Impact of the implementation of rest days in live bird markets on the dynamics of H5N1 highly pathogenic avian influenza

Abstract

Live bird markets (LBMs) act as a network 'hub' and potential reservoir of infection for domestic poultry. They may therefore be responsible for sustaining H5N1 highly pathogenic avian influenza (HPAI) virus circulation within the poultry sector, and thus a suitable target for implementing control strategies. We developed a stochastic transmission model to understand how market functioning impacts on the transmission dynamics. We then investigated the potential for rest days-periods during which markets are emptied and disinfected-to modulate the dynamics of H5N1 HPAI within the poultry sector using a stochastic meta-population model. Our results suggest that under plausible parameter scenarios, HPAI H5N1 could be sustained silently within LBMs with the time spent by poultry in markets and the frequency of introduction of new susceptible birds' dominant factors determining sustained silent spread. Compared with interventions applied in farms (i.e. stamping out, vaccination), our model shows that frequent rest days are an effective means to reduce HPAI transmission. Furthermore, our model predicts that full market closure would be only slightly more effective than rest days to reduce transmission. Strategies applied within markets could thus help to control transmission of the disease.

Figure 1.

Illustration of the within-market model. (a) Probability that a bird introduced at time t = 0 remains in the market as a function of time. In green: period during which the market is open for trading. In red: period during which the market is closed for trading. (b) Within-market SEI model. (c) Assumed distribution of the latent period (solid line, latent period). (d) Assumed distribution of the infectious period (dotted line, infectious period).

Figure 2.

Diagram of the poultry sector and routes of disease transmission in the meta-population model. (a) Diagram of the poultry sector. All farms (red square) are clustered around regional markets (yellow circle). Poultry movements are from farms to the wholesale market (red circle) and from the wholesale to the retail markets. (b) A given farm can be infected by farms located in the same cluster (red dotted arrow, relative strength of mixing: γ_ff), by farms in other clusters (red dashed arrow, Γ_ff), by a market in the same cluster (yellow dotted arrow, γ_fm) and by markets in other clusters (yellow dashed arrow, Γ_fm). (c) A given regional market can be infected by the introduction of infected birds from the wholesale market, by markets in other clusters (orange dotted–dashed arrow, Γ_mm), by farms in the same cluster (yellow dotted arrow, γ_fm) and by farms in other clusters (yellow dashed arrow, Γ_fm).

Figure 3.

Prevalence of infectious and symptomatic birds in the market under four different scenarios. (a) T_r = 1.5 days, introduction of a cohort every day. (b) T_r = 1.5 days, introduction of a cohort every 2 days. (c) T_r = 2.5 days, introduction of a cohort every day. (d) T_r=2.5 days, introduction of a cohort every 2 days. In all scenarios the mean prevalence P^m is 19.5%, the corresponding reproduction number assuming the same β and η is equal to (a) 16.8, (b) 35.1, (c) 6.4, (d) 9.2. Each graph shows the mean prevalence of infectious birds at each time step (solid blue line), the mean prevalence of infectious birds during the open period (dotted violet line) and the mean prevalence of symptomatic birds for each time step (solid orange line). 95% bounds from the stochastic realizations closely follow the mean and so are not presented.

Figure 4.

Comparison of the effects of stamping out, vaccination and rest days on the reproduction number, r. (a,d,g) For f/m = 0.05 almost all farm outbreaks are owing to market-to-farm transmission. (b,e,h) For f/m = 0.5 most farm outbreaks are owing to market-to-farm transmission. (c,f,i) For f/m = 2 most farm outbreaks are owing to farm-to-farm transmission. (a,b,c) For market 1, the mean prevalence at the market P^m is 19.5%, and the corresponding basic reproduction number assuming the same β and η is 16.8. (d,e,f) For market 2, P^m = 37.8% and = 40. (g,h,i) For market 3, P^m =43.1% and = 80. Cull., culling out; vacc., vaccination; 6m, rest days every six months; 1w, rest days once a week; cl, the market chain is closed.