[Human metapneumovirus]

Hypoxemic Human Metapneumovirus Pneumonia in a Young Immunocompetent Man in the First Autumn After the Reclassification of Coronavirus Disease

Although human metapneumovirus (hMPV) infection can induce severe symptoms in older adults or immunocompromised patients, it usually causes mild symptoms in young immunocompetent adults. The prevalence of hMPV infectious disease is highest during the late winter and early summer. We report a hypoxemic case of hMPV infection in a young immunocompetent man that occurred in the first autumn after the reclassification of coronavirus disease (COVID-19) from Class 2 to Class 5. A multiplex-nested polymerase chain reaction (MN-PCR) test using a sputum sample was useful for rapid detection of co-infection with hMPV and Haemophilus influenzae (H. influenzae) as causes of pneumonia and acute respiratory failure. In early October 2023, a 24-year-old healthy man without comorbidities visited our hospital with a fever, sore throat, and nasal congestion. The antigen test results for influenza and COVID-19 were negative. After 6 days, he presented with a productive cough and exertional dyspnea. He was immediately admitted to our hospital due to hypoxemia with a percutaneous arterial oxygen saturation of 90% breathing room air. Although bacterial pneumonia was suspected based on bilateral ground-glass opacities and infiltrative shadows on chest computed tomography (CT), the MN-PCR test using a nasopharyngeal swab indicated hMPV infection. On the 2nd day of admission, the MN-PCR test using a sputum sample (Group 5 in the Geckler classification) indicated that hMPV and H. influenzae were the causative pathogens for pneumonia. We initiated oxygen supplementation and administered intravenous ceftriaxone sodium hydrate and oral azithromycin hydrate. The patient was discharged on the 7th day of admission with improved pneumonic shadows and hypoxemia. hMPV infection can occur in any season in the post-COVID-19 era. Even in young immunocompetent adults, hMPV infection can cause pneumonia which may progress to acute respiratory failure with other bacterial co-infection. Conversely, it is clinically important to pay attention to the involvement of hMPV and other respiratory viruses in pneumonia cases that initially appear to be bacterial based on CT findings. MN-PCR test using a good-quality sputum sample is valuable not only for rapid detection of causative bacteria and viruses for pneumonia but also for identifying these co-infections.

Interleukin-4 Promotes Human Metapneumovirus Replication Through the JAK/STAT6 Pathway

Respiratory virus infections are the main causes of pediatric diseases. Human metapneumovirus (hMPV) is an enveloped RNA virus similar to severe acute respiratory syndrome coronavirus type 2, both of which have emerged as important new respiratory viruses. Recent studies have found that interleukin-4 (IL-4) is involved in the replication of a variety of viruses, and its role differs in different viruses. The purpose of this study was to investigate the effect of IL-4 on hMPV and to elucidate its mechanism of action. We found that hMPV infection promoted the expression of IL-4 in human bronchial epithelial cells. The replication of the virus was reduced using small interfering RNA knockdown of IL-4 expression, while the addition of exogenous recombinant human IL-4 to IL-4 knockdown cells restored viral replication ability. These results demonstrate that the expression of IL-4 is closely related to the replication of hMPV; moreover, further experiments revealed that IL-4 promotes the replication of hMPV through a mechanism dependent on the Janus kinase/signal transductor and transcription activator 6 signaling pathway. Therefore, anti-IL-4 strategies may be a promising avenue for the treatment of hMPV infection, representing an important breakthrough for children at risk from hMPV infection.

Characteristics of human metapneumovirus infection prevailing in hospital wards housing patients with severe disabilities

Epidemics of infectious diseases often occur at long-term inpatient facilities for patients with severe motor and intellectual disabilities. However, the pathogens causing these infections remain unknown in approximately half of such epidemics. Two epidemics of respiratory tract infection occurred in 2 wards in the National Hospital Organization Ehime Hospital (prevalence 1, 34 infected out of 59 inpatients in the A ward in September 2011; prevalence 2, 8 infected out of 58 inpatients in the B ward in June 2012). Human metapneumovirus (HMPV) was detected from the nasal (and some pharyngeal) swabs from 17 patients. Based on phylogenetic analysis of viral genomes, the virus was grouped in subgroup A2 (prevalence 1) and B2 (prevalence 2). We considered that the viruses had spread through the 2 wards. The average duration of high fever in the 42 patients was 6.8 days, with the majority of fevers exceeding 38℃ (79%) and being accompanied by a productive cough. Ten out of 17 patients (59%) in whom HMPV was detected had decreased lymphocyte and increased monocyte counts in the blood. Eleven cases (65%) had elevated-C reactive protein levels and fever protraction as well as images of bronchitis or pneumonia on chest radiographs approximately 1 week after onset. Anti-HMPV antibody in the blood was positive in 95% of patients (151 of 159 inpatients), indicating no relation between HMPV infection and antibody titer but revealing recurrent infections. In view of the fever protraction and frequent co-occurrence of bronchitis and pneumonia at long-term inpatient facilities for immunocompromised patients such as the ones in this study, the prevalence of HMPV must be carefully monitored, and preventive measures and early-stage treatments are required.