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169 Chlamydia psittaci (psittacosis)
Chlamydia psittaci was identified simultaneously by three investigators in 1930. It is one of four species within the genus Chlamydia. Based on RNA sequencing, it is currently considered distinct from Chlamydia pneumoniae and C. trachomatis, despite phenotypic and physiologic similarities that have taxonomically bound them for many years. The organism is an obligate intracellular pathogen that contains both RNA and DNA but lacks a classic cell wall. These characteristics contribute to both the clinical manifestations and the determination of therapeutic options. C. psittaci has a wide range of host species, including birds, humans, and lower mammals. C. pneumoniae, however, is found only in humans and C. trachomatis only in humans and mice.
The systemic illness associated with Chlamydia psittaci has been termed psittacosis because of its association with parrots and psittacine birds. Subsequently, many avian species have been found to harbor C. psittaci and to transmit the organism to humans, causing disease. The term ornithosis would be more appropriate; however, it is not traditional. The organism can be carried for years in birds, remaining latent and causing disease many years after acquisition. Transmission to humans can occur even in the absence of disease in the bird. Excretion in the feces with aerosolization is the typical mode of transmission. Human-to-human transmission has been documented rarely and usually in the setting of severe disease. Healthcare workers have acquired the disease, but it is not felt warranted to isolate patients when hospitalized. Cases of mammal-to-human acquisition have been described in the setting of placental aeration at birth, but these cases are likely caused by the now separate species Chlamydophila abortus.
Individuals epidemiologically at risk for Chlamydia psittaci infection include abattoir and veterinary workers as well as those exposed to aviaries. Poultry breeders (particularly turkey farmers) are at significant risk, accounting for most outbreaks. A variable degree of illness exists in the birds infected with C. psittaci, ranging from asymptomatic to full-blown disease manifested by anorexia, dyspnea, and diarrhea. Birds may resolve the illness spontaneously, and a waxing and waning clinical course is not unusual. Therefore, a history of contact with birds is pertinent even if the bird is seemingly healthy. Up to 20% of patients may not recall bird exposure, but contact as innocuous as mowing a lawn or being exposed to airborne feces is adequate exposure. It remains a distinctly unusual cause of pneumonia with only four cases reported to the Centers for Disease Control and Prevention (CDC) in 2010, compared to an average of 16 cases per year in the preceding decade. New criteria for the diagnosis also contributed to this drop. Underreporting in the setting of accepted clinical criteria may play a role in this lack of recognition.
Vigilance in the workplace and prevention in at-risk bird populations play an increasing role. Quarantine and treatment of imported birds is a mainstay.
Clinical syndromes
Following inhalation in aerosol form, the organism travels to the alveoli and then disseminates to regional lymph nodes and the reticuloendothelial system. Dissemination does not always occur, limiting disease to the chest. The organism invades and even multiplies successfully in a wide range of host cells, including macrophages and neutrophils. Multiple organ involvement is not uncommon, and the systemic nature of this disease cannot be overstated.
The classic presentation is one of an atypical pneumonia, although systemic infection in the absence of pneumonia has been well described as a typhoidal disease, even with cutaneous manifestations such as Horder’s spots. Cough is often a later clinical sign, preceded by fever, malaise, and often severe headache by a number of days. The presence of severe headache is felt to be a sentinel component of the disease. The incubation period ranges from 5 to 21 days, and in up to 20% of patients no history of exposure to a bird can be elicited. Diarrhea is very common.
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