Abstract

Background: Understanding the epidemiological distribution of legionellosis across demographic groups, exposure histories, and temporal patterns is essential for targeted public health interventions.

Methods: We conducted a descriptive epidemiological analysis of 224 confirmed Legionnaires’ disease and Pontiac fever cases reported between 2020 and 2024. Case distributions were examined by demographics, symptoms, and 35 potential exposure variables. Time series analysis assessed seasonality and trends using lab collection date as a proxy for disease onset.

Results: Cases were predominantly male (70.5%) and aged 50-74 years (65.6%). The most prevalent symptoms were fatigue (92.2%), fever (87.4%), and cough (86.6%). Among exposures with adequate response rates, the top five were: national travel (24.1%), showering away from home (23.6%), out-of-county travel (19.3%), clinic/dentist visits (16.5%), and CPAP/BiPAP use (13.0%). Overall, 24.6% of cases reported any travel exposure. Time series analysis revealed significant seasonality (p=0.001), with 53.4% of cases in summer months and peak incidence in June. No significant trend was detected across the 5 years of the study (p=0.888).

Conclusions: This analysis identifies key demographic and exposure risk factors for Legionellosis. Travel-related exposures, particularly domestic travel, represent a significant proportion of cases. The strong summer seasonality aligns with known Legionella ecology. These findings in this data support targeted interventions focusing on travel-associated risks, respiratory equipment users, and enhanced summer surveillance.

1. Introduction

Legionella bacteria can cause Legionnaires’ disease and Pontiac fever, collectively referred to as legionellosis. Legionnaires’ disease is a severe form of pneumonia, typically acquired through inhalation of contaminated water aerosols. Pontiac Fever is an inflammatory response to Legionella endotoxin (CDC, 2024). Understanding the epidemiological distribution of cases—including demographic characteristics, exposure histories, and temporal patterns—is essential for developing effective public health interventions.
Common exposure sources include cooling towers, hot water systems, whirlpool spas, decorative fountains, and respiratory therapy equipment. Travel-associated cases represent a recognized risk factor, with exposure often occurring in hotels, hospitals, or other buildings with complex water systems (Garrison et al., 2016).

The objectives of this analysis were:

  1. To describe case distribution by demographic variables;
  2. To evaluate statistical differences in case distribution among demographic groups;
  3. To characterize symptom distribution overall and by sex;
  4. To conduct descriptive analysis of exposure variables and identify key risk factors; and
  5. To assess temporal patterns including seasonality and trends.

2. Methods

2.1 Study Population and Data Source

On April 29, 2025, a query for Legionnaires’ disease of EpiTrax, the surveillance management system at the Kansas Department of Health and Environment (KDHE), retrieved a total of 591 records reported during 2020-2024. After deduplication and other data cleaning processes there were 224 records left for this analysis. Most of the remaining records (96%) were labeled Legionnaires’ disease, Pontiac Fever(1 or 0.5%), Other or the label was missing. Each case record included demographic information, clinical symptom data, laboratory collection date, and responses to 35 standardized exposure questions covering healthcare, travel, water/recreation, residential, occupational, and commercial/public exposure categories.

2.2 Variables and Definitions

Demographic variables included sex, age category (18-29, 30-49, 50-74, 75+ years), race, and ethnicity. Exposure variables were coded as YES, NO, or UNKNOWN/missing. Travel-related exposures included international travel, national travel, out-of-county travel, and U.S. travel.

Exposures were grouped into six categories: Healthcare/Medical (CPAP, nebulizer, healthcare facilities), Travel (all travel variables), Water/Recreation (pools, spas, water parks), Residential (long-term care, humidifiers, showering away from home), Occupational (construction, plumbing, food service), and Commercial/Public (cooling towers, stores, clinics).

2.3 Statistical Analysis

Descriptive statistics included frequencies and percentages for categorical variables. Exposure prevalence was calculated as the proportion of cases reporting YES among those with valid (YES/NO) responses. Response rates were calculated as the proportion of cases with non-missing data.

For identification of top exposure factors, analysis was restricted to variables with response rates ≥30% to ensure adequate data quality. Chi-square goodness-of-fit tests assessed demographic distributions. Mann-Whitney U tests compared prevalence distributions between travel and non-travel exposures.

