Stanisław Żaryn, a deputy government minister in charge of security services, said investigators wanted to rule out the possibility of foul play, including any possible involvement by Russia.
"We see that Russia is trying to sow panic," Żaryn told reporters. “This is another factor that has made us take steps to check some leads."
Private radio broadcaster RMF FM reported on Friday that officers were checking local water systems for any signs of sabotage.
The bacteria that causes the illness, Legionella pneumophila, has so far been found in 113 patients hospitalised in Rzeszów and the surrounding area in Poland’s southeastern Podkarpackie region near Ukraine, news agencies reported.
The seven people who have died were aged 64 to 95 and had other health conditions, health authorities said.
Two infected patients have recovered and been discharged from hospitals in Rzeszów, public broadcaster Polish Radio’s IAR news agency reported.
A 86-year-old man has been successfully treated at the Rzeszów City Hospital, and a 70-year-old man with Legionnaires' disease has recovered at the city’s University Clinical Hospital, according to doctors.
Authorities said earlier this week they were testing water samples for Legionella and that the results would be known on Monday.
Rzeszów Mayor Konrad Fijołek said on Thursday that the city was increasing the levels of chlorine, UV rays and ozone used for water treatment.
He added that the city’s water system would be rinsed and disinfected over the weekend.
According to the World Health Organisation, “the most common form of transmission of Legionella pneumophila is inhalation of contaminated aerosols produced in conjunction with water sprays, jets or mists of contaminated water sources.”
Moreover, "infection can also occur by aspiration of contaminated water or ice, particularly in susceptible hospital patients,” the WHO said.
The bacteria is not transmitted from person to person, virologist Włodzimierz Gut has said.
The Rzeszów region, in Poland’s southeast, is a key transit hub for international military support for Ukraine, following Russia's invasion in February last year, the AP news agency has reported.
Some 10,000 US troops are also stationed in the area, it noted.
(gs)
Source: IAR, PAP, AP, Reuters