Legionnaire’s Disease in Compromised Hosts

Severe Legionnaires' disease

BACKGROUND

Legionnaires' disease (LD) is a common but under-diagnosed cause of community-acquired pneumonia (CAP), although rapid detection of urine antigen testing (UAT) and advances in molecular testing have improved the diagnosis. LD entails intensive care unit (ICU) admission in almost one-third of cases, and the mortality rate ranges from 4% to 40%. This review aims to discuss recent advances in the study of this condition and to provide an update on the diagnosis, pathogenesis and management of severe LD.

RESULTS

The overall incidence of LD has increased worldwide in recent years due to the higher number of patients with risk factors, especially immunosuppression, and to improvements in diagnostic methods. Although LD is responsible for only around 5% of all-cause CAP, it is one of the three most common causes of CAP requiring ICU admission. Mortality in ICU patients, immunocompromised patients or patients with a nosocomial source of LD can reach 40% despite appropriate antimicrobial therapy. Regarding pathogenesis, no Legionella-specific virulence factors have been associated with severity; however, recent reports have found high pulmonary Legionella DNA loads, and impairments in immune response and lung microbiome in the most severe cases. The clinical picture includes severe lung injury requiring respiratory and/or hemodynamic support, extrapulmonary symptoms and non-specific laboratory findings. LD diagnostic methods have improved due to the broad use of UAT and the development of molecular methods allowing the detection of all Lp serogroups. Therapy is currently based on macrolides, quinolones, or a combination of the two, with prolonged treatment in severe cases.

CONCLUSIONS

Numerous factors influence the mortality rate of LD, such as ICU admission, the underlying immune status, and the nosocomial source of the infection. The host immune response (hyperinflammation and/or immunoparalysis) may also be associated with increased severity. Given that the incidence of LD is rising, studies on specific biomarkers of severity may be of great interest. Further assessments comparing different regimens and/or evaluating host-directed therapies are nowadays needed.

Legionnaires Disease in Solid Organ Transplant Recipients: A Decade-Long Nationwide Study in France

BACKGROUND

Legionnaires disease (LD) is a rare, life-threatening opportunistic bacterial infection that poses a significant risk to patients with impaired cell-mediated immunity such as solid organ transplant recipients. However, the epidemiologic features, clinical presentation, and outcomes of LD in this population are poorly described.

RESEARCH QUESTION

What are the clinical manifestations, radiologic presentation, risk factors for severity, treatment, and outcome of LD in solid organ transplant recipients?

STUDY DESIGN AND METHODS

In this 10-year multicenter retrospective cohort study in France, where LD notification is mandatory, patients were identified by hospital discharge databases. Diagnosis of LD relied on positive culture findings from any respiratory sample, positive urinary antigen test (UAT) results, positive specific serologic findings, or a combination thereof. Severe LD was defined as admission to the ICU.

RESULTS

One hundred one patients from 51 transplantation centers were eligible; 64 patients (63.4%) were kidney transplant recipients. Median time between transplantation and LD was 5.6 years (interquartile range, 1.5-12 years). UAT results were positive in 92% of patients (89/97). Among 31 patients with positive culture findings in respiratory samples, Legionella pneumophila serogroup 1 was identified in 90%. Chest CT imaging showed alveolar consolidation in 98% of patients (54 of 57), ground-glass opacity in 63% of patients (36 of 57), macronodules in 21% of patients (12 of 57), and cavitation in 8.8% of patients (5 of 57). Fifty-seven patients (56%) were hospitalized in the ICU. In multivariate analysis, severe LD was associated with negative UAT findings at presentation (P = .047), lymphopenia (P = .014), respiratory symptoms (P = .010), and pleural effusion (P = .039). The 30-day and 12-month mortality rates were 8% (8 of 101) and 20% (19 of 97), respectively. In multivariate analysis, diabetes mellitus was the only factor associated with 12-month mortality (hazard ratio, 3.2; 95% OR, 1.19-8.64; P = .022).

