Depopulation or culling is one of the primary control measures for fast moving diseases such as avian influenza virus (AIV) or Exotic Newcastle disease (END). Vaccination can be used to reduce clinical signs, but experience in Asia during the recent H5N1 outbreaks supports evidence that vaccination alone will not be sufficient.

Depopulation may also be required after structural collapse of poultry housing after a major storm event (hurricane, tornado, etc.). In those situations, birds often survive the collapse, but the feeding and watering systems are damaged.

Depopulation or culling is used to remove infected birds, birds suspected of infection, or to create a poultry free zone during an infection. During a mass emergency such as this, depopulation needs to balance human health, bird welfare, and disease management. Procedures suitable for depopulating individual animals in laboratory conditions are not suitable for mass emergency events. During mass emergency events, a distinction is made between euthanasia, depopulation, and culling.

The 2007 AVMA Guidelines on Euthanasia devotes only one paragraph on mass euthanasia which states “Under unusual conditions, such as disease eradication and natural disasters, euthanasia options may be limited. In these situations, the most appropriate technique that minimizes human and animal health concerns must be used.”

Gassing, typically using carbon dioxide (CO2) gas, is one of the accepted mass emergency depopulation procedures. Gassing can be: a) whole house; b) partial house; or c) containerized.

In whole house gassing, the house is sealed and the entire house is filled with CO2 gas. Typically, the process take several hours to seal the house and inject the CO2 gas. Depending on the size of the house, it may take between 5 to 15 tons of CO2.Whole housing gassing is suitable for houses that can be tightly sealed, which are more common in colder regions. Ryan et al. (2006) and Wrigley et al. (2008) report on two whole house gassing experiments conducted in Ireland.

In partial house gassing, the area to be gassed is reduced. The birds may be herded (when possible) into a smaller area or a chamber created within the house to reduce the amount of gas required. Broilers and sick birds of most species do not herd well. Several approaches have been used to varying degrees of success, but they require large numbers of experienced workers to be successful. Kingston et al. (2005) reported on a panel enclosure method could be used for batches of up to 5,500 turkeys at a time, required an average of 6 min 20s for cessation of activity.  The procedure required 7 workers approximately 2 hours to construct and implement the procedure in a typical turkey house.

In contanerized gassing, birds are hand caught (the normal means of catching birds prior to processing) and the birds are placed in transport containers. The transport containers are then placed in a sealed container and gassed. This method requires workers to handle the birds repeatedly, but offers good control of the gas. UK DEFRA has standardized a containerized gassing method as part of emergency poultry disease response.

A number of other techniques have been suggested or used during poultry disease outbreaks.  Overdose of anesthetic or poisonous agents in the feed or water can be used to sedate or kill poultry in houses or in the field.  Sick birds do not eat or drink as normal. Field reports indicate that this procedure may not be reliable.  In Israel in 2006, birds were culled by administering organophosphate poison in the flock’s drinking water after 24 hours of water deprivation.  This process was not satisfactory since some birds survived and had to be manually slaughtered.

One of the challenges of outbreak response is managing the human health concerns of the personnel involved.  Protecting the personnel involved requires minimizing the number and activity of those involved, and selecting appropriate personal protective equipment. 

Maximizing biosecurity is essential in containing an outbreak.  To maximize biosecurity, depopulation should be conducted on farm and in the animal housing facility.  Moving the birds outside the house or off farm decreases biosecurity and increases the chance of spread.  The depopulation method should be selected to be compatible with the disposal method.

There is no ideal depopulation method. Because of the problems associated with the current gassing methods, a water based foam depopulation method was developed.

Additional resources on mass emergency depopulation are available through a password protected portion of the Avian Bioscience Center. For access, email Dr. Eric Benson with your creditials and application.

In addition, depopulation training is included Emergency Poultry Disease Response certification course.