Probable psittacosis outbreak linked to wild birds

In autumn 2002, an outbreak of probable psittacosis occurred among residents of the Blue Mountains district, Australia. We conducted a case-control study to determine independent risk factors for psittacosis by comparing exposures between hospitalized patients and other residents selected randomly from the telephone directory. Of the 59 case-patients with laboratory results supportive of psittacosis, 48 participated in a case-control study with 310 controls. Independent risk factors were residence in the upper Blue Mountains (odds ratio [OR] 15.2, 95% confidence interval [CI] 5.6–41.7), age of 50–64 years (OR 3.9, 95% CI 1.5–10.5), direct contact with wild birds (OR 7.4, 95% CI 2.5–22), and mowing lawns without a grass catcher (OR 3.2, 95% CI 1.3–8.0). Protective equipment is recommended for residents in areas frequented by free-ranging birds if contact with birds and their droppings is likely when performing outdoor activities such as lawn mowing.

Psittacosis is a human disease caused by infection with the bacterium Chlamydophila psittaci. The bacterium also causes avian chlamydiosis, a disease reported in psittacine birds such as parrots, cockatiels, and parakeets. Chlamydophila psittaci can be present in large numbers in the droppings of sick birds and in dust contaminated by infected droppings. The organism can remain infectious in the environment for months. Human infection usually occurs when a person inhales the bacterium shed in feces and secretions of infected birds. Sheep, goats, cattle, and reptiles can also be infected, but these animals have rarely been linked to human cases.

Psittacosis has an incubation period of 1 to 4 weeks, and manifestations of disease can range from asymptomatic infection to systemic illness with severe pneumonia. Untreated psittacosis has a reported case-fatality rate of 15% to 20%. Psittacosis is most commonly reported among people in close contact with domestic birds, such as bird owners, poultry farmers, veterinarians, and workers within pet shops and poultry-processing plants. Sporadic cases and an outbreak in Australia linked to contact with free-ranging (wild) birds have been reported; however, little information is available on the role of wild birds in the transmission of Chlamydophila psittaci to humans.

Psittacosis became a notifiable disease in New South Wales (NSW), Australia, in 2001, and 38 laboratory notifications were received by the state health department that year, an incidence of 5.7 cases per 1,000,000 population for NSW. In May 2002, clinicians at the Blue Mountains Hospital (BMH), in the Wentworth Area Health Service, NSW, a 1-hour drive west of Sydney’s central business district, reported an increase in adult admissions for severe community-acquired pneumonia. From March to May 2002, a total of 160 persons with pneumonia were seen at the BMH emergency department, compared with 82 from March to May 2001. The population of the Blue Mountains is ˜80,000 persons, and the area includes a large national park. The lower Blue Mountains (altitude ˜160 m) is on the western outskirts of Sydney, and residences tend to have suburban-style yards. The upper Blue Mountains district (altitude ˜1,044 m) lies further west, receives more rain, and has more bush land; its residential areas have larger yards and are closer to bush land. Reports that patients had found increased numbers of dead free-ranging birds in their yards, handled dead birds, and occasionally mowed over dead bird carcasses prompted clinicians to suspect psittacosis, although no case had been confirmed by laboratory testing. We report on our investigation into the extent and most likely cause of this outbreak.

Emerging Infectious Diseases
March 29, 2005

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