Psittacosis, also known as bird disease, is a zoonotic disease caused by Chlamydia psittaci. Birds (poultry, pet birds, wild birds) are its natural hosts, but other animals such as cattle, goats, sheep, horses, pigs, foxes and dogs, including humans, can also be infected[3–5]. Whether humans are infected with psittacosis depends on a variety of factors, such as exposure intensity, microbial factors and route of transmission. High-risk groups include workers in poultry processing plants, pet shop staff, veterinarians and bird owners who keep infected pet birds[5]. The route of transmission includes contact with feces and respiratory secretions of infected birds or direct contact with them. Human-to-human transmission is rarely reported[6]. Most of the sick birds are asymptomatic, human infection can have different manifestations, a small number of patients can be asymptomatic, most patients are mainly pneumonia-related manifestations, accompanied by myalgia and fatigue and other symptoms, some patients can progress to severe pneumonia or even death. It is reported that in untreated cases, the mortality rate can be as high as 40%, while the mortality rate of co-infection may be even higher[5].
COVID-19 is the general name of atypical coronavirus that was first discovered and spread rapidly in Wuhan, China in December 2019. On January 12, 2020, the World Health Organization officially named COVID-19, who caused the pneumonia epidemic in Wuhan. It is highly contagious and spreads rapidly around the world, and there is widespread human-to-human and animal-to-human transmission[7, 8]. COVID-19 infection is the main source of infection, can also be seen in asymptomatic transmission, the main route of transmission is droplet transmission, contact transmission, aerosol and digestive tract transmission[8, 9]. Severe pneumonia caused by COVID-19 infection is more common in the elderly with more basic diseases, children and young adults rarely develop into severe pneumonia, and most of the elderly patients with severe pneumonia have a poor prognosis. Finally died of respiratory, circulatory, digestive system and other multiple organ failure.
The study found that [10–12], laboratory tests of Chlamydia psittaci infection showed that CRP was significantly increased, PCT and neutrophils could be increased, but WBC count was normal or slightly increased, and some patients were accompanied by hepatic and renal insufficiency. The laboratory examination of COVID-19 infection showed that CRP and ESR were increased, PCT was normal, WBC count was normal or decreased, severe patients were accompanied by the increase of D-dimer and progressive decrease of blood lymphocytes, and the increase of liver enzyme, myoglobin and myoglobin could be found in some patients. The results of laboratory examination in case 1 were basically consistent with previous literature reports. in addition, case 1 was accompanied by a small amount of pleural effusion and pericardial effusion, and the liver enzyme and creatine kinase were 3 times higher than the upper limit of the normal value. at the same time, it shows that the patient is also associated with heart and liver function damage, and the liver enzyme and muscle enzyme index decreased to normal after treatment, which is consistent with previous studies[13]. However, the laboratory index of case 2 is not as obvious as that of case 1 because of hematological diseases. It is considered that the immune function of the patient is decreased due to hematological diseases, and the inflammatory reaction is not severe. In addition, there is a great overlap in imaging signs between Chlamydia psittaci pneumonia and COVID-19. Lung parenchyma may be involved in severe patients, and non-specific signs such as "white lung" and "air bronchial sign" can be shown in imaging. Pleural effusion can also be accompanied by a small amount of pleural effusion, which often means that the patient's condition is more serious and the probability of requiring ventilator support is higher[14]. During the course of the disease, 2 patients had pneumonia such as fever, cough and expectoration, chest tightness and apnea. Chest CT showed consolidation of both lungs and pleural effusion, and both were supported by invasive ventilator, which met the diagnostic criteria of severe pneumonia[15].