INTERPRETATION

LD is a late and severe complication occurring in solid organ transplant recipients that may present as pulmonary nodules on which diabetes impacts its long-term prognosis.

Legionnaires' Disease in Occupational Settings: A Cross-Sectional Study from Northeastern Italy (2019)

In Italy, Legionnaires' Disease (LD) causes >1000 hospital admissions per year, with a lethality rate of 5 to 10%. Occupational exposures could reasonably explain a substantial share of total cases, but the role of Occupational Physicians (OPs) in management and prevention of LD has been scarcely investigated. The present survey therefore evaluates the knowledge, attitudes and practices (KAP) regarding LD from a convenience sample of Italian OPs, focusing on their participation in preventive interventions. A total of 165 OPs were recruited through a training event (Parma, Northeastern Italy, 2019), and completed a specifically designed structured questionnaire. The association between reported participation in preventive interventions and individual factors was analyzed using a binary logistic regression model, calculating corresponding multivariable Odds Ratio (aOR). Overall, participants exhibited satisfactory knowledge of the clinical and diagnostic aspects of LD, while substantial uncertainties were associated epidemiological factors (i.e., notification rate and lethality). Although the majority of participating OPs reportedly assisted at least one hospital (26.7%) and/or a nursing home (42.4%) and/or a wastewater treatment plant, only 41.8% reportedly contributed to the risk assessment for LD and 18.8% promoted specifically designed preventive measures. Working as OPs in nursing homes (aOR 8.732; 95% Confidence Intervals [95%CI] 2.991 to 25.487) and wastewater treatment plants (aOR 8.710; 95%CI 2.844 to 26.668) was associated with participation in the risk assessment for LD, while the promotion of preventive practice was associated with working as an OP in hospitals (aOR 6.792; 95%CI 2.026 to 22.764) and wastewater treatment plants (aOR 4.464, 95%CI 1.363 to 14.619). In other words, the effective participation of the OP in the implementation of preventive measures appears uncommon and is limited to certain occupational settings. Collectively, these results highlight the importance of tailoring specifically designed information campaigns aimed to raise the involvement of OPs in the prevention of LD in occupational settings other than healthcare.

Hiding in the yolk: A unique feature of Legionella pneumophila infection of zebrafish

The zebrafish has become a powerful model organism to study host-pathogen interactions. Here, we developed a zebrafish model to dissect the innate immune response to Legionella pneumophila during infection. We show that L. pneumophila cause zebrafish larvae death in a dose dependent manner. Additionally, we show that macrophages are the first line of defence and cooperate with neutrophils to clear the infection. Immunocompromised humans have an increased propensity to develop pneumonia, when either macrophages or neutrophils are depleted, these "immunocompromised" larvae become lethally sensitive to L. pneumophila. Also, as observed in human infections, the adaptor signalling molecule Myd88 is not required to control disease in the larvae. Furthermore, proinflammatory cytokine genes il1β and tnf-α were upregulated during infection, recapitulating key immune responses seen in human infection. Strikingly, we uncovered a previously undescribed infection phenotype in zebrafish larvae, whereby bloodborne, wild type L. pneumophila invade and grow in the larval yolk region, a phenotype not observed with a type IV secretion system deficient mutant that cannot translocate effectors into its host cell. Thus, zebrafish larva represents an innovative L. pneumophila infection model that mimics important aspects of the human immune response to L. pneumophila infection and will allow the elucidation of mechanisms by which type IV secretion effectors allow L. pneumophila to cross host cell membranes and obtain nutrients from nutrient rich environments.

