Secondary Bacterial Infections Associated with Influenza Pandemics

An international perspective on hospitalized patients with viral community-acquired pneumonia

BACKGROUND

Who should be tested for viruses in patients with community acquired pneumonia (CAP), prevalence and risk factors for viral CAP are still debated. We evaluated the frequency of viral testing, virus prevalence, risk factors and treatment coverage with oseltamivir in patients admitted for CAP.

METHODS

Secondary analysis of GLIMP, an international, multicenter, point-prevalence study of hospitalized adults with CAP. Testing frequency, prevalence of viral CAP and treatment with oseltamivir were assessed among patients who underwent a viral swab. Univariate and multivariate analysis was used to evaluate risk factors.

RESULTS

553 (14.9%) patients with CAP underwent nasal swab. Viral CAP was diagnosed in 157 (28.4%) patients. Influenza virus was isolated in 80.9% of cases. Testing frequency and viral CAP prevalence were inhomogeneous across the participating centers. Obesity (OR 1.59, 95%CI: 1.01-2.48; p = 0.043) and need for invasive mechanical ventilation (OR 1.62, 95%CI: 1.02-2.56; p = 0.040) were independently associated with viral CAP. Prevalence of empirical treatment with oseltamivir was 5.1%.

CONCLUSION

In an international scenario, testing frequency for viruses in CAP is very low. The most common cause of viral CAP is Influenza virus. Obesity and need for invasive ventilation represent independent risk factors for viral CAP. Adherence to recommendations for treatment with oseltamivir is poor.

A Case of Influenza B and Mycoplasma pneumoniae Coinfection in an Adult

A 19-year-old woman was referred to our hospital because of a persistent fever and cough that lasted for over a week. Influenza B virus infection was diagnosed using the rapid test kit. Initially, the patient was diagnosed with influenza B infection associated with lobar pneumonia and treated with an anti-influenza virus drug and sulbactam/ampicillin. The patient's fever persisted, and her respiratory condition worsened. On day 5, a computed tomography (CT) scan revealed an extension of the consolidation areas in the left lung and new opacities in the right lung. The antibiotic treatment was changed to meropenem and levofloxacin, and the patient's physical condition gradually improved. A sputum sample revealed the presence of Mycoplasma pneumoniae-specific DNA. Both influenza B virus and M. pneumoniae infections were confirmed serologically. This was a case of coinfection with influenza B virus and M. pneumoniae in a healthy young woman. The M. pneumoniae pneumonia diagnosis was delayed because the predominant feature observed in the CT scan was dense consolidation. M. pneumoniae should be considered as one of the causative pathogens in influenza coinfection cases with CT scan images presenting dense consolidation.

Impact of bacterial and viral coinfection in community-acquired pneumonia in adults

Bacterial and viral coinfected community-acquired pneumonia (CAP) is poorly characterized in adults. The aim of this study was to investigate the influence of bacterial and viral coinfection in patients with CAP. A total of 235 adults who requested molecular tests of pneumonia and were diagnosed with CAP were enrolled in this study. Microbiological tests included blood and sputum cultures, PCR for bacterial and viral pathogens, antigen test for Streptococcus pneumoniae and the influenza virus, and antibody detection of Mycoplasma pneumonia. Of the 235 patients, 32 (13.6%) patients were coinfected with bacteria and virus. Among 64 severe CAP patients, the concurrent infections were confirmed in 14 patients (21.9%). The proportion of severe pneumonia was significantly higher in patients with coinfection, and they showed a significantly higher mortality rate. In conclusion, bacterial and viral coinfection in CAP is not a rare occurrence in adults. Viral and bacterial coinfections have an adverse impact on the severity of the pneumonia, and increase morbidity and mortality in patients with CAP.

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13.6% CAP patients and 21.9% severe CAP patients were coinfected with bacteria and virus in adults.

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Bacterial and viral coinfection may adversely impact the severity of pneumonia.

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Concurrent infection also increases the morbidity in adult patients with CAP.

Animal models used to assess influenza antivirals

INTRODUCTION

Influenza continues to be a major public health concern. Antivirals play an important role in limiting the burden of disease and preventing infection and/or transmission. The developments of such agents are heavily dependent on pre-clinical evaluation where animal models are used to answer questions that cannot be easily addressed in human clinical trials. There are numerous animal models available to study the potential benefits of influenza antivirals but each animal model has its own pros and cons. Areas covered: In this review, the authors describe the advantages and disadvantages of using mice, ferrets, guinea pigs, cotton rats, golden hamsters and non-human primates to evaluate influenza therapeutics. Expert opinion: Animals used for evaluating influenza therapeutics differ in their susceptibility to influenza virus infection, their ability to display clinical signs of illness following viral infection and in their practical requirements such as housing. Therefore, defining the scientific question being asked and the data output required will assist in selecting the most appropriate animal model.