Time series analysis included seasonal decomposition, Kruskal-Wallis tests for seasonality, Mann-Kendall trend tests, and autocorrelation analysis. Statistical significance was set at α=0.05. All analyses were conducted using R version 4.5.1.

2.4 Reproducibility of Results

The analysis code and data processing scripts are available upon request to ensure reproducibility of results. GitHub repository: https://github.com/SOK-KDHE/legion_desc_24.git. GitHub Copilot was used to assist in code generation and documentation.

3. Results

3.1 Demographic Characteristics

A total of 224 cases were analyzed. Males comprised 70.5% (n=158), with the 50-74 year age group accounting for 65.6% (n=147). White individuals represented 77.7% of cases. Chi-square tests show significant statistical differences among all demographic subgroups considered (all p<0.001).

Table 1. Demographic Characteristics of Cases (N=224)

Demographic characteristics of Legionnaires’ disease cases
Variable Category N %
Sex Male 158 70.5
Female 66 29.5
Age (years) 18-29 3 1.3
30-49 36 16.1
50-74 147 65.6
75+ 38 17.0
Race Black or African American 34 15.2
Other/Unknown 16 7.1
White 174 77.7
Ethnicity Hispanic or Latino 10 4.5
Not Hispanic or Latino 201 89.7
Unknown 13 5.8
Figure 1. Case distribution by demographic characteristics.

Figure 1. Case distribution by demographic characteristics.

Table 2. Chi-square Tests for Demographic Distribution

Chi-square goodness-of-fit tests for demographic variables
Variable Chi-square df P-value Significant
Sex 37.79 1 <0.001 Yes
Race 620.13 5 <0.001 Yes
Ethnicity 320.69 2 <0.001 Yes
Age Category 210.96 3 <0.001 Yes

3.2 Symptom Prevalence Among Cases

The most common symptoms were fatigue/malaise/weakness (92.2%), fever (87.4%), and cough (86.6%). Chills (64.9%) and headache (61.0%) were moderately prevalent. No significant differences were observed between sexes for any symptom (all p>0.05).

Table 3. Symptom Prevalence Overall and by Sex

Symptom prevalence among cases with valid responses
Symptom Overall (%) Male (%) Female (%)
Fatigue/Malaise/Weakness 92.2 91.1 94.2
Fever 87.4 86.8 88.7
Cough 86.6 84.1 91.5
Chills 64.9 65.0 64.6
Headache 61.0 60.7 61.5
Diarrhea 49.7 50.8 47.8
Abdominal Cramps 22.9 25.6 17.9

3.3 Exposure Analysis

3.3.1 Overview of Exposure Variables

A total of 35 exposure variables were analyzed, with response rates ranging from 4.5% to 75.9%. 28 variables had response rates ≥50%, and 28 variables had response rates ≥30%.

3.3.2 Top Five Exposure Factors

Among exposure variables with adequate response rates (≥30%), the five most prevalent were:

  1. National Travel: 24.1% (41/170 cases)
  2. Shower Away from Home: 23.6% (34/144 cases)
  3. Out-of-County Travel: 19.3% (32/166 cases)
  4. Clinic Dentist Visit: 16.5% (26/158 cases)
  5. CPAP BiPAP: 13% (21/162 cases)

Table 4. Prevalence of Top 10 Exposure Factors

Top 10 exposure factors by prevalence
Exposure Yes No Prevalence Response Rate
National Travel 41 129 24.1 75.9
Shower Away from Home 34 110 23.6 64.3
Out-of-County Travel 32 134 19.3 74.1
Clinic Dentist Visit 26 132 16.5 70.5
CPAP BiPAP 21 141 13.0 72.3
Store Mister 18 140 11.4 70.5
Swimming Pool 18 143 11.2 71.9
Healthcare Exposure 18 148 10.8 74.1
Construction Near Home 14 127 9.9 62.9
Sprinkler 12 131 8.4 63.8
Figure 2. Top 15 exposure factors by prevalence. Red bars indicate travel-related exposures.

Figure 2. Top 15 exposure factors by prevalence. Red bars indicate travel-related exposures.

3.3.4 Exposure by Category

Analysis of any exposure within predefined categories revealed that residential exposures (26.3%) had the highest prevalence, followed by travel (32.4%), commercial/public (28.6%), healthcare/medical (24%), water/recreation (19%), and occupational exposures (17.1%).