Legionnaires' disease in the EU/EEA*: increasing trend from 2017 to 2019

BackgroundThe burden of Legionnaires' disease (LD) in the European Union/European Economic Area (EU/EEA) has increased during the last decade, with notification rates increasing from 1.2 to 1.4/100,000 population in 2012-16, to 1.8-2.2 within 2017-19.AimTo measure weekly excess cases during 2017-19 based on previous trends and determine whether a significant change in trend occurred, and to examine any differences in age, sex or level of imported infections.MethodsWe collated 2012-19 annual surveillance data from The European Surveillance System (TESSy) reported by EU/EEA countries. A retrospective prediction by a dynamic regression model was created from 2012-16 data to assess excess cases in 2017-19. Interrupted time series (ITS) analysis was performed to determine if a significant change in trend occurred in 2017-19 compared with the previous 5 years.ResultsWe found a 33.9% increase in cases in 2017-19 compared with the number predicted. The ITS also found a significant trend increase in 2017-19 compared with 2012-16. A significant trend increase was observed from 2017 most strongly among older age groups (> 60 years) and non-imported cases.ConclusionOur study showed a significant increasing trend in LD cases in the EU/EEA during 2017-19 compared with the previous 5 years. The distribution of cases per week suggests an overall amplification of the seasonal trends. These findings underscore that LD continues to be an infectious disease of public health concern in the EU/EEA, warranting further research into determinants of the increase.

Legionnaires' Disease: Update on Diagnosis and Treatment

Legionellosis is the infection caused by bacteria of the genus Legionella, including a non-pneumonic influenza-like syndrome, and Legionnaires’ disease is a more serious illness characterized by pneumonia. Legionellosis is becoming increasingly important as a public health problem throughout the world; although it is an underreported disease, studies have consistently documented a high incidence. In addition, health costs associated with the disease are high. Diagnosis of Legionnaires’ disease is based mainly on the detection of Legionella pneumophila serogroup 1 antigen in urine. However, there have been advances in detection tests for patients with legionellosis. New methodologies show greater sensitivity and specificity, detect more species and serogroups of Legionella spp., and have the potential for use in epidemiological studies. Testing for Legionella spp. is recommended at hospital admission for severe community-acquired pneumonia, and antibiotics directed against Legionella spp. should be included early as empirical therapy. Inadequate or delayed antibiotic treatment in Legionella pneumonia has been associated with a worse prognosis. Either a fluoroquinolone (levofloxacin or moxifloxacin) or a macrolide (azithromycin preferred) is the recommended first-line therapy for Legionnaires’ disease; however, little information is available regarding adverse events or complications, or about the duration of antibiotic therapy and its association with clinical outcomes. Most published studies evaluating antibiotic treatment for Legionnaires’ disease are observational and consequently susceptible to bias and confounding. Well-designed studies are needed to assess the usefulness of diagnostic tests regarding clinical outcomes, as well as randomized trials comparing fluoroquinolones and macrolides or combination therapy that evaluate outcomes and adverse events.

SARS-CoV-2-Legionella Co-Infections: A Systematic Review and Meta-Analysis (2020-2021)

Legionnaires' Disease (LD) is a severe, sometimes fatal interstitial pneumonia due to Legionella pneumophila. Since the inception of the SARS-CoV-2 pandemic, some contradictory reports about the effects of lockdown measures on its epidemiology have been published, but no summary evidence has been collected to date. Therefore, we searched two different databases (PubMed and EMBASE) focusing on studies that reported the occurrence of LD among SARS-CoV-2 cases. Data were extracted using a standardized assessment form, and the results of such analyses were systematically reported, summarized, and compared. We identified a total of 38 articles, including 27 observational studies (either prospective or retrospective ones), 10 case reports, and 1 case series. Overall, data on 10,936 SARS-CoV-2 cases were included in the analyses. Of them, 5035 (46.0%) were tested for Legionella either through urinary antigen test or PCR, with 18 positive cases (0.4%). A pooled prevalence of 0.288% (95% Confidence Interval (95% CI) 0.129-0.641), was eventually calculated. Moreover, detailed data on 19 co-infections LD + SARS-CoV-2 were obtained (males: 84.2%; mean age: 61.9 years, range 35 to 83; 78.9% with 1 or more underlying comorbidities), including 16 (84.2%) admissions to the ICU, with a Case Fatality Ratio of 26.3%. In summary, our analyses suggest that the occurrence of SARS-CoV-2-Legionella infections may represent a relatively rare but not irrelevant event, and incident cases are characterized by a dismal prognosis.

