Increased Risk of Legionella Pneumonia as Community-acquired Pneumonia after Heavy Rainfall in 2018 in West Japan

Background: Japan experienced a heavy rainfall event from June 28 to July 8, 2018, and many casualties were caused by both heavy rainfall and ooding. Few studies have investigated patients’ characteristics and causative pathogens of community-acquired pneumonia before and after heavy rainfall events. The aim of the present study was to evaluate the causative pathogens and clinical characteristics of hospitalised patients with community-acquired pneumonia before and after the heavy rainfall event using prospective cohort data. Methods: The study was divided into two periods: July to November 2013-2017 (pre heavy rainfall) and July to November 2018 (post heavy rainfall). The patients’ clinical characteristic and causative pathogens before and after the heavy rainfall were investigated. Regarding the causative pathogens, precipitation and seasonal patterns were adjusted. Results: There were no signicant differences in the number and clinical characteristics of patients pre and post heavy rainfall. However, the frequency of Legionella pneumonia was signicantly higher after than before the heavy rainfall event (8.9% vs 3.0%, P = 0.02) and remained signicant after adjusting for precipitation and season. Three of 7 Legionella pneumonia patients engaged in reconstruction work and 2 Legionella pneumonia patients had soil exposure. Conclusions: An increased risk of Legionella pneumonia after not only rainfall and serious ooding, but also following recovery work or soil exposure should be considered.


Background
Pneumonia is one of the most problematic infectious diseases worldwide because of its high morbidity and mortality. The number of patients hospitalised with pneumonia is much higher than that of patients hospitalised with other respiratory diseases such as bronchial asthma, bronchitis, chronic obstructive pulmonary disease (COPD), and lung cancer [1].
Previous reports demonstrated that acute respiratory infections are a major cause of disease after hydrologic events such as tropical cyclones, typhoons, and other heavy rainfall-related events [2][3][4][5][6], and numbers of both outpatients and inpatients with pneumonia increase after such disasters [7,8].
Regarding the aetiology of pneumonia, the rate of Haemophilus in uenzae infection was signi cantly higher after tsunami and earthquake disasters [7]. Both contact with oodwaters and post-ooding cleaning were reported to increase the risk of respiratory infections [4].
Western Japan experienced a heavy rainfall event from June 28 to July 8, 2018. This event was called the "west Japan heavy rainfall," and it caused signi cant ood damage in Kurashiki city, where our hospital is located. Few studies have investigated patients' characteristics and causative pathogens of CAP before and after this heavy rainfall event. Thus, the aim of this study was to investigate the frequency of CAP and any differences in causative pathogens before and after the west Japan heavy rainfall event using a prospective cohort database.

Study design and setting
A prospective cohort study was conducted to investigate the clinical characteristics of hospitalised pneumonia patients beginning in October 2010 in Kurashiki Central Hospital, a 1,166-bed tertiary hospital in Okayama Prefecture, Japan (UMIN000004353). In particular, CAP patients who were admitted between 2013 and 2018 were analysed. CAP was de ned as recommended by the Infectious Diseases Society of America/American Thoracic Society guidelines [9]. These guidelines required any of the following: presence of in ltrate on chest imaging; some clinical symptoms (cough, sputum production, fever, dyspnoea, and pleuritic chest pain); and physical ndings of coarse crackles on auscultation and elevated in ammatory biomarkers, including C-reactive protein and white blood cell count on laboratory testing. The exclusion criteria were age < 15 years and nursing and healthcare-associated pneumonia (NHCAP). NHCAP patients were classi ed according to the Japanese Respiratory Society guidelines [10].
Patient information (age, sex, underlying medical problems, antibiotic therapy, pneumonia severity, mortality rate) and causative pathogens were evaluated. Pneumonia severity was evaluated based on the CURB-65 [11] and the Pneumonia Severity Index (PSI) [12]. Patients' clinical characteristics and causative pathogens before and after the west Japan heavy rainfall event were compared using prospective cohort data.
This study was performed as part of a clinical study of pneumonia (trial registration: UMIN000004353), and it was approved by the institutional review board of Kurashiki Central Hospital (approval number 3379). All patients gave their informed consent to participate in this study. The research was conducted according to the principles of the World Medical Association Declaration of Helsinki.

