Rockmelons kill 30 in the US

With a final death toll of 30 people, including one miscarriage, the US’s most deadly outbreak ever of food poisoning has been declared over. In total, 146 people across 28 states became ill after eating Listeria monocytogenes contaminated whole rockmelon. While the number of deaths was not the largest ever recorded for a food poisoning outbreak, the greater than one in five death rate makes the outbreak of listeriosis the deadliest food poisoning incident in the US. The outbreak was also the first caused by whole, unprocessed produce.

Historically, Listeria monocytogenes contamination has been associated with ready-to-eat and processed food products, such as deli meat, unpasteurised cheese, raw milk, fresh-cut fruit and fresh-cut vegetables. The Gram-positive bacterium is typically thought of as an environmental contaminant of food plants. The bacterium is unlike most other foodborne bacteria in that it thrives in cold temperatures. Known reservoirs for Listeria monocytogenes include ruminant animals, decaying vegetation and cold, wet and difficult-to-clean environments.

Symptoms of listeriosis can take up to two months to develop in someone who has eaten contaminated food, so there can be quite a long lag time between consumption of the pathogen and the identification of an outbreak. Often the contaminated food has been consumed and is no longer available before an outbreak is identified.

In early September 2011, the Food and Drug Administration (FDA), in conjunction with the Centers for Disease Control and Prevention (CDC) and state health departments, began to investigate a multistate outbreak of listeriosis.

By comparing the foods eaten by the outbreak cases, scientists were able to very quickly and convincingly show that rockmelons were associated with the outbreak. Early in the investigation, rockmelons from Jensen Farms in south eastern Colorado were implicated and subsequently identified as the sole source of the contaminated produce.

Cases in other states were also quickly identified and linked to the growing outbreak through PulseNet, which is a national network of public health laboratories that conduct DNA fingerprinting on foodborne bacteria.

Multiple samples, including whole rockmelons and environmental (non-product) samples from within Jensen Farm, were collected for laboratory analysis to identify the presence of Listeria monocytogenes. Of the 39 environmental swabs collected from within the facility, 13 were confirmed positive for Listeria monocytogenes with pulsed-field gel electrophoresis (PFGE) pattern combinations that were indistinguishable from three of the four outbreak strains collected from affected patients. Of the 13 positive environmental swabs, 12 were collected at the processing line and one was collected from the packing area. Rockmelons collected from the firm’s cold storage during the inspection were also confirmed positive for Listeria monocytogenes with PFGE pattern combinations that were indistinguishable from two of the four outbreak strains.

Contributing factors

A number of factors that were likely to have contributed to the introduction, growth or spread of Listeria monocytogenes at Jensen Farms were identified.

In-field contamination

Officials determined that potential routes for contamination of the rockmelons in the field included:

• agricultural water
• soil amendments
• growing and harvesting practices
• animal intrusion
• adjacent land use
• employee health and hygiene practices

All the environmental samples collected in the growing fields were negative for Listeria monocytogenes. However, FDA determined that the growing environment could not be eliminated as a potential contributor in the introduction of contamination. Specifically, low-level sporadic Listeria monocytogenes contamination from the agricultural environment and incoming rockmelons may have allowed for the establishment of a harbourage or niche for Listeria monocytogenes in the packing facility and cold storage.

Packing facility and cold storage contamination risks

Potential routes of contamination during operations in the packing facility and cold storage included:

• facility and equipment sanitary design
• cleaning and sanitising practices
• washing and drying of rockmelons
• cooling of rockmelons
• storage of rockmelons
• transportation

Certain aspects of the packing facility, including the location of a refrigeration unit drain line, allowed water to pool on the packing facility floor in areas adjacent to packing facility equipment.

Also, the packing facility floor was constructed in a manner that was not easily cleanable. Specifically, the trench drain was not accessible for adequate cleaning. This may have served as a harbourage site for Listeria monocytogenes and, therefore, is a factor that may have contributed to the introduction, growth or spread of the pathogen.

Wet environments are known to be potential reservoirs for Listeria monocytogenes and the pooling of water in close proximity to packing equipment, including conveyors, may have extended and spread the pathogen to food contact surfaces. This pathogen is likely to establish niches and harbourages in refrigeration units and other areas where water pools or accumulates.

