The Effect of Age on Avian Influenza Viral Shedding in Mallards (Anas platyrhynchos)

Abstract

Avian influenza virus (AIV) prevalence in wild aquatic bird populations varies with season, geographic location, host species, and age. It is not clear how age at infection affects the extent of viral shedding. To better understand the influence of age at infection on viral shedding of wild bird-origin low pathogenicity avian influenza (LPAI) viruses, mallards (Anas platyrhynchos) of increasing age (2 wk, 1 mo, 2 mo, 3 mo, and 4 mo) were experimentally inoculated via choanal cleft with a 10(6) median embryo infectious dose (EID50) of either A/Mallard/MN/355779/00 (H5N2) or A/Mallard/MN/199106/99 (H3N8). Exposed birds in all five age groups were infected by both AIV isolates and excreted virus via the oropharynx and cloaca. The 1-month and older groups consistently shed virus from 1 to 4 d post inoculation (dpi), whereas, viral shedding was delayed by 1 d in the 2-wk-old group. Past 4 dpi, viral shedding in all groups varied between individual birds, but virus was isolated from some birds in each group up to 21 dpi when the trial was terminated. The 1-mo-old group had the most productive shedding with a higher number of cloacal swabs that tested positive for virus over the study period and lower cycle threshold values on real-time reverse-transcription PCR. The viral shedding pattern observed in this study suggests that, although mallards from different age groups can become infected and shed LPAI viruses, age at time of infection might have an effect on the extent of viral shedding and thereby impact transmission of LPAI viruses within the wild bird reservoir system. This information may help us better understand the natural history of these viruses, interpret field and experimental data, and plan future experimental trials.

Reassortant influenza A viruses in wild duck populations: effects on viral shedding and persistence in water

“…Because this individual may have been infected from contact transmission within the isolation unit rather than from the primary inoculation, this duck was excluded from the statistical analyses. Viruses were successfully isolated from both CL and OP swabs collected from all ducks inoculated with H3N8-C (control group) during the first 5 days postinfection, consistent with a previous study performed in one-month-old mallards with the same virus strain [38]. Overall, a substantial agreement was found between viruses isolated in embryonating chicken eggs and detected by RT-PCR (85.8% agreement, k ¼ 0.70), indicating that when a virus was detected by PCR it was also likely successfully isolated.…”

“…The absence of variation between virus genotypes contrasts with host-related effects that can significantly affect the outcome of virus shedding. Recent studies have for instance highlighted the effect of bird host species [40], age [38,41], body condition [42] and preexposure to other IAV [43,44], in the CL shedding duration and viral load. Such host-related variations may have direct implications in the transmission dynamics of IAV in wild duck populations.…”

“…The total shedding duration accounts for the intermittent viral shedding sometimes reported in Mallard [38], whereas the restricted shedding duration considers only the continuous excretion. Separate ANOVAs were performed for the CL and OP shedding durations.…”

Identifying crucial gaps in our knowledge of the life-history of avian influenza viruses—an Australian perspective

“…Also LPAI infections lacking clinical signs may nevertheless have an effect on the behaviour of free-living birds and, thus, transmission rate (van Gils et al 2007;Latorre-Margalef et al 2009). Further, the number of secondary infections caused by one infected individual (related to the amount of virus being shed and the duration of infection) may differ between individuals owing to variation in condition, immune function, infection history and age, although this has received little empirical attention (Sims 2006;Costa et al 2010;see above). Most contemporary epidemiological models assume a single transmission coefficient and, as such, assume uniform contact rates, susceptibility and infectiousness across all individuals in a population, but it is increasingly realised that this may be a too coarse an approximation (McCallum et al 2001;Beldomenico and Begon 2010).…”

Infectivity of Avian Influenza Virus-Positive Field Samples for Mallards: What Do Our Diagnostic Results Mean?

“…Oropharyngeal and cloacal swabs were collected from each Mallard on 0, 2, and 4 DPI and placed in separate vials containing 2.0 mL of BHI medium with antimicrobial drugs (250 mg/mL gentamicin, 500 mg/mL kanamycin, 1,000 mg/mL streptomycin, 1,000 units/mL penicillin G, and 25 mg/mL amphotericin B) for VI in embryonating chickens eggs . The timing of these sample collections was based on data from prior experimental challenge studies with lowpathogenic (LP) AI viruses in agematched Mallards (Costa et al, 2010;Brown, pers. comm.…”