What is Chlamydia psittaci?

What is Chlamydia psittaci?

Chlamydia psittaci is an obligate intracellular gram-negative bacterium that causes avian chlamydiosis in birds and psittacosis (also known as parrot fever or ornithosis) in humans, transmitted primarily through inhalation of aerosolized respiratory secretions or dried feces from infected birds. 1

Organism Characteristics

• C. psittaci is a bacterium with a unique biphasic developmental cycle that requires living cells for replication, distinguishing it from free-living bacteria 2
• The organism has been isolated from approximately 100 bird species, though it most commonly infects psittacine (parrot-type) birds including cockatiels, parakeets, parrots, macaws, and conures 1, 3
• Among non-psittacine birds, infection occurs most frequently in pigeons, doves, and mynah birds, with lower incidence in canaries and finches 1

Transmission Pathways

Animal-to-Human Transmission

• Primary transmission occurs when humans inhale aerosolized organisms from dried feces or respiratory secretions of infected birds 1
• Additional exposure routes include bird bites, mouth-to-beak contact, and handling infected birds' plumage and tissues 1
• Even brief, passing exposure to infected birds or contaminated droppings can lead to symptomatic infection, meaning patients may not recall bird contact 1
• Certain strains infect sheep, goats, and cattle, causing reproductive tract infections and abortion, with transmission to humans through exposure to birth fluids and placentas 1
• A feline strain (keratoconjunctivitis agent) rarely transmits from cats to humans 1

Human-to-Human Transmission

• Human-to-human transmission has been documented in China (2020), representing the first confirmed report of this transmission route, involving secondary and tertiary spread including transmission by asymptomatic carriers and healthcare workers 4
• Standard infection-control precautions are sufficient for patients with psittacosis; specific isolation procedures (private room, negative pressure air flow, masks) are not indicated for routine cases 1

Clinical Disease in Humans (Psittacosis)

Presentation

• The incubation period is typically 5-14 days, though longer periods have been reported 1
• Disease severity ranges from asymptomatic infection to severe pneumonia with multi-organ involvement 1, 5
• Symptomatic patients typically present with abrupt onset of fever, chills, headache, malaise, and myalgia 1
• A nonproductive cough develops, often accompanied by breathing difficulty and chest tightness 1
• Pulse-temperature dissociation (fever without elevated pulse), splenomegaly, and rash are sometimes observed and suggest psittacosis in community-acquired pneumonia 1

Complications

• C. psittaci can affect multiple organ systems beyond the respiratory tract, causing endocarditis, myocarditis, hepatitis, arthritis, keratoconjunctivitis, and encephalitis 1
• Severe illness with respiratory failure, thrombocytopenia, hepatitis, and fetal death has been reported in pregnant women 1
• Before antimicrobial agents, 15-20% mortality was reported; with proper treatment, less than 1% of patients now die 1

Epidemiology

Human Cases

• From 1988-1998, CDC received 813 reported cases of psittacosis in the United States, representing an underestimate due to diagnostic difficulty and underreporting 1
• During 2003-2014,112 human cases were reported through the Nationally Notifiable Diseases Surveillance System 3
• Approximately 70% of cases with known source resulted from exposure to pet birds during the 1980s 1

At-Risk Populations

• Bird fanciers and pet bird owners comprise the largest affected group (43% of cases) 1
• Pet shop employees account for an additional 10% of cases 1
• Other high-risk groups include poultry slaughtering/processing plant employees, veterinarians, veterinary technicians, laboratory workers, avian quarantine station workers, farmers, wildlife rehabilitators, and zoo workers 1
• Duck processing plant workers face significant occupational exposure, as demonstrated by outbreak investigations showing heavy asymptomatic shedding in mule duck flocks 4, 6

Diagnosis

Laboratory Confirmation

• A confirmed case requires clinical illness compatible with psittacosis plus one of three laboratory criteria: (1) culture of C. psittaci from respiratory secretions, (2) ≥4-fold antibody rise to reciprocal titer ≥32 by complement fixation (CF) or microimmunofluorescence (MIF) between paired sera collected ≥2 weeks apart, or (3) IgM antibody detection by MIF to reciprocal titer ≥16 1
• Probable cases include clinically compatible illness with either epidemiologic linkage to a confirmed case or single antibody titer ≥32 1
• Metagenomic next-generation sequencing and PCR-based approaches can detect C. psittaci in sputum, bronchoalveolar lavage fluid, and nasopharyngeal swabs 4

Diagnostic Challenges

• Psittacosis is difficult to diagnose and often goes unreported, contributing to underestimation of true case numbers 1
• The differential diagnosis includes Coxiella burnetii, Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella species, and respiratory viruses such as influenza 1
• Chlamydia CF antibody is not species-specific, so high CF titers may result from C. pneumoniae or C. trachomatis infection 1

Treatment

• Doxycycline is the drug of choice for psittacosis and is FDA-approved for this indication 7, 5
• The FDA label indicates doxycycline for "psittacosis (ornithosis) caused by Chlamydia psittaci" 7
• Tetracyclines are bacteriostatic and exert antimicrobial effect through inhibition of protein synthesis 7
• Treatment should be initiated immediately upon clinical suspicion, even before laboratory confirmation, as emphasized by the Infectious Diseases Society of America 8

Disease in Birds (Avian Chlamydiosis)

• In birds, C. psittaci infection is referred to as avian chlamydiosis (AC) 1, 3
• Infected birds shed bacteria through feces and nasal discharges, which can remain infectious for several months 1
• Heavy but asymptomatic C. psittaci shedding occurs in most mule duck flocks, particularly when reared in open range conditions 6
• Field studies suggest infections involve both horizontal and vertical transmission, with the environment playing an important role in maintaining infection on farms 6

Public Health Implications

• C. psittaci has demonstrated potential for human-to-human transmission via various routes and should be considered an elevated biosecurity and emergent risk 4
• The pathogen should be included as part of routine diagnosis globally, especially for high-risk populations 4
• Standardized procedures for controlling avian chlamydiosis in pet bird populations are essential steps in protecting human health 1, 3
• N95 respirators or higher should be used for protection during bird cleanup activities, not standard surgical masks 8