Epidemiology of Legionnaires' Disease in Italy, 2004-2019: A Summary of Available Evidence

Legionnaires’ disease (LD) incidence has been increasing in several European countries since 2011. Currently, Italy is experiencing high notification rates for LD, whose cause still remains scarcely understood. We sought to summarize the available evidence on the epidemiology of LD in Italy (2004–2019), characterizing the risk of LD by region, sex, age group, and settings of the case (i.e., community, healthcare, or travel-associated cases). Environmental factors (e.g., average air temperatures and relative humidity) were also included in a Poisson regression model in order to assess their potential role on the annual incidence of new LD cases. National surveillance data included a total of 23,554 LD cases occurring between 2004 and 2019 (70.4% of them were of male gender, 94.1% were aged 40 years and older), with age-adjusted incidence rates increasing from 1.053 cases per 100,000 in 2004 to 4.559 per 100,000 in 2019. The majority of incident cases came from northern Italy (43.2% from northwestern Italy, 25.6% from northeastern Italy). Of these, 5.9% were healthcare-related, and 21.1% were travel-associated. A case-fatality ratio of 5.2% was calculated for the whole of the assessed timeframe, with a pooled estimate for mortality of 0.122 events per 100,000 population per year. Poisson regression analysis was associated with conflicting results, as any increase in average air temperature resulted in reduced risk for LD cases (Incidence Rate Ratio [IRR] 0.807, 95% Confidence Interval [95% CI] 0.744–0.874), while higher annual income in older individuals was associated with an increased IRR (1.238, 95% CI 1.134–1.351). The relative differences in incidence between Italian regions could not be explained by demographic factors (i.e., age and sex distribution of the population), and also a critical reappraisal of environmental factors failed to substantiate both the varying incidence across the country and the decennial trend we were able to identify.

Environmental Surveillance of Legionella spp. in an Italian University Hospital Results of 10 Years of Analysis

The occurrence of Legionella spp. in the water distribution systems of large hospitals and other healthcare facilities is considered particularly dangerous, due to the critical nature of the hospitalized patients. The aim of this study is to present a pluri-annual environmental surveillance in a large university hospital assessing the prevalence of Legionella spp. and underlining its variability over the years. The samples of water were collected in accordance with the Italian National Guidelines and the sampling sites considered in this study were selected favoring wards with very high-risk patients and with patients at increased risk. The laboratory analyzed a total of 305 water samples deriving from 24 different sampling points. Legionella spp. were detected in 39.4% of samples, the majority of which were contaminated by Legionella pneumophila serogroups 2–14 (68.7%). Statistically significant differences were found among different seasons with a linear trend in positive proportion from summer to spring. Several experimental interventions to prevent and reduce Legionella colonization were attempted, but there is no a definitive method for the complete eradication of this microorganism. The permanent monitoring of hospital water distribution systems is fundamental to preventing the potential risk of nosocomial Legionellosis and to implementing procedures to minimize the risk of Legionella spp. colonization.