Coinfection With Influenza A Virus and Klebsiella oxytoca: An Underrecognized Impact on Host Resistance and Tolerance to Pulmonary Infections

Pneumonia is a world health problem and a leading cause of death, particularly affecting children and the elderly (1, 2). Bacterial pneumonia following infection with influenza A virus (IAV) is associated with increased morbidity and mortality but the mechanisms behind this phenomenon are not yet well-defined (3). Host resistance and tolerance are two processes essential for host survival during infection. Resistance is the host's ability to clear a pathogen while tolerance is the host's ability to overcome the impact of the pathogen as well as the host response to infection (4-8). Some studies have shown that IAV infection suppresses the immune response, leading to overwhelming bacterial loads (9-13). Other studies have shown that some IAV/bacterial coinfections cause alterations in tolerance mechanisms such as tissue resilience (14-16). In a recent analysis of nasopharyngeal swabs from patients hospitalized during the 2013-2014 influenza season, we have found that a significant proportion of IAV-infected patients were also colonized with Klebsiella oxytoca, a gram-negative bacteria known to be an opportunistic pathogen in a variety of diseases (17). Mice that were infected with K. oxytoca following IAV infection demonstrated decreased survival and significant weight loss when compared to mice infected with either single pathogen. Using this model, we found that IAV/K. oxytoca coinfection of the lung is characterized by an exaggerated inflammatory immune response. We observed early inflammatory cytokine and chemokine production, which in turn resulted in massive infiltration of neutrophils and inflammatory monocytes. Despite this swift response, the pulmonary pathogen burden in coinfected mice was similar to singly-infected animals, albeit with a slight delay in bacterial clearance. In addition, during coinfection we observed a shift in pulmonary macrophages toward an inflammatory and away from a tissue reparative phenotype. Interestingly, there was only a small increase in tissue damage in coinfected lungs as compared to either single infection. Our results indicate that during pulmonary coinfection a combination of seemingly modest defects in both host resistance and tolerance may act synergistically to cause worsened outcomes for the host. Given the prevalence of K. oxytoca detected in human IAV patients, these dysfunctional tolerance and resistance mechanisms may play an important role in the response of patients to IAV.

Natural Killer T Cells and Mucosal-Associated Invariant T Cells in Lung Infections

The immune system has been traditionally divided into two arms called innate and adaptive immunity. Typically, innate immunity refers to rapid defense mechanisms that set in motion within minutes to hours following an insult. Conversely, the adaptive immune response emerges after several days and relies on the innate immune response for its initiation and subsequent outcome. However, the recent discovery of immune cells displaying merged properties indicates that this distinction is not mutually exclusive. These populations that span the innate-adaptive border of immunity comprise, among others, CD1d-restricted natural killer T cells and MR1-restricted mucosal-associated invariant T cells. These cells have the unique ability to swiftly activate in response to non-peptidic antigens through their T cell receptor and/or to activating cytokines in order to modulate many aspects of the immune response. Despite they recirculate all through the body via the bloodstream, these cells mainly establish residency at barrier sites including lungs. Here, we discuss the current knowledge into the biology of these cells during lung (viral and bacterial) infections including activation mechanisms and functions. We also discuss future strategies targeting these cell types to optimize immune responses against respiratory pathogens.

Secondary Bacterial Pneumonia by Staphylococcus aureus Following Influenza A Infection Is SaeR/S Dependent

Staphylococcus aureus is a predominant cause of fatal pneumonia following influenza A virus (IAV) infection. Herein we investigate the influence of antecedent IAV infection on S. aureus virulence gene expression. Using a murine model, comparing the USA300 and USA300ΔsaeR/S strains, we demonstrate that S. aureus pathogenesis following IAV infection is SaeR/S dependent. Furthermore, we show that IAV modulates the lung environment to rapidly up-regulate S. aureus virulence factors containing the SaeR-binding domain. Data demonstrate that the pathogen response to IAV infection impacts host outcome and provides evidence that the ability of S. aureus to sense and respond to the lung environment determines severity of pneumonia.