Table 5. Exposure Prevalence by Category

Rate of any exposure within each category
Category Variables Any Exposure Valid N Prevalence (%)
Travel 5 59 182 32.4
Commercial/Public 5 48 168 28.6
Residential 5 44 167 26.3
Healthcare/Medical 6 41 171 24.0
Water/Recreation 5 31 163 19.0
Occupational 9 26 152 17.1
Figure 4. Exposure rate by category.

Figure 4. Exposure rate by category.

3.4 Time Series Analysis

3.4.1 Seasonality

Monthly case counts ranged from 0 to 15 cases (mean: 3.7, SD: 3.8). The Kruskal-Wallis test confirmed significant differences across months (χ² = 30.63, p = 0.001) and seasons (χ² = 24.67, p < 0.001). Summer months (June-August) accounted for 53.4% of all cases, with Jun showing the highest seasonal index (+7.88).

Table 6. Seasonal Distribution of Cases

Seasonal distribution of Legionnaires’ disease cases
Season Total Cases Mean Monthly SD % of Total
Winter 23 1.53 1.25 10.3
Spring 33 2.20 2.65 14.8
Summer 119 7.93 4.45 53.4
Fall 48 3.20 2.31 21.5
Figure 5. Monthly Legionnaire's disease cases (2020-2024) with smoothed trend line.

Figure 5. Monthly Legionnaire’s disease cases (2020-2024) with smoothed trend line.

Figure 6. Seasonal indices by month showing deviation from overall mean.

Figure 6. Seasonal indices by month showing deviation from overall mean.

Figure 7. Seasonal patterns by year showing consistent summer peaks.

Figure 7. Seasonal patterns by year showing consistent summer peaks.

3.4.2 Trend Analysis

The Mann-Kendall test showed no significant monotonic trend (Z = 0.14, p = 0.888), indicating stable rate of infection over the study period. Annual case counts ranged from 1 to 51, with no evidence of increasing or decreasing incidence.

4. Discussion

This analysis of 224 Legionnaires’ disease cases in Kansas reported from 2020 to 2024 provides insights into demographic patterns, exposure risk factors, and temporal trends. The demographic profile—predominantly male (70.5%) and older adults (65.6% aged 50-74 years)—is consistent with established risk factors (Falagas et al., 2007).

4.1 Key Exposure Findings

The exposure analysis identified several key findings. First, travel-related exposures were the single most prevalent category, with nearly a quarter (24.6%) of cases reporting domestic or international travel before illness onset. National travel (24.1%) and out-of-county travel (19.3%) were more common than international travel, suggesting that most travel-associated cases involve domestic rather than international exposure.

Second, showering away from home was the second most prevalent type of exposure (23.6%). This is epidemiologically significant because shower heads and hotel water systems are recognized sources of Legionella exposure, particularly in buildings with complex plumbing systems or inadequate water temperature maintenance (US EPA 2025).

Third, respiratory equipment use (CPAP/BiPAP) was reported by 13% of cases. Contaminated water reservoirs in respiratory equipment represent a known risk factor, particularly for individuals with underlying pulmonary conditions (Woo et al. 1992).

4.2 Travel-Associated Cases

The finding that 24.6% of cases reported any travel exposure has important public health implications. Legionnaires’ disease is reportable to the KDHE, and clusters linked to hotels, cruise ships, and other accommodations represent opportunities for intervention. The predominance of travel-associated cases suggests that prevention efforts should focus lodging facilities and an opportunity to collaborate with the Kansas Department of Agriculture which regulates the lodging industry in the state.

4.3 Seasonal Patterns

The pronounced summer seasonality, with 53.4% of cases occurring June through August, aligns with known Legionella ecology (Hicks et al. 2017). Warmer temperatures promote bacterial growth in water systems, cooling tower use increases during summer, and outdoor water features become more common. The consistent seasonal pattern across all five study years suggests this is a robust finding that should inform the timing of prevention efforts.

4.4 Limitations

Several limitations warrant consideration. First, exposure data were self-reported and subject to recall bias. Second, variable response rates may introduce selection bias for exposures with substantial missing data. Third, the absence of control data precludes calculation of odds ratios or attributable fractions. Finally, the small counts in some categories lead to unstable rates. Therefore, the reader should use caution in interpreting the statistics.

