Fig 1.
‘Single-event’ transmission from source farm i to receiver farm j, both located outside the compartment (i.e. represented by a dot), compared to ‘triple-event ‘ transmission from source farm i to receiver farm j via farms m and n, both located inside the compartment (i.e. represented by a star).
Thick lines represent HPAI transmission by the combination of neighbourhood transmission and by between-herd contact events. Thin lines represent HPAI transmission by neighbourhood transmission only. Circles represent the 10 km surveillance zone around the compartment farms m and n. DPPA means Densely Populated Poultry livestock Area. Shown are the possibilities that the compartment farms (m and n) are situated outside the DPPA (Fig 1A) and that one compartment farm (farm m) is located within the DPPA (Fig 1B).
Fig 2.
The location of the 5 layer farms and the hatchery of the compartment of VPI and that of other commercial poultry farms in the Netherlands (database of 2008, representative of the situation when the status of the example compartment VPI was granted and discussed in the Netherlands).
For more details on number, density and type of poultry farms in 2008, see [3]. The DPPA in the central part of the country has a density of 0.97 poultry farms per km2 (Gelderse Vallei).
Table 1.
Overview of between-farm contact types within the VPI compartment, their frequency, and the probability of HPAI transmission per contact, as estimated from the H7N7 HPAI epidemic in the Netherlands in 2003.
By multiplication of these two data, the transmission rate was calculated.
Table 2.
The risk of HPAI transmission jumps from one area to another, due to a compartment with only egg transports (i.e. no animal transports), and with no compartment farms situated in a DPPA.
Table 3.
The risk of HPAI transmission jumps from one area to another, due to a compartment with one farm located in the DPPA.
Table 4.
The risk of HPAI transmission jumps from one area to another, due to a compartment with one farm located in the DPPA and with transport of animals (chicken) and eggs.
Table 5.
Relative reduction (in %) of reproduction number R0,m of the VPI compartment farms as function of the reduction in transmission probability (10, 50, 90%) of a specific route, due to biosecurity measures within the compartment.
Fig 3.
Reproduction number R0,m of VPI compartment farms, when all transmission probabilities of the contact types of Table 1 are reduced with a certain percentage, due to improvement of biosecurity measures within the compartment.
A biosecurity improvement of 0% represents the biosecurity status of poultry farms during the HPAI epidemic in the Netherlands in 2003. A biosecurity improvement of 100% is a purely fictitious scenario of perfect biosecurity leading to zero risk.
Fig 4.
Risks of HPAI transmission jumps expressed as ratio between ‘triple-event’ transmission via the compartment and ‘single-event’ transmission, as function of the distance of the compartment farm to the nearest farm in the DPPA (here Gelderse Vallei, in the central part of the Netherlands).
Shown is a rolling average plot calculated with an averaging window of 5 km., based on a set of 10,000 pairs {distance, ratio} (see M&M section). The rolling average of the mean (blue line) and 95% percentile (green line) are presented. The 5% percentile is equal to 0 throughout.