In August 2017, the Minnesota Department of Health (MDH) initiated an outbreak of Legionnaires' Disease at a senior living community located in Maple Grove. The facility provides memory care, assisted living, and independent living services. Two outbreak associated cases were identified.

Case 1 resided in the memory care unit. He experienced onset of symptoms on August 22 and was hospitalized for 5 days. Case 2 experienced symptom onset on September 12. He resided in an assisted living unit, three floors directly above the memory care unit where Case 1 lived. Case 2 died of acute hypoxic respiratory failure due to Legionella pneumonia.

On September 7, the facility's existing water treatment contractor sampled two sites, the cooling tower and hot water from a kitchen outlet. The cooling tower sample was positive for Legionella pneumophila serogroup 1, and the kitchen sample was positive for non-pneumophila Legionella species. Both results were by culture. Water samples collected on September 21 from potable water system locations were all negative by both PCR and culture. Water samples from the pool and therapy pool were also negative. A water sample collected on September 21 from the cooling tower was positive by Legionella pneumophila PCR and grew Legionella pneumophila serogroup 1 upon culture. A post-remediation sample from the cooling tower collected on October 3 was negative by both Legionella pneumophila PCR and Legionella culture.

Although all premise potable water system samples collected were negative for Legionella, environmental assessment and epidemiological information - particularly the location of the cases' living units - suggested that the potable water system was the source of the outbreak. The presence of unoccupied units near the cases' units may have caused stagnancy within the potable water system and created favorable conditions for Legionella amplification.