Multistate Outbreak of Listeriosis Linked to Whole Cantaloupes from Jensen Farms, Colorado
Persons infected with an outbreak-associated subtype of Listeria monocytogenes, by date of illness onset (Figure A) and date of clinical specimen collection (Figure B), 2011 (147 cases reported to CDC as outbreak-associated)*
* Onset date not currently available for all persons with reported illness. Excludes a newborn diagnosed with listeriosis with a specimen date in November whose mother was reported as a case earlier in the outbreak.
Among persons for whom information was available, reported illness onset ranged from July 31, 2011 through October 27, 2011. Ages ranged from <1 to 96 years, with a median age of 78 years. Most ill persons were over 60 years old. Fifty-eight percent of ill persons were female. Among the 145 ill persons with available information on whether they were hospitalized, 143 (99%) were hospitalized.
Thirty-three outbreak-associated deaths were reported: Colorado (9), Indiana (1), Kansas (3), Louisiana (2), Maryland (1), Missouri (3), Montana (1), Nebraska (1), New Mexico (5), New York (2), Oklahoma (1), Texas (2), and Wyoming (2). Among persons who died, ages ranged from 48 to 96 years, with a median age of 81 years. In addition, one woman pregnant at the time of illness had a miscarriage.
Seven of the illnesses were related to a pregnancy; three were diagnosed in newborns and four were diagnosed in pregnant women. One miscarriage was reported.
The outbreak can be visually described with a chart showing the number of persons who became ill each day. This chart is called an epidemic curve or epi curve. Please see the description of the steps in a foodborne outbreak investigation for more details.
About 800 laboratory-confirmed cases of Listeria infection are reported each year in the United States and typically 3 or 4 outbreaks are identified. The foods that typically cause these outbreaks have been deli meats, hot dogs, and Mexican-style soft cheeses made with unpasteurized milk. Produce is not often identified as a source, but sprouts caused an outbreak in 2009, and celery caused an outbreak in 2010.
Interpretation of Epidemic Curves During an Ongoing Outbreak
The epidemic curve (epi curve) shows progression of an outbreak over time. The epi curve in Figure A above depicts by day when persons became ill.The horizontal axis represents the day when a person became ill, also called the date of illness onset. The vertical axis represents the number of persons with illness onset each day. The epi curve in Figure B above depicts by day when persons had a clinical specimen collected to be tested for listeriosis. The horizontal axis represents the day when the clinical specimen was collected, also called the date of clinical specimen collection. The vertical axis depicts the number of persons with clinical specimens collected. The epi curves are updated as new data comes in, and thus are subject to change. Several factors are important in understanding the epi curves and why they are slightly different from each other.
- Symptoms of listeriosis are non-specific. For some ill persons, it can be difficult to say exactly when the symptoms began. A person with listeriosis usually has fever and muscle aches, often preceded by diarrhea or other gastrointestinal symptoms. Since these initial symptoms occur in many common illnesses other than listeriosis, a person who became ill with listeriosis might have experienced them because of another recent illness and might not be sure when the listeriosis symptoms started.
- In this outbreak, the median amount of time between the day when a person reported developing symptoms and the day when a clinical specimen was collected for testing for listeriosis is 2 days. However, this time lag ranges from 0 days to 35 days. Because of this wide range and the non-specific nature of some symptoms of listeriosis, the epi curve for the date of clinical specimen collection presents a more accurate overall representation of when outbreak illnesses likely began.
- For some ill persons, the date when they became ill is not currently known either because they don’t remember, because they aren’t available to be interviewed by the health department, or because the health department hasn’t reached them yet. For the epi curve in Figure A, if the date that the person’s clinical specimen was collected is known, we estimate the date of illness onset as two days before that date.
- It can be difficult to determine when cases start to decline because of the reporting delay. This can become clearer as time passes.
- It can be difficult to say when the outbreak is over, because of the reporting delay. The delay means that the curve for the most recent three weeks always looks like the outbreak could be ending even during an active outbreak. The full shape of the curve is only clear after the outbreak is over.