Follow the poop to find the listeria.

I keep getting asked about confined animal feeding operations or CAFOs as the cause of the listeria-in-cantaloupe outbreak that has killed at least 18 and sickened 100.

I say, all animals poop.

The deer that caused E. coli O157:H7 outbreaks in Odwalla juice in 1996 that killed a 16-month-old child, or local Oregon strawberries in 2011 that killed one and sickened 14, had nothing to do with CAFOs.

Neither did the sheep in 1981, which were used to crapping on a cabbage field in Nova Scotia (that’s in Canada) and led to a listeria outbreak linked to coleslaw that sickened seven adults and led to 34 perinatal infections, according to a report on the outbreak published in 1983 in the New England Journal of Medicine (NEJM).

Lisa Schnirring of CIDRAP cites Dr. Lawrence (Larry) Goodridge, a food microbiologist in the department of animal sciences at Colorado State University in Fort Collins, as saying all potential sources of contamination are being considered, including irrigation water, soil, "biosolids," and contamination from animal incursions.

Goodridge said in the region of Colorado where cantaloupes are grown—though not necessarily at the farm implicated in the outbreak—sheep are often grazed on cantaloupe fields following harvest.

"If that practice was followed at Jensen Farms, then there is the possibility of sheep manure contaminating the cantaloupe with L monocytogenes," he said. A similar scenario occurred in Nova Scotia, Canada, in 1981 when a listeria outbreak caused by tainted cabbage was traced to the use of sheep manure as fertilizer, Goodridge added.

Goodridge said another puzzling aspect of the cantaloupe Listeria outbreak is that four different pulsed-field gel electrophoresis (PFGE) profiles have been identified, falling into two distinct serotypes, which could suggest multiple contamination events or a contamination event from multiple sources, such as different animals.