV are lower in this urban population. We also demonstrate that AstroV might be a larger burden to urban ducks than IAV, and should be better assessed to demonstrate the degree to which wild birds contribute to the epidemiology of these viruses. The presence of economically relevant viruses in urban Mallards highlights the importance of elucidating the ecology of wildlife pathogens in urban environments, which will become increasingly important for managing disease risks to wildlife, food production animals, and humans.

Competition between influenza A virus subtypes through heterosubtypic immunity modulates re-infection and antibody dynamics in the mallard duck

Our overall hypothesis is that host population immunity directed at multiple antigens will influence the prevalence, diversity and evolution of influenza A virus (IAV) in avian populations where the vast subtype diversity is maintained. To investigate how initial infection influences the outcome of later infections with homologous or heterologous IAV subtypes and how viruses interact through host immune responses, we carried out experimental infections in mallard ducks (Anas platyrhynchos). Mallards were pre-challenged with an H3N8 low-pathogenic IAV and were divided into six groups. At five weeks post H3N8 inoculation, each group was challenged with a different IAV subtype (H4N5, H10N7, H6N2, H12N5) or the same H3N8. Two additional pre-challenged groups were inoculated with the homologous H3N8 virus at weeks 11 and 15 after pre-challenge to evaluate the duration of protection. The results showed that mallards were still resistant to re-infection after 15 weeks. There was a significant reduction in shedding for all pre-challenged groups compared to controls and the outcome of the heterologous challenges varied according to hemagglutinin (HA) phylogenetic relatedness between the viruses used. There was a boost in the H3 antibody titer after re-infection with H4N5, which is consistent with original antigenic sin or antigenic seniority and suggest a putative strategy of virus evasion. These results imply competition between related subtypes that could regulate IAV subtype population dynamics in nature. Collectively, we provide new insights into within-host IAV complex interactions as drivers of IAV antigenic diversity that could allow the circulation of multiple subtypes in wild ducks.

No evidence for homosubtypic immunity of influenza H3 in Mallards following vaccination in a natural experimental system

The Mallard (Anas platyrhynchos) is an important reservoir species for influenza A viruses (IAV), and in this host, prevalence and virus diversity are high. Studies have demonstrated the presence of homosubtypic immunity, where individuals are unlikely to be reinfected with the same subtype within an autumn season. Further, evidence for heterosubtypic immunity exists, whereby immune responses specific for one subtype offer partial or complete protection against related HA subtypes. We utilized a natural experimental system to determine whether homo- or heterospecific immunity could be induced following experimental vaccination. Thirty Mallards were vaccinated with an inactivated H3, H6 or a sham vaccine and after seroconversion were exposed to naturally infected wild conspecifics. All ducks were infected within 2 days and had both primary and secondary infections. Overall, there was no observable difference between groups; all individuals were infected with H3 and H10 IAV. At the cessation of the experiment, most individuals had anti-NP antibodies and neutralizing antibodies against H10. Not all individuals had H3 neutralizing antibodies. The isolated H3 IAVs revealed genetic dissimilarity to the H3 vaccine strain, specifically substitutions in the vicinity of the receptor-binding site. There was no evidence of vaccine-induced homosubtypic immunity to H3, a likely result of both a poor H3 immune response in the ducks and H3 immune escape. Likewise, there was no observed heterosubtypic protection related to H6 vaccination. This study highlights the need for experimental approaches to assess how exposure to pathogens and resulting immune processes translates to individual and population disease dynamics.

Differential Viral Fitness Between H1N1 and H3N8 Avian Influenza Viruses Isolated from Mallards (Anas platyrhynchos)

