Skip directly to search Skip directly to A to Z list Skip directly to site content
CDC Home

Epidemiologic Notes and Reports Legionnaires' Disease Outbreak Associated with a Grocery Store Mist Machine -- Louisiana, 1989

On October 31, 1989, the Louisiana Department of Health and Hospitals received reports from two physicians of an outbreak of pneumonia among residents of Bogalusa (population 16,000) and the surrounding parish. An investigation confirmed 33 cases of Legionnaires' disease (LD) among persons hospitalized with pneumonia between October 10 and November 13. Patients ranged in age from 36 to 88 years (median: 64 years); 25 (76%) were female. Legionella pneumophila serogroup 1 (Lp1), monoclonal antibody subtype 1,2,5,6, was identified by direct fluorescent antibody tests of lung tissues from autopsies of two patients who died of pneumonia during the outbreak (1,2).

A case-control study of 28 cases and 56 controls, frequency matched by primary physician, age, and chronic-disease status, found that case-patients were no more likely than controls to live or travel within 200 meters of any identified cooling towers within the town in the 10 days before illness. However, case-patients were more likely to report shopping at one grocery store (grocery store A) in the 10 days before illness (25/27* vs. 28/54; odds ratio (OR)=11.6; 95% confidence interval (CI)=2.4-108.0). Among case-patients and controls who shopped at grocery store A, case-patients were more likely to spend greater than 30 minutes in the store (OR=8.6; CI=1.5-86.3) and to select produce items located close to an ultrasonic mist machine (OR=7.4; CI=1.3-75.0). In follow-up interviews of the three case-patients who did not report shopping at grocery store A in the 10 days before illness, two reported visiting the store but were unsure if their visits occurred within 10 days of illness, and one reported shopping there 12 days before onset of illness. No cases occurred among employees (median age: 23.5 years) of grocery store A.

Lp1 subtype 1,2,5,6 was isolated from water in the reservoir of the mist machine. The machine was installed in the store during October 1988 and continuously generated an aerosol over one section of the produce display. The mist was generated by ultrasonic transducers located in the machine's reservoir. In early December, the machine was turned off and removed from grocery store A. Under controlled conditions at CDC, Lp1 was added to the reservoir of the machine and viable Lp1 in respirable droplets ( less than 5 um) was isolated from mist produced by the machine. Reported by: W LaMaire, MD, H Jackson, MD; L McFarland, DrPh, State Epidemiologist, Louisiana Dept of Health and Hospitals. Respiratory Diseases Br, Div of Bacterial Diseases, Center for Infectious Diseases; Div of Field Svcs, Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: LD occurs primarily after inhalation of Lp1 contained in mists from aerosol-producing devices such as cooling towers, evaporative condensers, whirlpool baths, showers, and respiratory therapy equipment (2-6). Investigations of LD outbreaks can be challenging because of the potential exposure to many aerosol-producing sources during the disease's 2-10 day incubation period; these sources often contain L. pneumophila without being associated with disease (7).

Although the infectious dose of L. pneumophila for humans is unknown, one study suggests that disease can occur in susceptible persons exposed to one colony-forming unit of L. pneumophila per 50 L of air (2). Susceptible persons include the elderly and persons with a history of smoking or with underlying health conditions, including chronic lung or renal disease, malignancy, diabetes mellitus, and use of immunosuppressive medications. Employees from grocery store A, who did not develop LD, may have been less susceptible than affected persons.

Proliferation of Legionella, presumably introduced through water supplies, occurs most readily in systems with reservoirs (e.g., cooling towers and hot water systems) that are relatively stagnant and have temperatures of 25-42 C (77-108 F) (8). Systems similar to the one used in grocery store A (commonly referred to as "foggers") account for less than 10% of produce misting systems used by retail food stores nationwide (Food and Drug Administration (FDA), unpublished data). These systems generate mists by ultrasonic transducers located in reservoirs containing municipal water. They differ from other misting systems used more commonly in grocery stores that generally create mists in intermittent cycles by passing water directly through spray heads. These latter systems may generate larger, less respirable droplets than those produced by ultrasonic machines. No evidence exists that the more commonly used systems pose a risk of transmitting legionellosis.

Data suggesting that use of humidifiers may be associated with risk of LD have been limited to case reports (6,9) and a study in which subclinical infection occurred in laboratory animals exposed to aerosols from a humidifier contaminated with L. pneumophila (10). This investigation provides further evidence that an ultrasonic humidifier contaminated with Legionella can transmit LD to humans. Although some ultrasonic humidifiers used in other settings appear similar in design to the machine associated with this outbreak, their role in sporadic cases or outbreaks of LD is unknown.

