Heterosubtypic immunity increases infectious dose required to infect Mallard ducks with Influenza A virus

Naturally Acquired Antibodies to Influenza A Virus in Fall-Migrating North American Mallards

Although waterfowl are the primary reservoir for multiple subtypes of influenza A virus (IAV), our understanding of population immunity in naturally infected waterfowl is poorly understood. Population immunity may be an important driver of seasonal subtype predominance in waterfowl populations and may affect the potential for establishment of introduced IAV such as the Eurasian-like A/Goose/Guangdong/1/1996 lineage in these populations. Here, we examine the prevalence of naturally acquired antibodies to nucleoprotein (NP), hemagglutinin (H3, H4, H5), and neuraminidase (N1, N2, N6, N8) in early migrating mallards (Anas platyrhynchos) sampled in Northwest Minnesota during staging and early fall migration in September 2014, 2015, 2017, and 2018. Serologic results were compared to historic and contemporary virus isolation results from these same study sites. The prevalence of antibodies to NP ranged from 60.8−76.1% in hatch-year (HY) birds and from 86.0−92.7% in after-hatch-year (AHY, >1-year-old) mallards indicating a high level of previous infection with IAV early in the fall migration season. Neutralizing antibodies were detected against H3, H4, and H5 in all years as were antibodies to N1, N2, N6, and N8. A high proportion of NP seropositive ducks tested positive for antibodies to multiple HA and NA subtypes, and this was more common in the AHY age class. Antibody prevalence to the HA and NA subtypes included in this study were consistent with the predominance of H4N6 in these populations during all years and reflected a broadening of the antibody response with age. Additional work is needed to document the longevity of these immune responses, if and how they correlate with protection against IAV transmission, infection, and disease, and if, as detected in this study, they adequately describe the true extent of exposure to IAV or specific HA or NA subtypes.

Comparative micro-epidemiology of pathogenic avian influenza virus outbreaks in a wild bird population

Understanding the epidemiological dynamics of highly pathogenic avian influenza virus (HPAIV) in wild birds is crucial for guiding effective surveillance and control measures. The spread of H5 HPAIV has been well characterized over large geographical and temporal scales. However, information about the detailed dynamics and demographics of individual outbreaks in wild birds is rare and important epidemiological parameters remain unknown. We present data from a wild population of long-lived birds (mute swans; Cygnus olor) that has experienced three outbreaks of related H5 HPAIVs in the past decade, specifically, H5N1 (2007), H5N8 (2016) and H5N6 (2017). Detailed demographic data were available and intense sampling was conducted before and after the outbreaks; hence the population is unusually suitable for exploring the natural epidemiology, evolution and ecology of HPAIV in wild birds. We show that key epidemiological features remain remarkably consistent across multiple outbreaks, including the timing of virus incursion and outbreak duration, and the presence of a strong age-structure in morbidity that likely arises from an equivalent age-structure in immunological responses. The predictability of these features across a series of outbreaks in a complex natural population is striking and contributes to our understanding of HPAIV in wild birds.

This article is part of the theme issue ‘Modelling infectious disease outbreaks in humans, animals and plants: approaches and important themes’. This issue is linked with the subsequent theme issue ‘Modelling infectious disease outbreaks in humans, animals and plants: epidemic forecasting and control’.

Are Microneutralization and Hemagglutination Inhibition Assays Comparable? Serological Results from Influenza Experimentally Infected Mallard Ducks