Details of the heavy rainfall event in western Japan
Severe ooding occurred during the so-called west Japan heavy rainfall event between June 28 and July 8,2018. Due to the in uence of the seasonal rain front and typhoons, this storm produced recordbreaking heavy rainfall nationwide, particularly in western Japan. In the Chugoku and Shikoku regions, including Okayama Prefecture, there was approximately two to three times the normal rainfall for the whole of July based on averages from 1982 to 2018 [13]. A total of 123 observation points in Japan recorded the maximum 72-hour precipitation amount, and Kurashiki city was included among them [13].
The rainfall amount was 270.5 mm just over the period of July 5 to 7, whereas rainfall for all of July was 323.5 mm in Kurashiki city. This is 2.8 times the average rainfall for this period of 115 mm. Subsequent river ooding and landslides occurred in many areas of Japan. A total of 223 persons died, and 8 were declared missing nationwide; in Kurashiki city alone, there were 52 victims [14].
In addition, the heavy rainfall caused the Oda River, which is a tributary of the Takahashi River that runs through Kurashiki city, to ood and subsequently breach its banks. As a result, Mabi town in Kurashiki city, which is located 10 km from our hospital, was wholly under water ( Fig. 1). Patients affected by the ood visited our hospital because it is the largest hospital covering the affected area.
After the heavy rain, many people came to assist with reconstruction work from inside and outside the affected area. There were 17,287 volunteers in July, and 4,640 to 24,958 volunteers worked from August to November. They removed mud and debris from houses still standing and helped clear the signi cant wreckage. The removal of major debris in the Mabi town area to a temporary depot was completed on August 25, 2018, but work at the temporary depot has been on-going.

Study period
To investigate the impact of the disaster on hospitalised CAP patients, the study period covered the period from July 1 to November 30 in 2018 for the reasons described below, and the data were compared for the same period from 2013 to 2017.
The rst reason for the chosen study period was the effect of in uenza virus as a causative pathogen in CAP. Klein et al. reported that bacterial coinfection with in uenza is common in admitted patients, and that the most common causative bacteria is Streptococcus pneumoniae, followed by Staphylococcus aureus [15]. In Japan, in uenza virus activity increases from December to March. Since it was necessary to exclude the effect of coinfection with in uenza virus, the period spanning week 47 to week 15 was omitted [16].
The second reason for the chosen study period was that it was necessary to evaluate patients who had engaged in reconstruction work, primarily that involving the removal of rubble, earth, and sand after several months. After the west Japan heavy rainfall event, the Kurashiki municipal o ce announced the completion of primary debris removal on August 25, although work at the temporary debris storage facilities continued after that date. An estimated 5,000 to 25,000 volunteers participated in the clean-up from July to November.

Microbiologic investigation
To identify causative pathogens, sputum and blood samples were used for culture and serology, as well as a urinary antigen test to detect S. pneumoniae and Legionella pneumophila serogroup 1. Other Legionella species were identi ed using culture on Wadowsky-Yee-Okuda-α medium. Causative pathogens were de ned based on a previous report [17] if the following criteria were met: (1) positive sputum culture of more than 1 + on a qualitative test or 10 5 on a quantitative test, with reference to the sputum Gram stain; (2) positive blood culture (excluding contaminating normal skin ora); (3) positive pleural uid culture; (4) positive urinary antigen test for S. pneumoniae and L. pneumophila; (5) seroconversion or 4-fold increase in antibodies against Mycoplasma pneumoniae and Chlamydophila pneumoniae; and (6) ≥ 1:320 on a single antibody test for M. pneumoniae PA antibody (FUJIREBIO, Tokyo, Japan) or a cut-off index of ≥ 2.0 on a C. pneumoniae IgM antibody test using the Hitazyme assay (Hitachi Chemical, Tokyo, Japan).