COVID-19 co-infection is more common, and it has been reported at home and abroad that it can be complicated with mycoplasma pneumoniae, respiratory syncytial virus, adenovirus, soil mold and so on[16, 17]. However, Chlamydia psittaci complicated with COVID-19 infection has not been reported. It has been reported in a domestic guide for imaging diagnosis of pneumonia caused by COVID-19 infection that the pulmonary lesions caused by COVID-19 infection can be associated with tissue necrosis and form cavity. However, it has not been reported in other literature and guidelines[7, 18, 19]. Case 1 showed signs of cavity in both lungs during the reexamination of chest CT. The author initially thought it was caused by fungal infection and was given empirical antifungal therapy, but it was excluded after re-examination of sputum culture, fungal triple and mNGS examination, because the patient was also complicated with Klebsiella pneumoniae infection, and in previous studies, it was found that Klebsiella pneumoniae could cause liver abscess. Klebsiella pneumoniae can also cause extrahepatic infections such as pulmonary abscess, endophthalmitis and meningitis through blood flow, but it is rare in our country at present[20]. And the patient did not show liver abscess and blood flow infection during the whole course of the disease, so the pulmonary cavity caused by Klebsiella pneumoniae infection was temporarily excluded, but it was not clear whether it was chlamydia psittaci infection, or COVID-19 infection, or the combined infection of both. This has also become the most important controversial point for the patient at present, because Chlamydia psittaci complicated with COVID-19 infection is already rare, and the re-emergence of pulmonary cavity signs makes this case even more special, but case 1 does show lung cavity after double infection. Although the signs of infection gradually improved after reexamination of chest imaging features, there are still complications.
With regard to the treatment of severe pneumonia caused by psittacosis caused by COVID-19, according to the recommendation of relevant literature and extensive clinical experience, the first antibiotic of psittacosis is doxycycline, the secondary antibiotic is azithromycin, and the treatment duration is recommended to be at least 10 days. Because quinolones have in vitro activity to psittacosis, severe patients can be combined with macrolides and quinolones[15]. COVID-19 infection is routinely recommended for antiviral, hormone anti-inflammatory, low molecular weight heparin anticoagulation (except for contraindications), and even immune agents and human immunoglobulin, while antibiotics are non-essential treatment. At the same time, prone position ventilation is used to improve oxygenation[21, 22]. According to the relevant guidelines and consensus recommendations, 2 patients were finally treated with azithromycin combined with moxifloxacin and symptomatic support therapy such as antiviral, hormone anti-inflammatory, low molecular weight heparin anticoagulation and prone position ventilation. Although 2 patients were infected with Chlamydia psittaci and COVID-19 and developed into severe pneumonia, and the treatment of anti-infective treatment was basically the same, the sequence number of 2 patients infected with Chlamydia psittaci and COVID-19 was not the same. this also indicates that the severity of the two conditions are not the same, resulting in the final outcome is completely opposite. This shows that most human infections with Chlamydia psittaci can occur, but the severity also depends on previous physical conditions and whether there are other pathogen infections. When the patient foundation is poor and combined with other pathogens infection, the prognosis is significantly worse, and the mortality is significantly increased.
Although Chlamydia psittaci was discovered and reported as early as the 19th century, the diagnosis of Chlamydia psittaci is still difficult today, and traditional pathogen culture or serological tests often have a lag, while polymerase chain reaction (PCR) needs to be carried out in a special laboratory, and the specificity and sensitivity are poor[23, 24]. With the clinical application of mNGS, the number of cases of Chlamydia psittaci infection is relatively increased. In this paper, 2 patients were diagnosed as Chlamydia psittaci infection by mNGS, and case 1 also depended on mNGS to evaluate the therapeutic effect. In a word, mNGS can not only provide the basis of early diagnosis, but also provide reference for the follow-up treatment effect and treatment plan. Sending mNGS as soon as possible is helpful to accurate medicine and avoid the waste of early medical resources. However, at present, genetic testing is not included in medical insurance reimbursement and the cost is relatively high, while the efficiency of serology or PCR diagnosis is low. It is expected that the cost of mNGS testing can be reduced or the efficiency of traditional testing can be improved in the future to assist in the diagnosis of atypical pathogen infection. With the help of mNGS, we look forward to seeing more and more comprehensive case reports of Chlamydia psittaci infection.
To sum up, there is a great overlap in laboratory examination and imaging signs of Chlamydia psittaci complicated with COVID-19 infection, which leads to difficulties in clinical diagnosis. Early and timely mNGS detection can help clinicians to make rapid and accurate diagnosis, so as to give accurate medical treatment in time, and finally make patients get a better clinical outcome. However, for such patients with poor foundation and complicated with other pathogens infection, the prognosis is often poor.