The hemagglutination inhibition (HI) assay is commonly used to assess the humoral immune response against influenza A viruses (IAV). However, the microneutralization (MN) assay has been reported to have higher sensitivity when testing sera from humans and other species. Our objective was to determine the agreement between MN and HI assays and compare the proportion of positive samples detected by both methods in sera of mallards primary infected with the A/mallard/MN/Sg-000169/ 2007 (H3N8) virus and subsequently inoculated with homosubtypic or heterosubtypic IAV. Overall, we found poor to fair agreement (prevalence-adjusted bias-adjusted kappa [PABAK], 0.03-0.35) between MN and HI assays in serum samples collected 2 weeks after H3N8 inoculation; the observed agreement increased to moderate or substantial in samples collected 4 to 5 weeks postinoculation (WPI) (PABAK, 0.52-0.75). The MN assay detected a higher proportion of positive samples compared with HI assays in serum samples collected 2 WPI (P = 0.01). This difference was not observed in samples collected 4 WPI. Also, a boosting effect in MN and HI titers was observed when birds were subsequently inoculated with IAV within the same H3 clade. This effect was not observed when birds were challenged with viruses that belong to a different HA clade. In summary, the agreement between assays varies depending on the postinfection sample collection time point and the similarity between the antigens used for the assays. Additionally, subsequent exposure of ducks to homosubtypic or heterosubtypic strains might affect the observed agreement.

LIMITED DETECTION OF ANTIBODIES TO CLADE 2.3.4.4 A/GOOSE/GUANGDONG/1/1996 LINEAGE HIGHLY PATHOGENIC H5 AVIAN INFLUENZA VIRUS IN NORTH AMERICAN WATERFOWL

During 2014, highly pathogenic (HP) influenza A viruses (IAVs) of the A/Goose/Guangdong/1/1996 lineage (GsGD-HP-H5), originating from Asia, were detected in domestic poultry and wild birds in Canada and the US. These clade 2.3.4.4 GsGD-HP-H5 viruses included reassortants possessing North American lineage gene segments; were detected in wild birds in the Pacific, Central, and Mississippi flyways; and caused the largest HP IAV outbreak in poultry in US history. To determine if an antibody response indicative of previous infection with clade 2.3.4.4 GsGD-HP-H5 IAV could be detected in North American wild waterfowl sampled before, during, and after the 2014-15 outbreak, sera from 2,793 geese and 3,715 ducks were tested by blocking enzyme-linked immunosorbent assay and hemagglutination inhibition (HI) tests using both clade 2.3.4.4 GsGD-HPH5 and North American lineage low pathogenic (LP) H5 IAV antigens. We detected an antibody response meeting a comparative titer-based criteria (HI titer observed with 2.3.4.4 GsGD-HP-H5 antigens exceeded the titer observed for LP H5 antigen by two or more dilutions) for previous infection with clade 2.3.4.4 GsGD-HP-H5 IAV in only five birds, one Blue-winged Teal (Spatula discors) sampled during the outbreak and three Mallards (Anas platyrhynchos) and one Canada Goose (Branta canadensis) sampled during the post-outbreak period. These serologic results are consistent with the spatiotemporal extent of the outbreak in wild birds in North America during 2014 and 2015 and limited exposure of waterfowl to GsGD-HP-H5 IAV, particularly in the central and eastern US.

Innate Immune Responses to Avian Influenza Viruses in Ducks and Chickens

Mallard ducks are important natural hosts of low pathogenic avian influenza (LPAI) viruses and many strains circulate in this reservoir and cause little harm. Some strains can be transmitted to other hosts, including chickens, and cause respiratory and systemic disease. Rarely, these highly pathogenic avian influenza (HPAI) viruses cause disease in mallards, while chickens are highly susceptible. The long co-evolution of mallard ducks with influenza viruses has undoubtedly fine-tuned many immunological host–pathogen interactions to confer resistance to disease, which are poorly understood. Here, we compare innate responses to different avian influenza viruses in ducks and chickens to reveal differences that point to potential mechanisms of disease resistance. Mallard ducks are permissive to LPAI replication in their intestinal tissues without overtly compromising their fitness. In contrast, the mallard response to HPAI infection reflects an immediate and robust induction of type I interferon and antiviral interferon stimulated genes, highlighting the importance of the RIG-I pathway. Ducks also appear to limit the duration of the response, particularly of pro-inflammatory cytokine expression. Chickens lack RIG-I, and some modulators of the signaling pathway and may be compromised in initiating an early interferon response, allowing more viral replication and consequent damage. We review current knowledge about innate response mediators to influenza infection in mallard ducks compared to chickens to gain insight into protective immune responses, and open questions for future research.