Investigators Pinpoint Contamination Cause

Federal officials are now saying that processed food from a Michigan plant may have become contaminated with Listeria because of construction work on the plant's air conditioning system. The outbreak of the microorganism was traced to hot dogs and cold cuts at the Bil Mar Foods Zeeland, MI, plant in December 1998. It has killed at least 12 people and made more than 80 sick in 19 states, according to the Centers for Disease Control and Prevention.

"The plant's air conditioning system was being repaired and that may have generated dust, which may have contaminated the plant," Tom Skinner, a CDC spokesman, said this week. The centers' Dr. Paul Mead has said the "working hypothesis is that construction led to dust that contaminated the equipment."

The CDC expects to release a report on what happened at the plant in the next several weeks.

The maintenance construction at the plant occurred over the July 4 weekend, Mead said. But he added, "We do not have an absolute answer about what happened."

A USDA memo said testing for Listeria on floors and equipment at the plant's hot dog lines after the July 4 construction showed a spike in which 27 of 34 samples taken by investigators were positive. The samples were taken from July 5 through November 5.

In comparison, only three of 12 samples taken between May 26 and July 4 were positive. A similar result was detected on the chicken processing lines, CDC officials said.

However, USDA officials stress it is unclear what the spike in positive tests means because the analysis was a general Listeria assay and did not indicate whether Listeria monocytogenes was present, the species that caused the outbreak.

A USDA memo by investigators dated December 21 said Listeria "has never been documented to be transmitted in airborne particles," and that "airborne contamination of both the frank and chicken lines is difficult to imagine."

However, Ryser said, in general, it would be possible for the bacteria to blow out through an air duct and settle on hot dogs after they are cooked, infecting the meat before it is packaged and sealed.

Plant officials at Bil Mar voluntarily recalled about 35 million pounds of hot dogs and lunch meat on Dec. 22 in what agriculture officials are calling one of the largest recalls ever.