Listeria Outbreak Solved After Five Years Through Shoe-Leather
Epidemiology
A unique retroactive
combination of laboratory and epidemiologic investigations have
identified the source of a five year old multi-state Listeria
outbreak, according to the Centers for Disease Control and Prevention.
According to a report first posted in September 2015, 24 persons
infected with one or the other of five closely related strains of
Listeriamonocytogenes have been reported from 9 states since 2010.
CDC
investigators detected a cluster of these cases in August 2015 when
an increase in the number of isolates from one of these rare strains
occurred. Four other rare strains in the specimens collected since
2010 were found to be closely related to the cluster strain. All the
illnesses going back to 2010 were then seen as connected and counted
as part of a single multi-state Listeria outbreak. A remarkably high
percentage of these cases, 91%, were hospitalized and there has been
one death.
Shoe-Leather Findings
Shoe-leather interviews and investigations have sought
to obtain retrospective food histories for the month before illness
among affected persons. This sleuthing has found that most ill persons
ate soft cheese before becoming ill. Furthermore, investigations found
that 63% of the ill persons were of Middle Eastern or Eastern European
descent or shopped at Middle Eastern style markets. Four of the 7 ill
people who specified the brand of cheese consumed reported eating
brands distributed by Karoun Dairies of San Fernando, California.
Epidemiologist
Reactions
When questioned about the delayed investigation by Food Safety News,
CDC epidemiologist Brendan Jackson said “We saw there was a
Middle East connection back then, but there was just too little
information… Nothing was really standing out back then, so we stopped
following that cluster at that point." Jackson added that after
detecting the latest cluster and seeing that it was related to earlier
findings, “Suddenly, we went from just a few cases…to upwards of
20…Once we had those numbers, it was fairly easy to see that there was
a signal for soft cheese.” Follow up interviews with some of the more
recent cases identified a couple of cases who were able to remember
the brand of cheese they had eaten---Karoun Diaries. The types of
cheeses this company distributes fit the pattern identified in the
investigation
Proven Value
The utility of epidemiologic investigations of
lab-identified cases was proven again recently when an epidemiologist
was able to identify caramel apples as a source of a more recent
Listeria outbreak earlier in 2015. Jackson told Food Safety News that
a really astute epidemiologist in Texas thought to ask about caramel
apples because it had been mentioned by another patient earlier. It
led to the source of the outbreak.
“There is a perception among some people out there that
whole-genome sequencing will make epidemiology obsolete. Investigation
after investigation shows that’s just not the case. They have to be
used together to be most effective,” Jackson said.
■
|