Psittacine beak and feather disease (PBFD)
Psittacine beak and feather disease (PBFD) is caused by a circovirus. It has a wide species range although it appears to be a natural virus infection of cockatoos in Australasia where it occurs in wild flocks. It has been known in wild cockatoos in Australia for many years and recently Ducorps cockatoos from the Solomon Islands have also been found to be infected. Old world parrots seem most prone to clinical disease although other psittacines can also be affected. A related virus can cause disease in pigeons and doves.
Circoviruses are small non enveloped icosahedral viruses which have circular single stranded DNA.
Incubation of the virus can be as short as three weeks, depending on the amount of virus the bird was infected by, the age and the health status of the bird.
Primary replication of the virus is thought to occur in the Bursa of Fabricius and the gastrointestinal tract lymphoid tissue. It is the destruction of the bursa which weakens the immune system and causes the bird to be more susceptible to secondary infections. Secondary viral replication then occurs in the liver and thymus.
The PBFD virus is extremely infectious. It can be passed through a colony of birds in two ways:
- Ingestion or inhalation from infected material such as faeces, feather dust, crop secretions and from infected surfaces or equipment.
- Vertical transmission from hen to egg embryo.
The severity of the disease is dependant on the age, health, and breed of the bird. Young birds tend to get an acute form of the disease, or a peracute form if they are really unlucky. Older birds with weak immune systems are most likely to get a chronic infection, older birds with a healthy immune system may simply have a transient infection.
The symptoms vary depending on which form of disease the bird has. In acute cases the main action of the disease is to destroy the cells of the beak, feather and immune system. this causes abnormal feather loss and replacement feathers to be malformed, with lesions on the feather shaft. Skin cells grow more rapidly and the outermost layer of skin can become thickened. In peracute cases birds can suffer additional symptoms such as depression, anorexia, crop stasis and diarrhoea.
Symptoms presented by birds with a chronic infection can be observed through a succession of moults. Feathers stop growing shortly after emerging from the follicle. Birds with sub-clinical or transient infections may not present any symptoms.