Statistical analysis
Continuous variables are expressed as medians and interquartile range, whereas categorical variables are presented as percentages. Continuous variables were analysed using the non-parametric Mann-Whitney U-test, and categorical variables were compared using Fisher's exact test. Regarding the comparison of hospitalised pneumonia patients from 2013 to 2017 and 2018, the data were evaluated using Poisson regression analysis. An interrupted time series analysis and segmented Poisson regression with offset terms to detect changes in levels were performed. The rate of causative pathogens per month was also evaluated if the causative pathogens showed signi cant differences. Autocorrelation was tested using the autocorrelation function and partial autocorrelation function, with a threshold of 0.2. Previous studies reported that CAP patients with S. pneumoniae are more numerous in winter and that those with L. pneumophila are the most numerous in summer [18]. In addition, increased precipitation is associated with an increased risk for legionellosis [19][20][21]. Therefore, seasonality was adjusted based on Fourier terms, and the model was adjusted by monthly rainfall. As mentioned above, the duration of the intervention was determined to be between July and November 2018. A P value < 0.05 was considered signi cant. Analyses were performed using R software (version 3.0.3, Vienna, Austria).  Table 1 shows the clinical characteristics of patients in the pre-and post-disaster groups. Males comprised approximately 70% of the subjects. The most common comorbidity was COPD. There were no signi cant differences in age or other comorbidities between the two groups. The CURB-65 scores, PSI, and 30-day mortality rates between the two groups also showed no signi cant differences (Table 2).  Aetiology of pneumonia Table 3 shows a comparison of the distribution of causative pathogens between the pre-and postdisaster groups. The most common causative pathogen was S. pneumoniae in both groups, followed by H. in uenzae in the pre-disaster and L. pneumophila in the post-disaster groups. Only the prevalence of L. pneumophila increased signi cantly post-disaster. Legionella pneumonia patients after adjusting for seasonal patterns and precipitation

Patients' characteristics
After adjusting for seasonal patterns and precipitation in each month, the number of Legionella pneumonia patients was also signi cantly higher post-disaster than pre-disaster (estimate 1.344; standard error 0.464; P = 0.004). The locations and exposure of risk for Legionella infection were investigated for all of the Legionella pneumonia patients after the heavy rainfall event. Residence and exposure associated with reconstruction work or contaminated soil of Legionella pneumonia patients post-disaster are shown in Fig. 3. Three of 7 (42.9%) Legionella pneumonia patients were engaged in reconstruction work.

