The Legionnaires' form of pneumonia found in a months-long outbreak in the Pittsburgh VA system might appear disproportionately in Allegheny County, which accounts for a fifth of state-recorded cases in Pennsylvania.

But doctors warn there's no clear explanation for county figures that show reported rates of the disease at more than twice state averages. They said testing practices for pneumonia patients might be more thorough in Pittsburgh, an historical leader in Legionnaires' prevention, or environmental factors in the region could encourage the waterborne Legionella bacteria that cause the ailment.

"It's not an easy question to answer," said Dr. Ronald Voorhees, the acting county health director. "The possibilities are, it's either a high level of real cases or a high level of reported cases."

He said detailed studies to explore the discrepancies with other regions "just haven't been done," though county health officials intend to do so and are exploring strategies for Legionella monitoring and prevention.

State rules mandate that hospitals and other health care providers report Legionnaires' cases to public health officials within 24 hours of identification, said Department of Health spokeswoman Kait Gillis. The requirement depends on health care providers to test aggressively for the bacteria.

Department reports show 417 of at least 1,910 Legionnaires' cases statewide since 2008 sickened Allegheny County residents. Nearly 8 percent of the local cases were fatal, above the statewide death rate of about 5 percent.

Eighty Legionnaires' patients were in neighboring Westmoreland County, which ranked fourth statewide in cases.

"If there is a higher rate of Legionnaires' in your region, there is not a specific explanation that we are aware of," Gillis wrote in an email. She said some areas "have a higher awareness and reporting of Legionella" and that the disease "most often affects middle-aged or older men, particularly those who smoke, have chronic lung disease or drink heavily."

Public health officials identified a number of suspected Legionnaires' "hot spots" over the years, including in Barcelona; Halifax, Nova Scotia; and eastern Ohio, said Dr. Paul Edelstein of the University of Pennsylvania.

He said factors ranging from water treatment and disinfection methods to underground pipes and virulent Legionella strains could be to blame for reported regional outbursts. …