Legionellosis after hematopoietic stem cell transplantation

Limited data are available on legionellosis after hematopoietic stem cell transplant (HSCT). The aim of this study was to report the cases of legionellosis and to identify predictors of legionellosis, legionellosis-associated death, and non-relapse mortality (NRM). All cases of post-HSCT legionellosis from the EBMT registry were included and matched with controls in a 3:1 ratio for the analyses of risk factors. In the years 1995-2016, 80 cases from 52 centers in 14 countries were identified (mainly from France, Italy, and Spain). Median time from HSCT to legionellosis was 203 days (range, 0-4099); 19 (23.8%) patients developed early legionellosis (within-day +30 post-HSCT). Patients were mainly male (70%), after allogeneic HSCT (70%), with acute leukemia (27.5%), lymphoma (23.8%), or multiple myeloma (21.3%), and the median age of 46.6 (range, 7.2-68.2). Predictors of legionellosis were allogeneic HSCT (OR = 2.27, 95%CI:1.08-4.80, p = 0.03) and recent other infection (OR = 2.96, 95%CI:1.34-6.52, p = 0.007). Twenty-seven (33.8%) patients died due to legionellosis (44% after early legionellosis), NRM was 50%. Predictors of NRM were female sex (HR = 2.19, 95%CI:1.13-4.23, p = 0.02), early legionellosis (HR = 2.24, 95%CI:1.13-4.46, p = 0.02), and south-eastern geographical region (HR = 2.16, 95%CI:1.05-4.44, p = 0.036). In conclusion, legionellosis is a rare complication after HSCT, mainly allogeneic, occurring frequently within 30 days after HSCT and associated with high mortality.

Atypical Pneumonia: Definition, Causes, and Imaging Features

Pneumonia is among the most common causes of death worldwide. The epidemiologic and clinical heterogeneity of pneumonia results in challenges in diagnosis and treatment. There is inconsistency in the definition of the group of microorganisms that cause "atypical pneumonia." Nevertheless, the use of this term in the medical and radiologic literature is common. Among the causes of community-acquired pneumonia, atypical bacteria are responsible for approximately 15% of cases. Zoonotic and nonzoonotic bacteria, as well as viruses, have been considered among the causes of atypical pneumonia in a patient who is immunocompetent and have been associated with major community outbreaks of respiratory infection, with relevant implications in public health policies. Considering the difficulty of isolating atypical microorganisms and the significant overlap in clinical manifestations, a targeted empirical therapy is not possible. Imaging plays an important role in the diagnosis and management of atypical pneumonia, as in many cases its findings may first suggest the possibility of an atypical infection. Clarifying and unifying the definition of atypical pneumonia among the medical community, including radiologists, are of extreme importance. The prompt diagnosis and prevention of community spread of some atypical microorganisms can have a relevant impact on local, regional, and global health policies. ^©RSNA, 2021.

[Legionelosis en España, 2010-2015]

Introduction: Legionellosis is a bacterial respiratory disease with an environmental origin in the community or in hospitals; it is usually associated with devices, facilities, and buildings. The most common clinical form is the pneumonic, known as legionnaires’ disease. Objective: To determine the evolution of legionellosis cases in Spain from 2010 to 2015. Materials and methods: This was a descriptive study of time series with an analysis of the legionellosis cases notified to the Centro Nacional de Epidemiología (Government of Spain). Case distribution was determined according to sex, the autonomous community of origin, month, and age groups differentiating in the latter between men and women. Results: Case count in men was more than double compared to that in women. The cases notified by nine of the autonomous communities showed an increase at the end of the period, especially in Castilla y León, Navarra, and the Basque Country but also notable in Castilla-La Mancha. A seasonal pattern was identified with an epidemic peak in July-September and a greater number of cases among 50-years old people from both sexes. Conclusion: Despite its low prevalence compared to other respiratory diseases, legionellosis has a great impact on public health. Its distribution in Spain is global and heterogeneous with cases increasing in the last two years. Therefore, better disease prevention and control measures are recommended.

Slowly or Nonresolving Legionnaires' Disease: Case Series and Literature Review

BACKGROUND

Rarely, Legionnaires' disease (LD) can progress into a slowly or nonresolving form.

