Quantitative Risk Assessment of Wind-Supported Transmission of Highly Pathogenic Avian Influenza Virus to Dutch Poultry Farms via Fecal Particles from Infected Wild Birds in the Environment

Abstract

A quantitative microbial risk assessment model was developed to estimate the probability that the aerosolization of fecal droppings from wild birds in the vicinity of poultry farms would result in the infection of indoor-housed poultry with highly pathogenic avian influenza virus (HPAIv) in the Netherlands. Model input parameters were sourced from the scientific literature and experimental data. The availability of data was diverse across input parameters, and especially parameters on the aerosolization of fecal droppings, survival of HPAIv and dispersal of aerosols were uncertain. Model results indicated that the daily probability of infection of a single poultry farm is very low, with a median value of 7.5 × 10^-9. Accounting for the total number of poultry farms and the length of the bird-flu season, the median overall probability of at least one HPAIv-infected poultry farm during the bird-flu season is 2.2 × 10^-3 (approximately once every 455 years). This is an overall estimate, averaged over different farm types, virus strains and wild bird species, and results indicate that uncertainty is relatively high. Based on these model results, we conclude that it is unlikely that this introduction route plays an important role in the occurrence of HPAIv outbreaks in indoor-housed poultry.

Keywords: HPAI; QMRA; aerosolization; fecal droppings; wild birds; wind.

Figure 1

Outline of the quantitative microbial risk assessment model to estimate the HPAI transmission risk from wild birds to domestic poultry via aerosolized fecal droppings.

Figure 2

Box-and-whisker plot of model results for the daily probability of infection of a single poultry farm ($P_{inf,pf,d}$) and the overall probability of at least one infected poultry farm during the bird-flu season ($P_{inf}$).

Figure 3

Spider plot showing the relation between the median overall probability of at least one infected poultry farm during the bird-flu season ($P_{inf}$) and the percentile values of input parameters that had a correlation coefficient > |0.1| with $P_{inf}$. These input parameters were: fraction of virus retained after the dispersion of aerosols over a short distance ($F_{disp}$); concentration of HPAIv in wild bird feces (${AI}_{fec\_conc}$); fraction of the day that wild birds spent at the farm yard ($F_{pf}$); number of wild birds at the farm yard on a day that birds are present ($N_{wb}$); bird infectious dose (${BID}_{50}$); ventilation rate of poultry house ($VR$); and daily probability that wild birds are present at the farm ($P_{wb}$).

Figure 4

Tornado chart showing the relative increase or decrease (expressed as log₁₀ difference) in the overall probability of at least one infected poultry farm during the bird-flu season ($P_{inf}$) compared to the baseline scenario for 10 what-if scenarios. Parameters considered in the what-if scenarios were: apparent HPAI prevalence in wild birds (${Prev}_{wb}$); daily amount of feces excreted by wild birds (${Fec}_{wb\_dry,d}$); concentration of HPAIv in wild bird feces (${AI}_{fec\_conc}$); survival of HPAIv during the drying of feces ($F_{surv\_dry}$); survival of HPAIv during air transport ($F_{surv\_trans}$); fraction of virus retained after the dispersion of aerosols over a short distance ($F_{disp}$); ventilation rate of poultry house ($VR$); and bird infectious dose (${BID}_{50}$). The arrows indicate an increase (↑) or a decrease (↓) of the input parameter’s value. A more detailed description of each scenario is given in Table 2.