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Legionella Activities


The Emerging Infections Program (EIP), in partnership with the Connecticut Department of Public Health (CT DPH) and Centers for Disease Control and Prevention (CDC), is conducting activities to better understand legionellosis in Connecticut.

Legionella was discovered after an outbreak in 1976 among people who attended a Philadelphia convention of the American Legion. Those who were affected suffered from a type of pneumonia (lung infection) that eventually became known as Legionnaires' disease.

Legionellosis includes three clinically distinct illnesses, Legionnaires' disease, Pontiac fever, and extrapulmonary legionellosis, that are all caused by exposure to Legionella bacteria. Legionnaires' disease most commonly presents as pneumonia, Pontiac fever is a milder, influenza-like illness, and extrapulmonary legionellosis happens when there is disease outside the lungs, (e.g. heart) and diagnostic testing reveals evidence of Legionella at that site.

Legionella are a group of naturally occurring bacteria that are found in fresh water environments such as rivers, lakes, streams, and wet soils. It is not unusual to find levels of Legionella bacteria inside of building water distribution systems. Following best practice guidelines are necessary to limit conditions inside of piping that would encourage this bacteria from growing, multiplying, and travelling throughout the water distribution system. Some of these best practices include keeping water moving (avoiding stagnation such as what is found in plumbing dead ends), maintain adequate disinfectant residual levels (chlorine), and storing and distributing very hot water (140F or greater) before it is tempered down at the point of use (faucet, shower, tub, etc.). These measures are important to prevent people from breathing in water mist containing Legionella that might cause illness.

The CT DPH uses the national surveillance case definition to determine case status. Legionellosis is reportable to the CT DPH by physicians and laboratories, and to the patient's local health department. To identify possible common sources of exposure, staff of the CT DPH Epidemiology and Emerging Infections Program conduct follow-up on all confirmed legionellosis cases by contacting the healthcare provider of record and conducting patient interviews.

Goals and Objectives

  • To better understand the trends in Legionella infection in Connecticut and identify potential spatial clusters of disease for further investigation.


  • To interview all legionellosis cases reported to the CT DPH in order to gather clinical and epidemiologic information to assist in cluster and outbreak detection.
  • Perform spatial analyses of legionellosis cases to identify clusters of infections and potential environmental reservoirs.

Estimating the True Burden of Legionnaires' Disease. Cassell K, Gacek P, Rabatsky-Ehr T, Petit S, Cartter M, Weinberger DM. Am J Epidemiol. 2019 Sep 1;188(9):1686-1694: 10.1093/age/kwz142. PMID: 31225857.

An Increase in Legionellosis, Connecticut, 2018. Gacek P, Cassell K. CT Epidemiologist. 2018 Sept;38(3):9-10.

Association Between Sporadic Legionellosis and River Systems in Connecticut. Cassell K, Gacek P, Warren JL, Raymond PA, Cartter M, Weinberger DM. J Infect Dis. 2018 Jan 4;217(2):179-187. doi: 10.1093/infdis/jix531. PMID: 29211873.

For more information about Connecticut Legionellosis surveillance please visit: CT DPH Legionellosis Website.

Project Contact

Kelsie Cassell, MPH
PhD Student
Yale School of Public Health/Connecticut Emerging Infections Program

Paul Gacek MPH, CPH
Legionellosis Surveillance Coordinator & Waterborne Disease Coordinator
Connecticut Department of Public Health