METHODS

A nationwide retrospective study was conducted by the French National Reference Center for Legionella (2013-2017) including cases of slowly or nonresolving LD defined as persistent clinical symptoms, computed tomography (CT) scan abnormalities, and Legionella detection in lower respiratory tract specimens by culture and/or real-time (RT) polymerase chain reaction (PCR) >30 days after symptom onset.

RESULTS

Twelve cases of community-acquired slowly or nonresolving LD were identified among 1686 cases of culture-positive LD. Median (interquartile range [IQR]) age was 63 (29-82) years. Ten (83.3%) patients had ≥1 immunosuppressive factor. Clinically, 9 patients transiently recovered before further deterioration (median [IQR] symptom-free interval, 30 [18-55] days), 3 patients had uniformly persistent symptoms (median [IQR] time, 48 [41.5-54] days). Two patients had >2 recurrences. CT scan imagery found lung abscess in 5 (41.6%) cases. Slowly or nonresolving LD was diagnosed on positive Legionella cultures (n = 10, 83.3%) at 49.5 (IQR, 33.7-79) days. Two cases were documented through positive Legionella RT PCR at 52 and 53 days (cycle threshold detection of 21.5 and 33.7, respectively). No genomic microevolution and no Legionella resistance to antibiotics were detected. The median (IQR) duration of treatment was 46.5 (21-92.5) days. Two empyema cases required thoracic surgery. At a median (IQR) follow-up of 26 (14-41.5) months, LD-attributable mortality was 16.6% (n = 2).

CONCLUSIONS

Slowly or nonresolving LD may occur in immunocompromised patients, possibly leading to lung abscess and empyema.

The clinical presentation of Legionella arthritis reveals the mode of infection and the bacterial species: case report and literature review

BACKGROUND

While Legionella is a common cause of pneumonia, extrapulmonary infections like arthritis are scarce. Here, we describe a case of monoarthritis due to Legionella bozemanii, with no history of pneumonia. We provide a literature review of the 9 previously published Legionella arthritis and highlight a dichotomous epidemiology suggesting different physiopathological pathways leading to joint infection.

CASE PRESENTATION

A 56-year old woman under immunosuppressive treatment by oral and intra-articular corticosteroids, methotrexate, and tocilizumab for an anti-synthetase syndrome was hospitalized for worsening pain and swelling of the left wrist for 3 days. Clinical examination showed left wrist synovitis and no fever. The arthritis occurred a few days after an accidental fall on wet asphalt responsible for a cutaneous wound followed by a corticosteroid intra-articular injection. Due to both the negativity of conventional culture of articular fluid and suspicion of infection, 16S rRNA and specific PCRs were performed leading to the identification of L. bozemanii. Legionella-specific culture of the articular fluid was performed retrospectively and isolated L. bozemanii. The empiric antibiotic therapy was switched for oral levofloxacin and rifampin and the patient recovered after a 12-week treatment.

CONCLUSION

We report a case of L. bozemanii monoarthritis in an immunosuppressed woman, following a fall on wet asphalt and intra-articular corticosteroid injection. The review of the literature found that the clinical presentation reveals the mode of infection and the bacterial species. Monoarthritis more likely occurred after inoculation in patients under immunosuppressive therapy and were associated with non-Legionella pneumophila serogroup 1 (Lp1) strains that predominate in the environment. Polyarthritis were more likely secondary legionellosis localizations after blood spread of Lp1, the most frequently found in pneumonia. In both settings, 16S rRNA and Legionella-specific PCR were key factors for the diagnosis.