The packing facility floor where water pooled was directly under the packing facility equipment from which FDA collected environmental samples that tested positive for Listeria monocytogenes with PFGE pattern combinations that were indistinguishable from outbreak strains. Therefore, this aspect of facility design is a factor that may have contributed to the introduction, growth or spread of Listeria monocytogenes.

Another potential means for introduction of Listeria monocytogenes contamination into the packing facility was a truck used to haul culled rockmelons to a cattle operation. This truck travelled to and from a cattle operation and was parked adjacent to the packing facility where contamination may have been tracked via personnel or equipment, or through other means, into the packing facility.

Based on the positive results from the environmental samples collected from the packing facility and from rockmelons collected from cold storage, it is likely that the contamination occurred in the packing facility. It is also likely that the contamination proliferated during cold storage.

Equipment design

FDA evaluated the design of the equipment used in the packing facility to identify factors that may have contributed to the growth or spread of Listeria monocytogenes. In July 2011, the firm purchased and installed equipment for its packing facility that had been previously used at a firm producing a different raw agricultural commodity.

The design of the packing facility equipment, including equipment used to wash and dry the rockmelons, did not lend itself to be easily or routinely cleaned and sanitised. Several areas on both the washing and drying equipment appeared to be uncleanable, and dirt and product build-up was visible on some areas of the equipment, even after it had been disassembled, cleaned and sanitised. Corrosion was also visible on some parts of the equipment.

Further, because the equipment is not easily cleanable and was previously used for handling raw potatoes with different washing and drying requirements, Listeria monocytogenes could have been introduced as a result of past use of the equipment.

Environmental samples collected from the packing facility equipment tested positive for Listeria monocytogenes with PFGE pattern combinations that were indistinguishable from three of the four outbreak strains. After the firm discarded portions of the packing facility equipment and cleaned and sanitised the remaining packing equipment, environmental samples tested negative for Listeria monocytogenes.

The design of the packing facility equipment, especially that it was not easily amenable to cleaning and sanitising and that it contained visible product build-up, is a factor that likely contributed to the introduction, growth or spread of Listeria monocytogenes. Rockmelon that is washed, dried and packed on unsanitary food contact surfaces could be contaminated with the bacteria or could collect nutrients for Listeria monocytogenes growth on the rockmelons’ rind.

Postharvest practices

In addition, free moisture or increased water activity of the rockmelons’ rind from postharvest washing procedures may have facilitated Listeria monocytogenes survival and growth. After harvest, the rockmelons were placed in cold storage. The rockmelons were not precooled to remove field heat before cold storage. Warm fruit with field heat potentially created conditions that would allow the formation of condensation, which is an environment ideal for Listeria monocytogenes growth.

The combined factors of the availability of nutrients on the rockmelons’ rind increased rind water activity, and lack of pre-cooling before cold storage may have provided ideal conditions for Listeria monocytogenes to grow and out-compete background microflora during cold storage. Samples of rockmelons collected from refrigerated cold storage tested positive for Listeria monocytogenes.

Auditing

Five weeks before the FDA conducted its inspection at Jensen Farms, a third-party audit by Primus Labs gave the facility 96 out of 100 possible points. The facility was found to have well-cleaned areas and no standing water. Everything was documented as easy to clean.

This raises interesting problems for the US as it introduces its Food Safety Modernization Act, which incorporates third-party audits as standard procedure. Who will audit the auditors and will they be good enough?

Don’t let it happen to you

The FDA’s findings regarding this particular outbreak highlight the importance for the industry to employ good agricultural and management practices in their packing facilities as well as in growing fields. Specific recommendations include:

• Assess produce facility and equipment design to ensure adequately cleanable surfaces and eliminate opportunities for introduction, growth and spread of Listeria monocytogenes and other pathogens.
• Assess and minimise opportunities for introduction of Listeria monocytogenes and other pathogens in packing facilities.
• Implement cleaning and sanitising procedures.
• Verify the efficacy of cleaning and sanitising procedures.
• Periodically evaluate the processes and equipment used in packing facilities to ensure they do not contribute to fresh produce contamination.
• Ensure third-party auditors are up to the task.