5. Conclusions

This analysis provides an epidemiological profile of Legionnaires’ disease in Kansas, identifying key demographic, exposure, and temporal patterns. The findings support several actionable recommendations:

  1. Enhanced travel-associated surveillance: Given that a quarter of the cases reported travel, continued vigilance for travel-associated clusters is warranted.

  2. Respiratory equipment education: Patients using CPAP/BiPAP devices should receive guidance on proper cleaning and water reservoir maintenance.

  3. Seasonal preparedness: Enhanced surveillance and cooling tower inspections would be beneficial during May-August.

  4. Targeted outreach: Prevention messaging should focus on high-risk groups (males, older adults, those with respiratory conditions).

  5. Water management: Building owners and facility managers should maintain water management programs, particularly in hotels and healthcare facilities.

References

CDC. (2024). Legionella (Legionnaires’ Disease and Pontiac Fever). Centers for Disease Control and Prevention.

Garrison, L. E., et al. (2016). Vital Signs: Deficiencies in environmental control identified in outbreaks of Legionnaires’ disease—North America, 2000–2014. MMWR Morb Mortal Wkly Rep, 65(22), 576-584.

Falagas, M. E., Mourtzoukou, E. G., & Vardakas, K. Z. (2007). Sex differences in the incidence and severity of respiratory tract infections. Respiratory Medicine, 101(9), 1845-1863.

Klein, S. L., & Flanagan, K. L. (2016). Sex differences in immune responses. Nature Reviews Immunology, 16(10), 626-638.

U.S. Environmental Protection Agency. (2025, August 27). Legionella in the indoor environment. https://www.epa.gov/indoor-air-quality-iaq/legionella-indoor-environment.

Hicks, L. A., et al. “Weather-Dependent Risk for Legionnaires’ Disease, United States.” Emerging Infectious Diseases, vol. 23, no. 10, Oct. 2017, pp. 1714–16. PubMed Central, https://pmc.ncbi.nlm.nih.gov/articles/PMC5652433/

Woo, A. H., et al. “Transmission of Legionella by Respiratory Equipment and Aerosol-Generating Devices.” Chest, vol. 102, no. 5, 1992, pp. 1586–1590. https://journal.chestnet.org/article/S0012-3692(16)59107-7/abstract

Appendix: Complete Exposure Variable Summary

Complete exposure variable summary (sorted by prevalence)
Exposure Yes No Valid N Prevalence (%) Response Rate (%)
U.S. Travel 41 0 41 100.0 18.3
CPAP Humidifier 14 4 18 77.8 8.0
National Travel 41 129 170 24.1 75.9
Shower Away from Home 34 110 144 23.6 64.3
Out-of-County Travel 32 134 166 19.3 74.1
Clinic Dentist Visit 26 132 158 16.5 70.5
CPAP BiPAP 21 141 162 13.0 72.3
Store Mister 18 140 158 11.4 70.5
Swimming Pool 18 143 161 11.2 71.9
Healthcare Exposure 18 148 166 10.8 74.1
Construction Near Home 14 127 141 9.9 62.9
Sprinkler 12 131 143 8.4 63.8
Clinic Dentist Visit 2 2 22 24 8.3 10.7
Another Spa Exposure 1 1 11 12 8.3 5.4
Longterm Care Facility 13 148 161 8.1 71.9
International Travel 3 36 39 7.7 17.4
Hospital Exposure 2 1 12 13 7.7 5.8
Use Humidifier 12 145 157 7.6 70.1
Spa 12 149 161 7.5 71.9
Work in Construction 9 138 147 6.1 65.6
Building Cooling Tower 8 127 135 5.9 60.3
Other Type of Work 7 141 148 4.7 66.1
Water Feature 7 152 159 4.4 71.0
Convention Reception 6 137 143 4.2 63.8
Nebulizer 6 154 160 3.8 71.4
Work Involves Industrial Spray 5 139 144 3.5 64.3
Plumbing Maintenance Work 5 145 150 3.3 67.0
Work as a Trucker 4 144 148 2.7 66.1
Congragate Living 3 141 144 2.1 64.3
Custodial Work 2 146 148 1.4 66.1
Water Park 2 159 161 1.2 71.9
Waste Water Work 1 146 147 0.7 65.6
Therapy Equipment Humidifier 0 10 10 0.0 4.5
Works in a Kitchen 0 150 150 0.0 67.0
Work in the Leisure Industry 0 149 149 0.0 66.5

Report generated on December 17, 2025 at 09:29

R version 4.5.1