Chlamydiosis (Psittacosis, Ornithosis)
Dr. Branson Ritchie
Chlamydia psittaci is the causative agent of psittacosis. It is classified as a bacteria even though it can only replicate when inside a cell, which is characteristic of a virus. Most species of free-ranging and domestic birds are considered to be susceptible to some strain of C. psittaci, and some strains of C. psittaci have also been associated with disease in humans, cats, koala bears, marine mammals, sheep and many other mammals.
Humans are susceptible to at least three different species of chlamydia (C. psittaci, C. pneumonia and C. trachomatis). Until the 1980s, C. psittaci was considered a relatively common cause of upper respiratory tract disease in humans. In the 1980s, researchers found that most cases of suspected C. psittaci in humans were actually caused by C. pneumonia. Thus, literature concerning C. psittaci infections in humans prior to the 1980s is of limited value. C. psittaci and C. pneumonia both cause a flu-like illness in humans.
With respect to birds, the organism is most commonly recovered from psittacine birds, pigeons and doves. Were it not for the potential for this organism to occasionally be passed from birds to humans, it is probable that C. psittaci would be viewed as any other treatable bacteria in birds. Chlamydia infections can cause varied problems and/or clinical changes depending on the strain and quantity of organism to which a bird is exposed and the species, age and condition of the bird.
The fact that many healthy birds have antibodies against this organism suggests that most birds exposed to chlamydia mount an appropriate immune response that prevents the organism from causing progressive disease. Some infected birds will die shortly after clinical changes are first noted, while others can develop a mild disease and shed the organism for an extended period.
Chlamydiosis causes symptoms that are similar to those associated with many bacterial, fungal or viral infections. Mycobacteriosis, aspergillosis and mycoplasmosis cause particularly similar clinical changes. Toxins that affect the liver or digestive tract can also cause clinical signs similar to chlamydiosis. Chlamydiosis should be considered in any bird with clinical signs that suggest damage to the respiratory or digestive tracts. These include difficult breathing, depression, weight loss, loss of appetite, diarrhea, nasal discharge, swelling of the tissues around the eyes or discharge from the eyes. Yellowish or greenish discoloration of the urates and urine is a common symptom. This discoloration is not specific to Chlamydia, however, since it can be seen anytime cells in the liver are damaged.
Chlamydia is most frequently transmitted through direct contact with feces, or ocular or nasal secretions from an actively infected bird. This organism can remain infectious in dried feces for several months, and contaminated enclosures, toys, perches and food bowels can serve as a source for indirect exposure to the organism. The most severe outbreaks of disease typically occur in crowded conditions where numerous birds congregate. It has been suggested that chlamydia may be shed in the feces for several days to a week before a bird develops obvious signs of disease. The reported incubation period ranges from 5 to 42 days.