viruses Article Exacerbation of Influenza A Virus Disease Severity by Respiratory Syncytial Virus Co-Infection in a Mouse Model Junu A. George 1 , Shaikha H. AlShamsi 2 , Maryam H. Alhammadi 3 and Ahmed R. Alsuwaidi 1, *   Citation: George, J.A.; AlShamsi, S.H.; Alhammadi, M.H.; Alsuwaidi, A.R. Exacerbation of Influenza a Virus Disease Severity by Respiratory Syncytial Virus Co-Infection in a Mouse Model. Viruses 2021, 13, 1630. https://doi.org/10.3390/v13081630 Academic Editor: Stacey Schultz-Cherry Received: 28 June 2021 Accepted: 16 August 2021 Published: 18 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Pediatrics, College of Medicine and Health Sciences, United Arab Emirates University, Al Ain 17666, United Arab Emirates; junugeorge@uaeu.ac.ae 2 Department of Medical Education, Sheikh Khalifa Medical City, Abu Dhabi Health Services Company (SEHA), Abu Dhabi 51900, United Arab Emirates; shhalshamsi@seha.ae 3 Department of Medical Affairs, Sheikh Shakhbout Medical City, Abu Dhabi Health Services Company (SEHA), Abu Dhabi 11001, United Arab Emirates; mhalhammadi@seha.ae * Correspondence: alsuwaidia@uaeu.ac.ae Abstract: Influenza A virus (IAV) and respiratory syncytial virus (RSV) are leading causes of child- hood infections. RSV and influenza are competitive in vitro. In this study, the in vivo effects of RSV and IAV co-infection were investigated. Mice were intranasally inoculated with RSV, with IAV, or with both viruses (RSV+IAV and IAV+RSV) administered sequentially, 24 h apart. On days 3 and 7 post-infection, lung tissues were processed for viral loads and immune cell populations. Lung functions were also evaluated. Mortality was observed only in the IAV+RSV group (50% of mice did not survive beyond 7 days). On day 3, the viral loads in single-infected and co-infected mice were not significantly different. However, on day 7, the IAV titer was much higher in the IAV+RSV group, and the RSV viral load was reduced. CD4 T cells were reduced in all groups on day 7 except in single-infected mice. CD8 T cells were higher in all experimental groups except the RSV-alone group. Increased airway resistance and reduced thoracic compliance were demonstrated in both co-infected groups. This model indicates that, among all the infection types we studied, infection with IAV followed by RSV is associated with the highest IAV viral loads and the most morbidity and mortality. Keywords: IAV; RSV; co-infection; mortality; viral titer 1. Introduction Respiratory infections are major causes of both morbidity and mortality worldwide [1]. Diagnostic and surveillance studies have revealed that co-infections with more than one pathogen in the respiratory tract are common [2,3]. Pathogens involved in co-infections interact with the host and with themselves, thereby profoundly affecting replication of each pathogen, disease pathogenesis, immune responses and most notably the disease outcome [4,5]. Influenza A virus (IAV) and respiratory syncytial virus (RSV) are common causative agents of respiratory tract infections among all age groups worldwide. RSV is the most common cause of lower respiratory tract infections in children. IAV causes seasonal in- fluenza that affects 5–10% of adults and 20–30% of children every year [6–8]. IAV is a negative strand, enveloped RNA virus that infects airway epithelial cells [9], whereas RSV is an enveloped virus that contains a linear negative-sense RNA genome. Simultaneous co- infections with these two viruses are frequently associated with lower respiratory tract illness in infants [10]. Pediatricians are often challenged when it comes to discriminating between the two viruses, as they have similar clinical presentations. RSV and influenza might have a common ecologic niche in the respiratory tract. In vitro experiments sug- gest that while RSV grows in MDCK cells, co-infection with IAV leads to a reduction of progeny RSV [11]. Consistently, epidemiologic studies show that when RSV infections are Viruses 2021, 13, 1630. https://doi.org/10.3390/v13081630 https://www.mdpi.com/journal/viruses