Background Information on Psittacosis

Psittacosis is an infection of birds caused by the bacterium Chlamydophila (formerly called Chlamydia) psittaci. It has been described in over 130 species of birds, most commonly in psittacines (for example, parrots, parakeets, budgerigars and cockatiels). Other commonly affected birds include pigeons and doves. Chlamydial infections in birds are important as they represent a biological hazard to human health, as well as economic loss to the poultry industry and the pet bird retail trade. C. psittaci can be transmitted from birds to humans; the resulting infection is also known as psittacosis, ornithosis or parrot fever.

Transmission

Apparently healthy birds may shed Chlamydophila which can then infect both other birds and humans. Infectious organisms in avian respiratory secretions or faeces can remain viable for several months. Transmission of disease occurs mainly through inhalation of aerosols, respiratory secretions, or dried faecal or feather dust, but oral infection and handling of infected birds' plumage and tissues are alternative routes. Those at greatest risk of contracting the disease include bird fanciers and owners of pet birds. Pet shop employees, and others whose occupation places them at risk of exposure (e.g., poultry slaughter-house workers, veterinarians, wildlife and zoo workers) are also at risk. Because human infection can result from brief, passing exposure to infected birds or their dried contaminated droppings, people with no identified occupational or recreational risk can become infected.

Infection in humans

Psittacosis typically causes influenza-like symptoms, and can lead to severe pneumonia and non-respiratory health problems. It is an acute generalised respiratory disease with an incubation period of between one and four weeks. Clinical presentations may be variable, with fever, headache, myalgia (muscle aches) and upper or lower respiratory tract symptoms. An extensive atypical pneumonia may be demonstrated radiologically. This usually has a characteristic distribution in lung fields which is distinct from other causes of pneumonia.. Although human disease may often be mild or moderate, it can be severe especially when untreated in elderly or immunocompromised individuals. Psittacosis can be difficult to diagnose and cases, especially of mild infection, often go unreported.

Epidemiology of the disease in humans

As with many zoonotic (acquired from animals) infections, psittacosis is likely to be under diagnosed and laboratory reports of Chlamydophila psittaci are likely to underestimate the true prevalence of human infection. Domestic bird contact is considered to be responsible for a significant number of the cases reported by laboratories in England and Wales, although the likely source of infection may be difficult to identify even following full local investigation. Growth of the worldwide trade in pet birds after World War II may have served to increase the incidence of human psittacosis from that source. The increase in reports of human C. psittaci infection observed during the 1980's coincided with a number of reports of human cases associated with duck processing plants.

There has been a steady decline in reported cases in England and Wales since 2000. Sixty-one laboratory confirmed cases of psittacosis were reported in 2008.

Diagnosis of the disease in humans

Diagnosis of the disease in humans is confirmed by serological testing in which paired sera (blood samples collected two weeks apart) are tested for chlamydial antibodies. A diagnosis of psittacosis should be considered in patients with appropriate symptoms who have a history of exposure to birds and elevated or increasing antibody titres.

Because the antibody test is not species-specific, high titres can result from infections with related organisms including Chlamydophila pneumoniae and Chlamydia trachomatis. Acute phase serum specimens should be collected as soon as possible after onset of symptoms, with 'convalescent phase' samples two weeks later. If the patient's epidemiological and clinical history indicates a possible diagnosis of psittacosis, molecular assays (polymerase chain reaction [PCR]) can be used to distinguish C. psittaci infection from that due to other chlamydial species.

Laboratory services

The disease in birds

There are 8 known serovars (variants) of Chlamydophila psittaci, and each is associated with a different Order or group of birds e.g., serovars A and F in psittacines, B in pigeons and doves, and C in ducks and geese.

Psittacines are physiologically able to cope adequately with the infection (unlike other Orders of birds where there is often a history of the disease in the birds themselves) only developing signs of the disease when severely stressed. C. psittaci is excreted in the faeces and nasal discharges of infected birds. The organism is resistant to drying and can remain infectious for several months. Some infected birds can appear healthy and shed the organism intermittently or at low levels. Shedding can be activated by stress factors, including transport, relocation, crowding, chilling, and breeding.

In birds, the incubation period is usually in the range from three days to several weeks, although active disease can appear years after initial exposure. Signs of avian chlamydiosis include lethargy, anorexia and ruffled feathers, which may also be similar to other signs of systemic illness. They may also have nasal or ocular discharge and green to yellow diarrhoea.

Bird owners should be aware that even treated birds may present a future health risk to bird or human contacts. Euthanasia of infected birds may need to be considered as an option under these circumstances.

Control of psittacosis