Chlamydia (Chlamydophila) psittaci (Psittacosis)

The Pathogen

Chlamydia psittaci (also known as Chlamydophila psittaci ) is a gram-negative obligate intracellular zoonotic bacterium that causes both systemic infection and pneumonia, often referred to as psittacosis or ornithosis ( Fig. 168.1 ). Originally named parrot fever because of its association with psittacines (e.g., parrots, parakeets), psittacosis can be transmitted by virtually all domestic and wild birds. ^,

Epidemiology

Psittacosis was first described in 1879. It became a nationally notifiable disease in the US in 1944. As of 2021, psittacosis was reportable in the District of Columbia and all states except Connecticut and Texas. From 1992 through 2018, a total of 607 psittacosis cases were reported to the Centers for Disease Control and Prevention (CDC). The number of reported cases has decreased since the early 1990s ( Fig. 168.2 ). Although surveillance case definitions have changed over the years, the decrease in reported cases also may be due to improvement in laboratory tests that distinguish between C. psittaci and C. pneumoniae . ^, Overall, less than 10% of the reported psittacosis cases were among persons aged <20 years. The number of reported cases, especially among children, is likely an underestimate of the actual number because of difficulties in diagnosis; symptoms are generally mild and non-specific, and healthcare providers may not consider testing for psittacosis.

C. psittaci is transmitted through inhaled aerosols from the respiratory tract secretions, eye secretions, or droppings of infected birds. Mouth-to-beak contact, handling of plumage, and lawn-mowing have been reported modes of exposure. Illness can occur after even brief exposure to infected birds or their droppings. Most reported psittacosis cases have a known exposure to pet birds. However, outbreaks associated with domiciliary birds, wild birds, poultry processing, veterinary clinics, and aviaries have been described. Infected birds can be asymptomatic or only mildly ill and can shed the organism for prolonged periods after recovery. Because C. psittaci resists drying and can remain infectious in the environment for months, bird exposure may not be reported in some cases. Although biologically plausible, reports of person-to-person transmission are rare.

Clinical Manifestations

Psittacosis classically causes an “atypical” pneumonia, characterized as nonproductive cough, fever, headaches, and chest radiograph images that are more dramatic than would be suggested from clinical findings. However, acute disease manifestations can range from a mild, influenza-like illness to severe systemic disease ( Box 168.1 ). ^, Psittacosis is rarely fatal in humans if treated properly. ^, Symptoms generally develop after an incubation period of approximately 5–14 days but can occur up to 1 month after exposure. Because signs and symptoms of C. psittaci infection are similar to those caused by other community-acquired pneumonia pathogens (e.g., Mycoplasma pneumoniae and C. pneumoniae ), psittacosis should be considered in any child with pneumonia who has had close exposure to birds. ^,