The current outbreaks of H5N8 and H5N2 in Europe, North America and Asia it is important to implement high level bio security measures at farms to prevent production animals becoming infected. Once a farm is infected, culling entire flock is the only option to prevent further spreading with devastating consequences for the industry.
During the 2003 outbreaks of H7N7 in Holland the preferred culling technique was whole house gassing, also known as stable gassing.
Practical experience has shown that, in whole house gassing, the birds start to die after approximately 35 minutes and the entire operation ends after 2 to 3 hours. Killing by whole house gassing is most suited to large flocks in floor management systems. Under certain circumstances, it is possible to use whole house gassing in cage or aviary housing.
From the 1.100 farms that have been eradicated, stable gassing has been the technique of choice on 568 farms (51,6% of all farms), culling in total 21.740.000 birds (74,3% of all poultry). On 55 farms (10,7%), Carbon Monoxide (CO) has been used; on 513 farms (90,3%), Carbon Dioxide (CO2) was used.
Before the technique can be applied, the house must be well sealed. The screens and shuttering in an open house mean that it is also possible to make these houses gas-tight in order to carry out whole house gassing. The ventilation is switched off immediately before gassing.
The principle of stable gassing using CO2 is Hypoxia: displacement of atmospheric air by at least 70% , CO2 by volume. The gas is pumped into the house at high pressure and slowly fills the space. Stable gassing is a complicated technique to apply because it is difficult to measure whether the minimum concentration is achieved the animals can be stunned and killed and it is difficult to measure the concentration accurately. Therefore the total amount of gas that is pumped in resembles at least 2 to 3 time the volume of the house.
In order to fill the house with gas, the air in the house has to be replaced, exiting through any channel it can find. Most of the time, that is through ventilation at the top of the building or through cracks in the walls and the roof. The displaced air also causes (possibly) contaminated farm-dust particles and feathers deposited into the open air.
After the poultry is culled and before the dead birds can be collected safely, the house must be ventilated for approximately three hours to ensure rapid and complete removal of the gas from the house air. This ventilation also causes contaminated farm-dust particles from inside the house to deposit to the open air.
To understand the risks of spreading contaminated materials caused by stable gassing, a quantitative understanding of the spread of contaminated farm dust between locations is a prerequisite for obtaining much-needed insight into one of the possible mechanisms of disease spread between farms.
The researchers Amos Ssematimba, Thomas J. Hagenaars, Mart C. M. de Jong of the Dutch Department of Epidemiology, Crisis Organization and Diagnostics, Central Veterinary Institute (CVI) part of Wageningen University and Research Centre, Lelystad, The Netherlands, and Quantitative Veterinary Epidemiology, Department of Animal Sciences, Wageningen University, Wageningen, The Netherland developed a model to calculate the quantity of contaminated farm-dust particles deposited at various locations downwind of a source farm and apply the model to assess the possible contribution of the wind-borne route to the transmission of Highly Pathogenic Avian Influenza virus (HPAI) during the 2003 epidemic in the Netherlands.
The model is obtained from a Gaussian Plume Model by incorporating the dust deposition process, pathogen decay, and a model for the infection process on exposed farms. Using poultry- and avian influenza-specific parameter values we calculate the distance-dependent probability of between-farm transmission by this route. A comparison between the transmission risk pattern predicted by the model and the pattern observed during the 2003 epidemic reveals that the wind-borne route alone is insufficient to explain the observations although it could contribute substantially to the spread over short distance ranges, for example, explaining 24% of the transmission over distances up to 25 km.