Homosubtypic and heterosubtypic immunity in mallards (Anas platyrhynchos) play an important role in the avian influenza virus (AIV) diversity. The mechanisms of AIV replication among wild birds and the role of immunity in AIV diversity have thus not been completely clarified. During the monitoring of AI circulation among wild waterfowl in 2007-2008, two viruses (H3N8 and H1N1) were isolated from ducks caught in a funnel trap located in La Hulpe wetland in Belgium. H3N8 viruses were revealed to be more prevalent in the mallard population than was H1N1, which might suggest a better adaptation to this species. In order to investigate this hypothesis, we characterized both isolated viruses biologically by experimental inoculation. Virus excretion and humoral response induced by both isolated viruses were evaluated in mallards after a first infection followed by a homo- or heterosubtypic reinfection under controlled experimental conditions. The H1N1 virus had a delayed peak of excretion of 4 days compared to the H3N8, but the virus shedding was more limited, earlier, and shorter after each reinfection. Moreover, the H3N8 virus could spread to all ducks after homo- or heterosubtypic reinfections and during a longer period. Although the humoral response induced by both viruses after infection and reinfection could be detected efficiently by competitive ELISA, only a minimal H1 antibody response and almost no H3-specific antibodies could be detected by the HI test. Our results suggest that the H3N8 isolate replicates better in mallards under experimental controlled conditions.

Evaluation of the kinetics of anti-NP and anti-HA antibody after infection of Pekin ducks with low pathogenic avian influenza virus

Serological monitoring is a feature of surveillance programmes for the detection of the circulation of notifiable low pathogenic avian influenza (LPAI) viruses in commercial poultry holdings. Commercial multispecies nucleoprotein (NP) enzyme‐linked immunosorbent assays (ELISAs) have been replacing the haemagglutination inhibition (HI) test as pre‐screening tools. Few comparative studies have been conducted to test sera from domestic ducks for diagnostic purposes. Therefore, we evaluated the correlation between commercial NP ELISAs and the HI test. Anti‐NP and anti‐haemagglutinin (HA) antibodies were measured in sera from domestic ducks that had undergone serological screening and from juvenile domestic Pekin ducks that were experimentally infected with LPAI viruses. The findings highlight an absence of a correlation between NP ELISA and HI results with both field and experimental duck sera. Dissimilar kinetics of the antibodies detected during the follow‐up evaluation of the humoral immune responses in experimentally infected ducks may explain this lack of correlation. Indeed, anti‐NP titres decreased over time, whereas anti‐HA titres remained unchanged after inoculation with the H3N1 LPAI virus isolated from domestic duck or the H7N1 LPAI virus isolated from chicken. Despite these differences, the NP ELISA may serve as a valid pre‐screening tool to detect circulating LPAI viruses in domestic duck populations at the flock level.

Influenza A virus H5-specific antibodies in mute swans (Cygnus olor) in the USA

The use of serologic assays for influenza A virus (IAV) surveillance in wild birds has increased because of the availability of commercial enzyme-linked immunosorbent assays (ELISAs). Recently, an H5-specific blocking ELISA (bELISA) was shown to reliably detect H5-specific antibodies to low- and high-pathogenic H5 viruses in experimentally infected waterfowl. Mute Swans (Cygnus olor) were frequently associated with highly pathogenic H5N1 outbreaks in Europe and may have a similar role if highly pathogenic H5N1 is introduced into North America. We measured the prevalence of antibodies to the nucleoprotein and H5 protein in Mute Swans using three serologic assays. We collected 340 serum samples from Mute Swans in Michigan, New Jersey, New York, and Rhode Island, US. We detected antibodies to the IAV nucleoprotein in 66.2% (225/340) of the samples. We detected H5-specific antibodies in 62.9% (214/340) and 18.8% (64/340) using a modified H5 bELISA protocol and hemagglutination inhibition (HI) assay, respectively. The modified H5 bELISA protocol detected significantly more positive samples than did the manufacturer's protocol. We also tested 46 samples using virus neutralization. Neutralization results had high agreement with the modified H5 bELISA protocol and detected a higher prevalence than did the HI assay. These results indicate that North American Mute Swans have high nucleoprotein and H5 antibody prevalences.

