The Colorado Listeria outbreak has produced one confirmed case every 24 hours for the 10 days through the Labor Day weekend, bringing  the total to 13, including two deaths, since Aug. 1

The source of the outbreak remains unknown, according to the Colorado Department of Public Health and Environment.

The current outbreak started slowly with two cases earlier in the month, but since Aug. 29 the pace has quickened with eight counties up and down Colorado’s Front Range reporting  cases in the growing outbreak, which is now at a case-per-day pace.

Adams, Arapahoe, Boulder, Denver, Douglas, El Paso, Jefferson, and Weld counties have all reported one or more cases of listeriosis.    

Colorado did experience a small but deadly Listeria outbreak in June that  killed two of the three who were then infected.  On June 2, Colorado health officials said a man in his 30s and a woman in her 60s died after being infected with Listeria. 

The reports mark a sharp increase in the number of Listeria monocytogenes infections for the Centennial State, which ordinarily sees just 10 such cases a year.

Public health officials in Colorado are continuing to investigate both the June outbreak and current one.  Thus far, the two are not seen as connected.   The June outbreak was limited to Denver.

“Until we have more information about the sources of this outbreak, it is important for people to follow the standard CDC guidance about Listeria,” says Alicia Cronquist, a state epidemiologist.

The Centers for Disease Control and Prevention (CDC) suggests people who are at high risk for Listeria avoid hot dogs and deli meats unless they are cooked to an internal temperature of 165 degrees, and stay away from pates, meat spreads, refrigerated smoked seafood, and soft cheeses unless they are made with pasteurized milk.

People age 60 and older, pregnant women, and those with weakened immune systems are viewed as high risk for Listeria.

The incubation period for Listeria, meaning the time from exposure to showing symptoms, can be as long as 70 days, making investigations requiring people to remember what they’ve eaten more difficult.