After analyzing four of the five types of Listeria implicated in last year’s deadly cantaloupe outbreak, scientists have discovered that a new outbreak strain was among those that contributed to the nearly 150 illnesses and 33 deaths caused by contaminated melons. The researchers, an international team of government and university scientists, also compared the strains involved in the 2011 outbreak to those that had caused other outbreaks or been collected from other food facilities. By doing this, they identified two new sets of “epidemic clones” –  strains isolated in different times and places that appear to have common ancestors. Only five epidemic clones, or ECs, of Listeria had been identified prior to this study, which raises that number to seven. One of these categories was discovered when the scientists found that some strains linked to the 2011 cantaloupe outbreak were genetically related to two strains isolated from U.S. chicken plants in the early 2000s, and to the strain that caused a 1996 outbreak linked to crab meat. Together, these clones were grouped into a new category: ECVI. The other new EC category was named when several strains from the Listeria cantaloupe outbreak were found to be related to two strains previously discovered in chicken plants, as well as a type of Listeria that caused a Canadian outbreak linked to whipping cream in 2000. This group was called ECVII. The authors say these findings could help explain why so many strains were linked to last year’s cantaloupe outbreak. “Different clones, particularly ECVI and ECVII, might have cocolonized niches or harborage sites within the cantaloupe processing facility.” This would explain the variety of strains that contaminated the melons, they note. The facility was owned and operated by Jensen Farms, a colorado company that declared bankruptcy in the aftermath of the outbreak. Looking at samples from other countries and other types of foods, say the authors, was important to discovering relatives among Listeria strains involved in the cantaloupe outbreak. “In contrast to focusing on isolates from a single outbreak, our findings demonstrate that to detect new ECs it is important to analyze isolates from many sources around the world,” they note in the study. The results of the study appear in the January 2013 issue of Emerging Infectious Diseases, a monthly publication put forth by the federal Centers for Disease Control and Prevention.