Further studies are needed to evaluate factors that influence colonization of Legionella in misting machines and humidifiers and to identify design features that affect the potential for transmitting disease. Studies are also needed to determine the lowest concentration of L. pneumophila necessary for generation of respirable drop lets containing the bacteria. Until such information is available and prevention methods can be refined, ultrasonic mist machines and humidifiers should be drained and cleaned regularly, following manufacturers' latest recommendations. For ultrasonic mist machines used in produce sections of grocery stores, FDA has issued guidelines that specify weekly disassembly and cleaning, which includes use of a hypochlorite solution (at least 50 ppm). General guidelines on the cleaning and maintenance of humidifiers in other settings have been issued by the Consumer Product Safety Commission (11).


  1. Barbaree J, Martin W. Rapid dot blot procedure for

identification of Legionella (Abstract). In: Programs and abstracts of the 88th Annual Meeting of the American Society of Microbiology. Washington, DC: American Society of Microbiology, 1988:340. 2. Breiman RF, Cozen W, Fields BS, et al. Role of air-sampling in an investigation of an outbreak of Legionnaires' disease associated with exposure to aerosols from an evaporative condenser. J Infect Dis 1990 (in press).

3. Dondero TJ, Rendtorff RC, Mallison GF, et al. An outbreak of Legionnaires' disease associated with a contaminated air-conditioning cooling tower. N Engl J Med 1980;302:365-70.

4. Vogt RL, Hudson PJ, Orciari L, Heun EM, Woods TC. Legionnaires' disease and a whirlpool spa (Letter). Ann Intern Med 1987;107:596.

5. Breiman R, Fields B, Volmer L, et al. Definitive association of shower use with Legionnaires' disease: possible role of amoebae (Abstract). In: Program and abstracts of the 89th Annual Meeting of the American Society for Microbiology. Washington, DC: American Society for Microbiology, 1989:126.

6. Arnow PM, Chou T, Weil D, Shapiro EN, Kretzschmar C. Nosocomial Legionnaires' disease caused by aerosolized tap water from respiratory devices. J Infect Dis 1982;146:460-7.

7. Redd SC, Cohen ML. Legionella in water: what should be done? (Editorial). JAMA 1987;257:1221-2.

8. Schulze-Robbecke R, Rodder M, Exner M. Multiplication and killing temperatures of naturally occurring legionellae. Zentralbl Bakteriol Mikrobiol Hyg (B) 1987;184:495-500.

9. Phillips SJ, Zeff RH, Gervich D. Legionnaires' disease (Letter). Ann Thorac Surg 1987;44:564. 10. Zuravleff JJ, Yu VL, Shonnard JW, Rihs JD, Best M. Legionella pneumophila contamination of a hospital humidifier. Am Rev Respir Dis 1983;128:657-61. 11. Consumer Product Safety Commission/American Lung Association. Biological pollutants in your home. Washington, DC: Consumer Product Safety Commission/American Lung Association (in press). Factor denominators exclude cases and controls who answered "don't know" to this question.

Use of trade names and commercial sources is for identification only and does not imply endorsement by the U.S. Department of Health and Human Services.

References to non-CDC sites on the Internet are provided as a service to MMWR readers and do not constitute or imply endorsement of these organizations or their programs by CDC or the U.S. Department of Health and Human Services. CDC is not responsible for the content of pages found at these sites. URL addresses listed in MMWR were current as of the date of publication.

All MMWR HTML versions of articles are electronic conversions from typeset documents. This conversion might result in character translation or format errors in the HTML version. Users are referred to the electronic PDF version ( and/or the original MMWR paper copy for printable versions of official text, figures, and tables. An original paper copy of this issue can be obtained from the Superintendent of Documents, U.S. Government Printing Office (GPO), Washington, DC 20402-9371; telephone: (202) 512-1800. Contact GPO for current prices.

**Questions or messages regarding errors in formatting should be addressed to The U.S. Government's Official Web PortalDepartment of Health and Human Services
Centers for Disease Control and Prevention   1600 Clifton Road Atlanta, GA 30329-4027, USA
800-CDC-INFO (800-232-4636) TTY: (888) 232-6348 - Contact CDC–INFO
A-Z Index
  1. A
  2. B
  3. C
  4. D
  5. E
  6. F
  7. G
  8. H
  9. I
  10. J
  11. K
  12. L
  13. M
  14. N
  15. O
  16. P
  17. Q
  18. R
  19. S
  20. T
  21. U
  22. V
  23. W
  24. X
  25. Y
  26. Z
  27. #