Local-scale diversity and between-year "frozen evolution" of avian influenza A viruses in nature

Influenza A virus (IAV) in wild bird reservoir hosts is characterized by the perpetuation in a plethora of subtype and genotype constellations. Multiyear monitoring studies carried out during the last two decades worldwide have provided a large body of knowledge regarding the ecology of IAV in wild birds. Nevertheless, other issues of avian IAV evolution have not been fully elucidated, such as the complexity and dynamics of genetic interactions between the co-circulating IAV genomes taking place at a local-scale level or the phenomenon of frozen evolution. We investigated the IAV diversity in a mallard population residing in a single pond in the Czech Republic. Despite the relative small number of samples collected, remarkable heterogeneity was revealed with four different IAV subtype combinations, H6N2, H6N9, H11N2, and H11N9, and six genomic constellations in co-circulation. Moreover, the H6, H11, and N2 segments belonged to two distinguishable sub-lineages. A reconstruction of the pattern of genetic reassortment revealed direct parent-progeny relationships between the H6N2, H11N9 and H6N9 viruses. Interestingly the IAV, with the H6N9 subtype, was re-detected a year later in a genetically unchanged form in the close proximity of the original sampling locality. The almost absolute nucleotide sequence identity of all the respective genomic segments between the two H6N9 viruses indicates frozen evolution as a result of prolonged conservation in the environment. The persistence of the H6N9 IAV in various abiotic and biotic environmental components was also discussed.

Interspecies transmission and emergence of novel viruses: lessons from bats and birds

As exemplified by coronaviruses and influenza viruses, bats and birds are natural reservoirs for providing viral genes during evolution of new virus species and viruses for interspecies transmission. These warm-blooded vertebrates display high species biodiversity, roosting and migratory behavior, and a unique adaptive immune system, which are favorable characteristics for asymptomatic shedding, dissemination, and mixing of different viruses for the generation of novel mutant, recombinant, or reassortant RNA viruses. The increased intrusion of humans into wildlife habitats and overcrowding of different wildlife species in wet markets and farms have also facilitated the interspecies transmission between different animal species.

Is low pathogenic avian influenza virus virulent for wild waterbirds?

Although low pathogenic avian influenza virus (LPAIV) is traditionally considered to have adapted to its wild waterbird host to become avirulent, recent studies have suggested that LPAIV infection might after all have clinical effects. Therefore, I reviewed the literature on LPAIV infections in wild waterbirds. The virulence of LPAIV was assessed in 17 studies on experimental infections and nine studies on natural infections. Reported evidence for virulence were reductions in return rate, feeding rate, body weight, long-range movement and reproductive success, as well as pathological changes in infected organs. However, major caveats in studies of experimental infections were unnatural route of LPAIV inoculation, animal husbandry not simulating natural stressors and low sensitivity of clinical assessment. Major caveats in studies of natural infections were incomplete measurement of LPAIV infection burden, quasi-experimental design and potential misclassification of birds. After taking these caveats into account, the only remaining evidence for virulence was that presence and intensity of LPAIV infection were negatively correlated with body weight. Based on this correlation, together with the demonstrated LPAIV tropism for the intestinal tract, I hypothesize that LPAIV reduces digestive tract function, and suggest how future studies could be directed to test this hypothesis.

Individual variation in influenza A virus infection histories and long-term immune responses in Mallards

Wild dabbling ducks (genus Anas) are the main reservoir for influenza A virus (IAV) in the Northern Hemisphere. Current understanding of disease dynamics and epidemiology in this virus-host system has primarily been based on population-level surveillance studies and infection experiments conducted in laboratory settings. Using a combined experimental-natural approach with wild-strain captive mallards (Anas platyrhynchos), we monitored individual IAV infection histories and immunological responses of 10 birds over the course of 15 months. This is the first detailed study to track natural IAV infection histories over several seasons amongst the same individuals growing from juvenile to adults. The general trends in the infection histories of the monitored birds reflected seasonal variation in prevalence at the population level. However, within the study group there were significant differences between individuals in infection frequency as well as in short and long term anti-IAV antibody response. Further observations included individual variation in the number of infecting virus subtypes, and a strong tendency for long-lasting hemagglutinin-related homosubtypic immunity. Specifically, all infections in the second autumn, except one, were of different subtypes compared to the first autumn. The variation among birds concerning these epidemiologically important traits illustrates the necessity for IAV studies to move from the level of populations to examine individuals in order to further our understanding of IAV disease and epidemiology.