Listeria monocytogenes Outbreak 

Linked to Pork Patty Products

Outbreak Investigation After-Action Review, Report 2019-02

Highlights

• This outbreak was linked to pork patty products 

and was only the second reported multistate 

outbreak of listeriosis linked to a Food Safety and 

Inspection Service (FSIS) regulated product since 

2005 (see FSIS After-Action Report 2018-16).

• Whole genome sequencing (WGS) analysis of 

routine and investigative FSIS isolates, and 

isolates from ill people, evaluated in conjunction 

with epidemiologic and traceback evidence, was 

instrumental in detecting and solving this outbreak. 

• Investigation revealed that the pork patty products 

were cooked using a process that was not 

validated to ensure all pieces were thoroughly 

cooked. Investigation also revealed a history of Lm

harborage and opportunities for contamination after 

cooking that were not considered by the 

establishment.

People infected with Listeria monocytogenes by state 

of residence

Outbreak at a Glance

Illnesses 4

States included 4

Hospitalizations 4

Deaths 0

Response at a Glance

Investigative 

samples tested

49

Recall or Public 

Health Alert

Yes

Process improvements

• FSIS may update Directive 

10,240.4, Verification 

Activities for the Lm 

Regulation and the RTE 

Sampling Program to 

include additional 

instructions for inspection 

personnel when verifying if 

a product is exposed to the 

environment after 

undergoing a cooking step.

• FSIS translated the recall 

release into Vietnamese 

and Cambodian. FSIS 

routinely posts recall 

releases in English and 

Spanish and will continue 

to consider the languages 

spoken by ill people and 

consumers when posting 

recall releases.
What happened?

From October through December 2018, public health officials in Louisiana, Michigan, 

Tennessee, Texas, the Centers for Disease Control and Prevention (CDC) and FSIS 

investigated an outbreak of four Listeria monocytogenes (Lm) illnesses linked to readyto-eat (RTE), Asian-style pork patty products manufactured in an FSIS-regulated 

establishment. The four ill people reported being of Asian ethnicity and eating Asianstyle foods. Purchase information was not available; however, investigators determined 

that the pork patty products were distributed from one FSIS establishment to three retail 

locations where three of the four ill people reported shopping. WGS analysis of routine 

FSIS product and environmental isolates indicated the historical presence of the 

outbreak strain at the establishment. In response to these positive samples, FSIS 

performed follow-up activities, including sampling, to verify that the establishment’s 

corrective actions were effective. Investigation at the establishment revealed that after 

an initial cooking step, the pork rolls were handled, and during this time they may have 

been cross-contaminated from the processing environment. After handling, the pork 

rolls were placed in a bag and re-cooked. The cook-in-bag process was not validated to 

ensure all pieces were thoroughly cooked so that bacteria that can make people sick 

(e.g., Salmonella and Lm) would not survive. The investigation also revealed that there 

was the potential for contamination after both cooking steps, even though the 

establishment did not consider the product exposed to the environment after the 

cooking step. An environmental sample collected at the establishment during the 

investigation was positive for the outbreak strain, which helped confirm the source 

establishment. On November 20, 2018, the establishment voluntarily recalled the 

affected pork patty products. 

What we learned from this outbreak

Sampling

 roduct and environmental samples collected before the outbreak and 

environmental samples collected during the outbreak investigation 

yielded the outbreak strain his helped identify the source of the 

outbreak and indicated harborage at the regulated establishment, 

highlighting the value of both routine and investigative sampling 

Whole Genome Sequencing

 his outbreak likely would not have been detected, nor the cause 

determined, without W W is a powerful tool for determining 

relatedness between bacterial isolates and is helping public health 

partners to detect and solve outbreaks with a small number of cases 

Sampling

 roduct and environmental samples collected before the outbreak and 

environmental samples collected during the outbreak investigation 

yielded the outbreak strain his helped identify the source of the 

outbreak and indicated harborage at the regulated establishment, 

highlighting the value of both routine and investigative sampling 

Sampling

 roduct and environmental samples collected before the outbreak and 

environmental samples collected during the outbreak investigation 

yielded the outbreak strain his helped identify the source of the 

outbreak and indicated Lm harborage at the regulated establishment, 

highlighting the value of both routine and investigative sampling 

 

 
 

 

What can you do to prevent or solve the next 

outbreak?

Helpful Links

Helpful Links

• ecall 116-2018, ovember 20, 2018

• ood afety lert, ovember 21, 2018, updated January 29, 2019

• irective 10,240 4, Verification ctivities for the Listeria monocytogenes 

 egulation and the eady-to- at ( ) ampling rogram

• fter- ction eview eport 2018-16: Listeria monocytogenes llness 

 utbreak ssociated with eady-to- at, ountry- ured Ham, 2017–2018

• Salmonella ompliance uidelines for mall and Very mall eat and 

 oultry stablishments that roduce eady-to- at ( ) roducts and evised 

 ppendix (June 2017) 

• eport a roblem with ood | ood afety and nspection ervice (usda gov)

• ind more information about ’ response to foodborne outbreaks on the 

 utbreak age

 stablishments that 

produce cook-in-bag 

 products should 

ensure that they have

sufficient scientific 

support that the chosen 

endpoint temperature 

results in a safe product 

and have gathered inplant validation data that 

demonstrates the 

desired endpoint 

temperature can be met 

throughout the product 

 he data should also 

support that the 

monitoring procedures 

are sufficient to identify 

a process deviation 

 ontinue to work closely 

with local, state, and 

federal partners to 

detect and investigate 

outbreaks ublic health 

partners should email 

 oodborne isease epo

rts@usda gov to notify 

 if -regulated 

products may be 

involved in an outbreak 

Public Health Partners

 f you think you have a 

foodborne illness, see a 

health care provider for 

treatment Your health 

care provider can test 

you and if the tests 

show you have a 

reportable foodborne 

illness, your state or 

local public health 

officials should be 

notified f you suspect 

an -regulated meat, 

poultry, or egg product 

made you sick, see the 

 eport a roblem with 

 ood page for 

information on how to 

notify 

Industry Consumers