The endotoxin-induced pulmonary inflammatory response is enhanced during the acute phase of influenza infection

Background

Influenza infections are often complicated by secondary infections, which are associated with high morbidity and mortality, suggesting that influenza profoundly influences the immune response towards a subsequent pathogenic challenge. However, data on the immunological interplay between influenza and secondary infections are equivocal, with some studies reporting influenza-induced augmentation of the immune response, whereas others demonstrate that influenza suppresses the immune response towards a subsequent challenge. These contrasting results may be due to the use of various types of live bacteria as secondary challenges, which impedes clear interpretation of causal relations, and to differences in timing of the secondary challenge relative to influenza infection. Herein, we investigated whether influenza infection results in an enhanced or suppressed innate immune response upon a secondary challenge with bacterial lipopolysaccharide (LPS) in either the acute or the recovery phase of infection.

Methods

Male C57BL/6J mice were intranasally inoculated with 5 × 10³ PFU influenza virus (pH1N1, strain A/Netherlands/602/2009) or mock treated. After 4 (acute phase) or 10 (recovery phase) days, 5 mg/kg LPS or saline was administered intravenously, and mice were sacrificed 90 min later. Cytokine levels in plasma and lung tissue, and lung myeloperoxidase (MPO) content were determined.

Results

LPS administration 4 days after influenza infection resulted in a synergistic increase in TNF-α, IL-1β, and IL-6 concentrations in lung tissue, but not in plasma. This effect was also observed 10 days after influenza infection, albeit to a lesser extent. LPS-induced plasma levels of the anti-inflammatory cytokine IL-10 were enhanced 4 days after influenza infection, whereas a trend towards increased pulmonary IL-10 concentrations was found. LPS-induced increases in pulmonary MPO content tended to be enhanced as well, but only at 4 days post-infection.

Conclusions

An LPS challenge in the acute phase of influenza infection results in an enhanced pulmonary pro-inflammatory innate immune response. These data increase our insight on influenza-bacterial interplay. Combing data of the present study with previous findings, it appears that this enhanced response is not beneficial in terms of protection against secondary infections, but rather damaging by increasing immunopathology.

Influenza

Influenza is an infectious respiratory disease that, in humans, is caused by influenza A and influenza B viruses. Typically characterized by annual seasonal epidemics, sporadic pandemic outbreaks involve influenza A virus strains of zoonotic origin. The WHO estimates that annual epidemics of influenza result in ~1 billion infections, 3–5 million cases of severe illness and 300,000–500,000 deaths. The severity of pandemic influenza depends on multiple factors, including the virulence of the pandemic virus strain and the level of pre-existing immunity. The most severe influenza pandemic, in 1918, resulted in >40 million deaths worldwide. Influenza vaccines are formulated every year to match the circulating strains, as they evolve antigenically owing to antigenic drift. Nevertheless, vaccine efficacy is not optimal and is dramatically low in the case of an antigenic mismatch between the vaccine and the circulating virus strain. Antiviral agents that target the influenza virus enzyme neuraminidase have been developed for prophylaxis and therapy. However, the use of these antivirals is still limited. Emerging approaches to combat influenza include the development of universal influenza virus vaccines that provide protection against antigenically distant influenza viruses, but these vaccines need to be tested in clinical trials to ascertain their effectiveness.

The nasopharyngeal microbiome

Human microbiomes have received increasing attention over the last 10 years, leading to a pervasiveness of hypotheses relating dysbiosis to health and disease. The respiratory tract has received much less attention in this respect than that of, for example, the human gut. Nevertheless, progress has been made in elucidating the immunological, ecological and environmental drivers that govern these microbial consortia and the potential consequences of aberrant microbiomes. In this review, we consider the microbiome of the nasopharynx, a specific niche of the upper respiratory tract. The nasopharynx is an important site, anatomically with respect to its gateway position between upper and lower airways, and for pathogenic bacterial colonisation. The dynamics of the latter are important for long-term respiratory morbidity, acute infections of both invasive and non-invasive disease and associations with chronic airway disease exacerbations. Here, we review the development of the nasopharyngeal (NP) microbiome over the life course, examining it from the early establishment of resilient profiles in neonates through to perturbations associated with pneumonia risk in the elderly. We focus specifically on the commensal, opportunistically pathogenic members of the NP microbiome that includes Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae and Moraxella catarrhalis. In addition, we consider the role of relatively harmless genera such as Dolosigranulum and Corynebacterium. Understanding that the NP microbiome plays such a key, beneficial role in maintaining equilibrium of commensal species, prevention of pathogen outgrowth and host immunity enables future research to be directed appropriately.