Psittacosis

Psittacosis is a zoonotic respiratory infection caused by Chlamydia psittaci, an obligate intracellular bacterium. It is transmitted to humans predominantly through inhalation of aerosolised particles from the respiratory secretions, faeces or feather dust of infected birds (particularly psittacine birds including parrots and budgerigars, though poultry and other bird species can also serve as reservoirs).

It produces a spectrum from a mild, self limiting illness through to severe atypical pneumonia with systemic complications.

A relevant bird exposure history is the essential clue prompting clinical consideration of this otherwise readily overlooked diagnosis among the broader differential of atypical pneumonia.

Pathophysiology and epidemiology

• Following inhalation, the organism disseminates from the respiratory tract via the bloodstream, with a particular tropism for the reticuloendothelial system before secondary seeding of the lungs, explaining the characteristic combination of systemic toxicity and pulmonary involvement disproportionate to the sometimes relatively modest chest radiographic findings.
• Occupational exposure (pet shop workers, poultry workers, veterinarians, aviary and zoo workers) and pet bird ownership represent the principal recognised risk factors. Infected birds frequently, though not universally, appearing clinically unwell themselves (lethargy, ruffled feathers, reduced appetite, diarrhoea). This provides an additional, though inconsistently present, clue supporting clinical suspicion when a relevant exposure history is elicited.

• Fever, often high and accompanied by prominent rigors, together with a severe headache (frequently a striking and disproportionately prominent feature relative to other symptoms) and myalgia, characterises the typical onset, developing after an incubation period of one to two weeks following relevant bird exposure.
• A relatively dry, non productive cough, together with pleuritic chest pain in a proportion of cases, reflects the pulmonary involvement. Chest examination and radiographic findings frequently less impressive than the degree of systemic toxicity would suggest. This pattern is shared with several other atypical pneumonia pathogens.
• Splenomegaly occurs, and in a minority of cases a faint macular rash (Horder spots). Relative bradycardia is similar to that described in several other atypical systemic infections.
• In more severe disease, hepatitis, myocarditis or endocarditis can occur, and, rarely, neurological involvement including encephalitis. These features reflect the broader systemic and multi organ potential of this infection beyond the respiratory tract alone.
• A specific and detailed bird exposure history, encompassing pet bird ownership, recent pet shop, aviary or poultry farm visits, and occupational exposure, should be actively and specifically sought in any patient presenting with an atypical pneumonia picture. This history is frequently the key element distinguishing this diagnosis from the numerous other, generally more common causes of atypical pneumonia.

• Diagnosis has historically relied predominantly on serological testing (complement fixation test or microimmunofluorescence), typically requiring paired acute and convalescent samples to demonstrate a significant rise in antibody titre, limiting its practical utility for immediate acute clinical decision making.
• Polymerase chain reaction testing on respiratory specimens offers more rapid diagnostic confirmation where available, increasingly preferred given the practical limitations of serological testing.
• Chest radiography typically shows patchy or lobar infiltrates, without any specific distinguishing radiographic pattern from other causes of atypical pneumonia, meaning the diagnosis relies fundamentally on maintaining appropriate clinical suspicion based on the specific exposure history rather than any distinguishing radiological or routine laboratory feature.
• Given the practical diagnostic challenges, empirical treatment based on a compatible clinical picture and relevant bird exposure history is frequently appropriate, without necessarily awaiting laboratory confirmation, particularly where standard empirical community acquired pneumonia treatment already provides appropriate antimicrobial cover for this organism.

• Doxycycline 100 mg orally or intravenously twice daily for ten to fourteen days is first line treatment, with reliable and rapid clinical response typically observed when appropriate treatment is initiated promptly.
• Macrolides (azithromycin or clarithromycin) represent an effective alternative where doxycycline is contraindicated or not tolerated, particularly relevant in pregnancy or in young children where tetracycline class agents are generally avoided.
• Standard empirical community acquired pneumonia regimens frequently incorporate a macrolide or, in some regimens, doxycycline for atypical pathogen coverage. As a result, psittacosis is often adequately treated even without a specific diagnosis having been established, provided appropriate empirical atypical pathogen coverage has been included in the initial treatment regimen for a patient presenting with pneumonia of uncertain specific cause.
• Severe disease with significant systemic complications (myocarditis, hepatitis, encephalitis) requires appropriate supportive management of the specific affected organ system alongside antibiotic treatment, delivered within an appropriate level of care according to overall severity.
• Any suspected case should be reported to relevant public health authorities given the recognised potential for common source exposure (particularly relevant in pet shop or aviary related clusters) and the animal health implications. Investigation and, where relevant, treatment of the identified source bird population is an important component of the broader public health response and prevention of further human cases.
• Prevention advice for those with relevant occupational or pet bird ownership exposure includes appropriate ventilation and hygiene measures when cleaning bird cages or handling birds. It also includes prompt veterinary assessment of any unwell bird, given the potential for this to represent an infective source for human transmission.