It is a disease which can affect most groups of birds, hence the veterinary preference to call it Chlamidiosis, although most cases of infection of humans have come from parrots. Since most aviculturalists are more at home with Psittacosis, I will continue to use that name.
Recognition of the disease in birds can be difficult without pathology testing. A bird sick with Psittacosis can show one or several of the following symptoms; eye discharge or swelling, laboured breathing, diarrhoea, poor appetite, lethargy, "fluffed up" appearance, or weakness. Most of these are shared by many other diseases, and identification of the problem is often mainly by a process of elimination. The one symptom which is suggestive of this rather than other diseases is eye discharge, and any bird with an eye discharge should be considered for Psittacosis. Pathology tests are available so it is vital that you consult your avian vet at the earliest convenience.
Not surprisingly with several hundred birds in stock at any one time, and from a wide variety of sources, we get the occasional case in our pet shop. Perhaps our first experience of the disease was a Bourke Parrot which looked a picture of health when purchased out of a holding cage. However when placed into a free-flight aviary I discovered it couldn't fly, even though it was not thin, had a clean vent, and appeared perfectly healthy. At that time it was isolated, and in-house tests done for worms, Coccidiosis, megabacteria and gram negative bacteria, all of which proved negative. It continued to eat well, had no diarrhoea, but gradually got thinner while tests by us and our vet continued to fail to find a reason for his decline. Finally an eye discharge appeared when it was near death's door, and a test for Psittacosis done, which proved positive. In hindsight of course we should have suspected Psittacosis sooner, but I had never experienced the disease until then, and our vet at that time was not an avian specialist. These days, with experience, and access to a good avian vet, we pick up any case much earlier.
Certain groups of birds seem more prone to the problem, particularly Neophemas, and especially Bourkes and Scarlet-chested Neophemas. Cockatiels are also very prone to Psittacosis, in Australia at least, but I have never personally had a case in either budgies, Rosellas or Asiatics, while it seems rare, again from my personal experience, in members of the Polytelis genus, lovebirds and cockatoos.
Infection is usually through the droppings of a carrier bird. The organism can be present in large numbers and can remain virulent for several months in dried droppings. It may also be passed through feather dust or even from hen to egg. A major problem in controlling the disease is that a bird may often be a carrier, capable of spreading the disease, but not itself showing any symptoms. Wild birds have been shown to commonly be carriers and are a possible infection source for outdoor aviaries. An outbreak of the disease is typically related to stress, when a carrier can start to shed large numbers of the organism. Overcrowding is a likely cause of such stress, and those pet shops with poor, overcrowded holding facilities for their birds are particularly liable to suffer outbreaks. Like many diseases, if your birds are kept in good condition, with minimal stress, then problems with Psittacosis are much less likely to happen.
Treatment is usually by the antibiotic doxycycline, however some newer drugs are also proving of value. Before the development of doxycycline the oxytetracycline and chlortetracycline group were used but have now largely been replaced. All of these anitbiotics are available only from a vet in Australia at least, so it is vital that you consult with your vet if the disease is suspected, both for diagnosis and treatment. Doxycycline may be given either through the drinking water or by weekly injection for 6 weeks. Treatment through the drinking water is for 45 days, and all sources of calcium need to be removed for this period, as calcium affects the uptake of the antibiotic. Treatment by weekly injection is a more effective way of delivering the drug, however it does cause some traumatising of the injection site, and care is needed.
Provided the bird is not too sick before treatment is sought, an apparently complete recovery usually results. Unfortunately the bird can still sometimes be a carrier even if seeming to be cured so further tests are needed at the end of the course of doxycycline to make sure the organism is completely eliminated before being allowed contact with the rest of your birds.
Prevention is probably most effective by using good aviary practices to ensure your birds are under as little stress as possible, as well as, if possible, quarantining all new birds for up to 45 days.
The symptoms in aviculturalists I can relate from first hand experience, having recently suffered an attack myself. They are usually similar to influenza, with lethargy, as well as fevers (mine up to 40oC, 104oF), chest pains (which I only had when coughing), headaches, dry cough and nausea. Unlike flu, sore throats, runny nose, and large amounts of mucus are not usually present. If you ever see a doctor with flu-like symptoms, be sure to mention that you are in regular contact with aviary or pet birds, and that Psittacosis is a possibility. A simple blood test can be done to positively diagnose the disease, though isn't 100% reliable, and treatment is very effective using antibiotics. Mine improved within a day of starting the antibiotic course. Be sure to mention Psittacosis as well as Chlamydia though, since a closely related type of Chlamydia is a common venereal transmitted disease, and you don't want to give the wrong impression! If you ignore the problem it can at times develop into pneumonia, and may be life threatening, although in many cases it probably eventually fades without treatment if you are the strong and healthy type. It could be more serious in children or the over 50's. Immunity is not given by having the disease, so a repeat dose is always possible.
So, Psittacosis is an important bird disease to be aware of. It is one of the very good reasons to quarantine any new birds from your existing flock for several weeks, and there is a slight risk you can catch the disease. I do emphasise the slight though, after all, I am surrounded in our shop by hundreds of birds, 7 days a week, with new birds constantly appearing, some of which have developed Psittacosis, and I have had only a single attack in the last 6 years. My family, also in constant contact with these birds, have never had an infection.
P.S. The little Bourke at death's door recovered after treatment, but never put on weight, probably due to liver damage, and never flew. He happily lived at home with us after testing showed him to be clear, and delighted us with the Bourke's beautiful song for over three years before a probable heart attack took him from us.