- The most common way in which the virus is spread is that it is shed from fowl pox wounds on affected birds and enters its next victim through skin wounds. (Normal healthy skin will act as an effective barrier to fowl pox virus).
- Biting insects can carry the virus from one bird to another.
- Pox viruses that infect mammals can be spread through the air and it is thought that this may be a way for fowl pox to be transmitted.
Photo of Fowl pox on a broiler chicken, courtesy of Paul E. Shkarlat. Aviagen LLC.Once in the skin the virus reproduces to cause ‘pocks’. These start off as raised areas which then burst releasing a tiny amount of pus to leave scab-like crusted raised areas which are often a brownish colour. These wounds are mostly seen on featherless skin such as the legs, comb, eye lids and wattles. This is called the ‘dry form’ of fowl pox. Signs of the ‘dry form’ are raised crusted areas on the featherless areas of the bird causing some mild irritation. These birds may stop laying and lose weight. In severe cases if the eye lids are affected the birds can find it hard to open their eyes. Birds with the ‘dry form’ rarely die. After about 14 days the pocks heal but can sometimes leave a tiny scar. Much more rarely the virus can infect the mouth, the oesophagus (food pipe) and the trachea (windpipe) initially to form white lumps in these areas but subsequently these areas join together to create a layer of mucus, pus and dead cells from the mouth lining. This is called the ’wet form’. This mucus/pus/dead cells can obstruct the trachea (windpipe) making it difficult for the birds to breathe (in severe cases choking the bird) and it can be very painful making it hard for the bird to eat. Birds with the ‘wet form’ if severe enough can die. Once a bird in a flock has been infected the virus will spread very slowly but because of this it may be several weeks before it has infected all the birds.