Chlamydia psittaci is a strain of chlamydia that can lead to a rare flu-like condition called psittacosis, ornithosis or "parrot fever". Humans usually get parrot fever from parrots (as the name implies) and other birds such as chickens, turkey, pigeons, and ducks when they handle the bird, breathe fine particles of the bird’s urine, dried faeces or other body excretions, touch their mouth to the bird’s beak, and/or get bitten by a bird. Exotic caged birds are a particular risk. The usual signs of parrot fever are: fever, headache and chills; nausea and vomiting; muscle, joint, and chest pain; weakness and fatigue; diarrhoea; dry cough; shortness of breath; intolerance to light. Infection can in principle be passed from man to man, but this results in a considerable decline in virulence.
Ornithosis is an infection communicable by animals to man (zoonosis), caused by an organism belonging to the Chlamydia group. As carriers, parrots were first recognized; later, procellarians, pigeons, song birds and poultry (ducks, chickens, turkeys) were also incriminated. However, neither from the microbiological nor from the clinical standpoint is it possible to differentiate between the carriers. Consequently today it is no longer justifiable to distinguish between psittacosis (the disease communicated by psittacine birds) and ornithoses (infections communicated by other birds). As a particular form of ornithosis, psittacosis is now largely of historical interest only.
The CDC reports that since 1996, less than 50 cases occur per year in the U.S., however, some instances may have been unreported or undiagnosed.
Amongst pet birds, cockatiels have the highest incidence of psittacosis. Ducks, turkeys and pigeons are also major sources of the disease in man. Infection from these sources has been reported among farmers, poultry processors and rendering-plant workers, some of whom have been severely infected more than once. Several outbreaks of an endemic nature have been reported in workers at poultry slaughterhouses, particularly in persons working in air containing plumage dust. Agents prevalent in birds are highly contagious and virulent for man. The fact that there are, in animals and man, serologically related strains of Chlamydia which show relatively low virulence may explain why, in spite of a very high incidence of Chlamydia infection in the population (30-40%), only relatively few cases of actual disease occur.