By: Heather Williams

As the Salmonella Outbreak investigation in papayas expands, another outbreak is added to the toll.  And with it comes another fatality.  This makes four separate outbreaks linked to imported Maradol papayas from Mexico.  In the latest investigation, so far 14 have been sickened across 3 states resulting in 5 hospitalizations and 1 death. Arizona and Colorado each have 1 reported case and California has 12 reported cases with 1 death.

Papayas have been an ongoing concern.  With recent DNA fingerprinting linking those who are sick with imported papayas, the concern is growing even greater.  With several outbreaks involving several different strains of the harmful bacteria Salmonella across several farms in Mexico, investigators are scrambling to keep up.

But how do investigators know that there is more than one outbreak if everyone is sick with salmonellosis?  What is the difference between one outbreak compared to another?  Why is there widespread concern?  To answer these questions, we must first explain how investigators distinguish one outbreak from another.  Then we can analyzed data from those who are sick and the indicated sources.

How are Outbreaks Separated?

An outbreak is defined as an incident where two or more people who have similar illness resulting from the ingestion of a common food.  Outbreaks are easier to track more than they ever have been with the onset of PulseNet in 1996.  PulseNet is an online national laboratory network database.  It uses DNA fingerprinting results, which identify specific strains of illness causing bacteria and viruses, allowing epidemiologists to track patterns in illnesses.  This allows them to detect local and even multi-state outbreaks.  This information allows investigators to identify a source faster, giving them the opportunity to alert the public sooner.

What exactly is DNA fingerprinting.  Just like no two people have the same fingerprint, no two people have the same DNA (unless you count identical twins).  This is also true for strains of viruses.  Just like our DNA consists of a specific order of bases pairs, so does that of bacteria.  No two versions of Salmonella have identical DNA.  This is how we tell them apart.  PulseNet allows the laboratory to identify the exact DNA fingerprint of the sick person.  During the course of investigation, potentially contaminated specimens are checked for their own DNA fingerprints.  When a cluster of people who are sick present the same DNA fingerprint, an investigation starts.  Those who are ill are often interviewed.  Some of the main questions that are asked involves where and what they may have eaten the week before illness.  This leads investigators to test products that infected people have in common.  If the potential food comes back positive for the particular strain detected in those who are sick, it is a good probability that the source has been found.  Knowing the origin of the infection helps in recall efforts and notifying the public to avoid particular foods and/or specific lots of foods.

Salmonella Anatum

The newest outbreak added to the investigation involves a strain not present in the other four investigations. Salmonella Anatum was reported in 3 states with illness onset dates ranging from December 20, 2016 to April 8, 2017.  This outbreak had not previously been linked to the existing papaya outbreak.  Initially 8 people were affected by the Salmonella Anatum outbreak that was already being investigated.  More recently a review the Center for Disease Control and Prevention (CDC) PulseNet database identified six more ill people with the same DNA fingerprint of the strain.  This allows the CDC and epidemiologists track illness and link outbreaks.  The CDC is currently investigating if these more recent illnesses are linked to the current papaya outbreak. However, on September 4, 2017 the FDA identified the same Salmonella strain Salmonella Anatum from a sample taken from an imported papaya at the U.S.-Mexico border.  With this new evidence it seems that this outbreak is linked.

The investigation based on epidemiologic, laboratory, and traceback evidence indicated that the Maradol papayas imported by Bravo Produce Inc. of San Ysidro, California are the likely source of this multistate outbreak.  These papayas were recalled on September 10, 2017 by Bravo Produce Inc.  The Maradol papayas packed by Frutas Selectas de Tijuana, S. de RL de CV and distributed to California from August 10 to August 29, 2017 had already been recalled.

Other Salmonella Outbreaks

As of September 1, 2017 the initial Salmonella outbreak involving strains of Salmonella Thompson, Salmonella Kiambu, Salmonella Agona, and Salmonella Gaminara has reached 201 cases across 23 states. This includes an additional 28 sick and 12 new states impacted between August 18, 2017 and September 1, 2017.  This outbreak was linked to papayas from Caraveo Produce and Tecomán, Mexico and El Zapotanio in La Huerta, Mexico with a matching DNA fingerprint.  These papayas originated from the Carica de Campech farm.

A separate outbreak attributed to the strains Salmonela Newport and Salmonella Infantis sickened 3 people in 3 states includeing Illinois, Massachusetts, and Michigan.  All three infected people reported eating papayas the week before they became sick.  Those two strains were identified in papaya samples linked to Caraveo produce in Tecomán, Mexico with a matching DNA fingerprint.

A third outbreak attributed to the Salmonella Urbana strain was also linked to imported papayas through DNA fingerprint analysis.  Of these, 6 have been infected from 3 states including New Jersey, New York, and Pennsylvania.  Of the 6 reported, 4 were interviewed.  Of these, 75% of those interviewed reported eating papayas in the week before they became ill.

Widespread Concern

The U.S. Food and Drug Administration (FDA) is continuing the testing process of papayas from Mexico and the CDC is continuing to monitor PulseNet to identify if any additional Salmonella infections are linked to any current outbreaks or additional strains found in papaya samples.

The biggest concern is that Salmonella contaminated papayas are being identified from multiple distributers and originating from multiple farms.  Additionally, there are many different strains identified as contaminants.  This presents the possibility that there is more going on in Mexico with Salmonella, which presents a large health risk.  At this time, investigations are ongoing to determine if any additional public warnings are needed beyond those specific to particular importers and farms.

 

Sources:

https://www.cdc.gov/mmwr/preview/mmwrhtml/ss4901a3.htm

https://www.cdc.gov/pulsenet/about/index.html

https://www.yourgenome.org/facts/what-is-a-dna-fingerprint

https://www.cdc.gov/salmonella/kiambu-07-17/index.html

https://www.cdc.gov/salmonella/anatum-9-17/index.html

https://www.cdc.gov/salmonella/anatum-9-17/map.html