During the largest psittacosis outbreak in the United States in 30 years, Chlamydia psittaci was best detected in lower respiratory specimens by using real-time polymerase chain reaction (PCR) assay, according to a case study published by the Centers for Disease Control and Prevention (CDC) in their Morbidity and Mortality Weekly Report. Although real-time PCR is not widely available, it provides a faster and more accurate psittacosis diagnosis than the more commonly used serologic tests.
From August to October 2018, the Virginia and Georgia departments of health identified 33 workers as having probable or confirmed psittacosis. All of them worked at 1 of 2 chicken slaughter plants in Virginia or Georgia that shared source farms. Probable cases were based on symptoms and epidemiologic exposure, and confirmed cases were based on detection of C psittaci via real-time PCR in at least 1 of the following specimens: nasopharyngeal swab, sputum, or stool.
Based on the 54 specimens submitted, 13 (39%) of the workers had confirmed psittacosis, and 20 (61%) workers had probable psittacosis. Workers with confirmed infection tended to be older men, have a diagnosis of pneumonia, require hospitalization, or require admission to an intensive care unit. The most common specimen type submitted was the nasopharyngeal swab, which confirmed 90% of probable infection and 61% of psittacosis cases.
C psittaci was most commonly detected in stool (4/5) and lower respiratory (10/17) specimens, and in 2 of 28 upper respiratory specimens. Cycle threshold (Ct) values were lower in lower respiratory C psittaci-positive specimens (mean, 29; range, 26-31), indicating a higher bacterial load compared with nasopharyngeal swabs (Ct values 31 and 33) and stool specimens (mean, 34; range, 32-37).
Of 6 patients with confirmed psittacosis who submitted nasopharyngeal swabs and sputum, all sputa and only 1 nasopharyngeal swab tested positive for C psittaci. Of 3 patients with confirmed psittacosis who submitted stool and sputum, all 3 sputa and 2 stool specimens tested positive for C psittaci, and C psittaci was detected in the stool specimens of 2 patients with C psittaci-negative nasopharyngeal swabs.
The study was limited by a small sample size and the difficulty of obtaining lower respiratory specimens from mildly ill workers because psittacosis is characterized by a dry cough; therefore, lower respiratory specimens were more likely submitted by severely ill workers who probably had a higher bacterial load.
The study authors warned that “Public health professionals and healthcare providers should be aware that C psittaci might not be detected if nasopharyngeal swab specimens alone are tested.” In addition, they noted that stool specimens may have utility in psittacosis diagnosis when using real-time PCR.
McGovern OL, Kobayashi M, Shaw KA, et al. Use of real-time PCR for Chlamydia psittaci detection in human specimens during an outbreak of psittacosis – Georgia and Virginia, 2018. MMWR Morb Mortal Wkly Rep. 2021;70(14):505-509. doi:10.15585/mmwr.mm7014a1