Discussion
In the present study, the effects of the disaster on the frequency and causative pathogens of CAP were examined. There were no signi cant differences in the number of admissions and clinical characteristics pre-and post-disaster. However, regarding the causative pathogens, there were signi cantly more patients with Legionella pneumonia post-disaster than pre-disaster, although there were no signi cant differences in the number of patients with CAP caused by other pathogens.
Some studies have reported that the number of patients with respiratory infections increases after a natural disaster [7,8,24,25]. Among these studies, one examining the Great East Japan earthquake in 2011 reported a 5-fold increase in the number of pneumonia hospitalisations compared with the usual number, along with an increased risk of death due to pneumonia [7]. Pneumonia deaths increased signi cantly during the 1st through 12th week after the earthquake and tsunami [7,26]. The authors of that study suggested that the tsunami increased the risk of pneumonia death. Regarding ooding-related disasters, there are some reports of increases in the occurrence of acute respiratory infections [2][3][4]. These reports included patients who were outpatients and had symptoms but had not been diagnosed with pneumonia. In the present study, there was no signi cant increase in the number of hospitalised patients in 2018 when compared with the pre-disaster period. One reason for this difference could be that the rainfall occurred in summer, and upper respiratory tract infections or outpatients with pneumonia were not analysed.
The most common causative pathogen of CAP after the heavy rainfall event was S. pneumoniae, similar to our previous report [17]. There was a signi cant increase in the number of pneumonia cases due to Legionella after the west Japan heavy rainfall event, although the total number of pneumonia patients and patients with other causative pathogens did not increase signi cantly. A previous study reported that the frequency of H. in uenzae as a causative pathogen of pneumonia increased after the Great East Japan earthquake, but the frequency of L. pneumophila was not signi cantly changed [7].
The following causes likely led to the increased prevalence of Legionella pneumonia from July to November of 2018. First, the increase in precipitation appeared to affect the number of cases of Legionella pneumonia. This possibility is supported by some reports indicating that not only a wet and humid environment, but also precipitation is signi cantly associated with increased rates of legionellosis [20,21,23,27]. The risk of legionellosis is signi cantly associated with increased rainfall 6-10 days before disease onset. This association was explained by observations showing that rainfall increases organic sediments and contamination with other microbes [21]. Another report suggested that vehicles could produce aerosols that contain Legionella as a result of driving on wet road surfaces [23].
However, in the present study, the number of Legionella pneumonia patients was signi cantly greater after adjusting for seasons and precipitation. Engaging in reconstruction work and soil exposure after the disaster were thought to have led to a higher risk of Legionella pneumonia. Van Heijnsbergen et al.
reported that garden soil serves as a reservoir of Legionella bacteria. They detected Legionella over a period of 1 year in some locations in the Netherlands. There was no difference in temperature or precipitation between the soils in which Legionella bacteria were isolated, and Legionella was detected throughout the year. They concluded that Legionella bacteria can survive in soil over a long period of time [28]. Wallis et al. reported a case of legionellosis in which the patient's sputum specimen was positive for L. pneumophila SG1, and the same genotype of L. pneumophila was detected in soil of the patient's workplace [29]. In this case, soil was considered the source of infection [29]. In Japan, there have been some reports of Legionella pneumonia in workers engaged in recovery efforts after a disaster. One case involved a patient doing decontamination work after the Great East Japan earthquake; this work included removal of the top layer of soil in mountainous areas and cleaning roads and roofs of residential buildings using high-pressure water [30]. In another study, Oda et al. reported a case of Legionella pneumonia diagnosed several days after the patient was working to remove rubble, earth, and sand dust after the west Japan heavy rainfall event [31]. One can hypothesise that the increased chance of inhaling Legionella-containing aerosols from the soil after the disaster was one of the causes of the increase in Legionella pneumonia cases. In the present study, 3 patients had been engaged in reconstruction work, and 2 patients had been exposed to garden soil. These results and those of other studies thus suggest that reconstruction work and soil exposure increase the risk of Legionella pneumonia after a natural disaster.
The present study has some limitations. First, this was a single-institution study, and it is unclear whether similar results would be obtained in other areas. Therefore, it is di cult to conclude with certainty that the prevalence of Legionella pneumonia increased as a result of the overall effect of the heavy rainfall event.
Second, data were collected only from patients who came to our hospital. The heavy rainfall event occurred over a large area, and many volunteers engaged in reconstruction work. Some of these individuals came from other prefectures and returned home after their work was completed. In case of illness, they would have visited a local hospital other than ours. Therefore, we may have underestimated the incidence of reconstruction work-related pneumonia because it was not possible to identify those patients who had visited other hospitals. Third, a urinary antigen test that detects only L. pneumophila serogroup 1 was used [32]. When patients are likely to have Legionella pneumonia diagnosed based on clinical data, other L. pneumophila serogroups are checked via sputum culture in addition to the urinary antigen test, but sometimes it is not possible to identify the pathogen. It should be noted that it has not been conclusively determined whether Legionella infection is associated with recovery work, since no genetic analyses have been performed on bacteria isolated from patients or soil.
On the other hand, the present study did have some strengths. First, the prospective pneumonia data were compared with data from after the west Japan heavy rainfall event, and there are no reports speci c to CAP similar to the present study published to date. Second, the large number of beds and large medical care area of our hospital made it possible to cover a reasonable number of patients, even though the study was conducted at a single centre.

Conclusions
There was a signi cant increase in the number of pneumonia cases due to Legionella after the west Japan heavy rainfall event, although the total number of pneumonia patients and patients with other causative pathogens did not increase signi cantly. The present data suggest an association between Legionella pneumonia and being engaged in recovery work or soil exposure after such a natural disaster. Therefore, it is important to take preventive measures such as wearing masks and gloves when engaging in reconstruction work or being exposed to soil after oods. Availability of data and materials The datasets used and analyzed during the current study are available from the corresponding author on reasonable request. Areas affected by the west Japan heavy rainfall event in Okayama Prefecture. Okayama Prefecture is located in central-west Japan, and Kurashiki city is located in the southwestern part of Okayama Prefecture. The Takahashi River is indicated by a blue line, and the ooded area is coloured light blue.
The disaster area data were obtained from the Geospatial Information Authority of Japan.