Environmental Surveillance of Legionellosis within an Italian University Hospital-Results of 15 Years of Analysis

Legionnaires' disease is normally acquired by inhalation of legionellae from a contaminated environmental source. Water systems of large and old buildings, such as hospitals, can be contaminated with legionellae and therefore represent a potential risk for the hospital population. In this study, we demonstrated the constant presence of Legionella in water samples from the water system of a large university hospital in Messina (Sicily, Italy) consisting of 11 separate pavilions during a period of 15 years (2004⁻2018). In total, 1346 hot water samples were collected between January 2004 and December 2018. During this period, to recover Legionella spp. from water samples, the standard procedures reported by the 2000 Italian Guidelines were adopted; from May 2015 to 2018 Italian Guidelines revised in 2015 (ISS, 2015) were used. Most water samples (72%) were positive to L. pneumophila serogroups 2⁻14, whereas L. pneumophila serogroup 1 accounted for 18% and non-Legionellapneumophila spp. Accounted for 15%. Most of the positive samples were found in the buildings where the following critical wards are situated: (Intensive Care Unit) ICU, Neurosurgery, Surgeries, Pneumology, and Neonatal Intensive Unit Care. This study highlights the importance of the continuous monitoring of hospital water samples to prevent the potential risk of nosocomial legionellosis.

Community acquired pneumonia by Legionella pneumophila: Study of 136 cases

OBJECTIVE

Most of the data on Legionella pneumonia in our country come from the Mediterranean area, and there are few studies from the Northwest area. This study tries to assess the situation of this infection in this area.

METHOD

Retrospective study of all patients with positive Legionella antigenuria treated at the University Hospital Lucus Augusti in Lugo (Spain) from 2001, the year in which this test was introduced in our centre, until 2015. We analysed epidemiological data, risk factors, clinical, radiological and biochemical findings, and clinical outcome.

RESULTS

The sampled included 136 patients. When comparing the first five years of the study with the last five, the incidence increased from 10.9 to 64.5 cases/1,000,000; the number of antigenuria requests increased 3.4 times, and compared to other pneumonia aetiologies Legionella increased from 0.9% to 15%. The mean age was 64.1years and 84.6% were males; 74.3% had comorbidities. Males were significantly younger (62.7±16.6 vs 71.9±17.3) and consumed more alcohol (26.1% vs 0%) and tobacco (67.8% vs 14.3%). Diagnosis was established within the first 72hours in 88.9% of cases and most received levofloxacin (95.6%). Hospitalisation was needed in 85% of cases, 11.7% in ICU and 4.4% died.

CONCLUSIONS

After the introduction of antigenuria there was an increase in the incidence of Legionella pneumonia recorded in our health area. Its rate in recent years has been one of the highest in our country. Despite the fact that the patients had advanced age and comorbidities, mortality was low.

Study of the radiologic features of Legionnaires' disease with mediastinal adenopathy: Legionella or lymphoma?

An index case of Legionnaires's disease with mediastinal adenopathy prompted us to review our recent experience with Legionnaires' disease to determine the incidence of mediastinal adenopathy of this finding in Legionnaires' disease. We reviewed the radiographic findings of 90 hospitalized adults with Legionnaires' disease from 2015 to 2017. Excluded were 11 patients with mediastinal adenopathy due to non-Legionnaires' disease causes, e.g., lymphoma. Thirty-seven of the remaining patients had both chest films and chest computed tomography (CT) scans. Of the 37 Legionnaires' disease cases, 13/37 (35%) had mediastinal adenopathy and 8/27 (24%) also had unilateral hilar adenopathy. These chest CT findings were not seen on chest films. Chest CT scans are needed to detect mediastinal adenopathy in Legionnaires' disease. Mediastinal adenopathy may be due to Legionnaires' disease or a malignancy. Some findings in Legionnaires' disease are also present in mediastinal adenopathy due to lymphomas, e.g., highly elevated erythrocyte sedimentation rate (ESR), lactate dehydrogenase (LDH), and ferritin. Hospitalized adults with Legionnaires' disease and mediastinal adenopathy should have serial chest CT scans to monitor resolution of the mediastinal adenopathy. In hospitalized adults with otherwise unexplained persistent mediastinal adenopathy, they should be considered as being due to another etiology, e.g., lymphoma, until proven otherwise.