Researchers said it is important to use validated disinfectant protocols and to clean and sanitize wire cutters, cutting boards, knives, and utensils after cutting each block or wheel of cheese.
They added cross-contamination is a vital consideration when investigating an outbreak with suspected links to cheese, even more so if a single type or brand is not identified early.
Public health officials determined two deaths were related to listeriosis and one fetal loss was reported was in the 2012 outbreak.
Imported Italian cheese
Frescolina Marte brand ricotta salata cheese imported from Italy and distributed by Forever Cheese to firms in 30 states, was the source.
In 2012, the Food and Drug Administration placed the exporter, Fattorie Chiarappa of Conversano, Italy, on Import Alert, meaning cheese was denied admission unless the importer showed it is not contaminated with Listeria.
Initial analyses identified an association between eating soft cheese and outbreak-related illness but no common brand.
Although nine out of 10 patients reported eating soft cheese, no single type was initially reported by more than five patients. Eight patients having yogurt but examination of brand information revealed no common source.
The researchers hypothesized early in the investigation that an intact, contaminated cheese (cheese X) could cross-contaminate multiple types or brands during cutting and repackaging at retail and distribution locations.
Three pieces of information reinforced this: (i) the outbreak strain was cultured from samples of cut and repackaged cheeses collected at distributor C and grocery chain A; (ii) distributor C did not ship cut and repackaged cheeses to grocery chain A; and (iii) grocery chain A received only intact wheels from its distributors.
This meant cheese X would have been cut and repackaged at both locations, which was the case.
At distributor C, the process of cutting and repackaging Marte Brand Frescolina Ricotta Salata likely cross contaminated the two partial wheels (blue and farmstead) that yielded the outbreak strain.
All four PFGE patterns of L. monocytogenes cultured from this cheese or from patients’ specimens were extremely rare or had not been identified in the US before.
MLVA analysis confirmed these PFGE patterns were closely related, comprising a single outbreak strain. Seven cases were linked directly to the ricotta salata, including a mother–infant pair.
Another patient probably ate ricotta salata or a cross-contaminated cheese. Many of the remaining 14 patients were probably exposed to L. monocytogenes in cheeses cross contaminated by Marte Brand Frescolina Ricotta Salata.
Cross contamination hypothesis
There are several reasons to think cross-contamination of other cut and repackaged cheeses occurred and caused illnesses, said the researchers.
“First, the index patient consumed two cut and repackaged cheeses from which the outbreak strain was isolated but did not report consuming ricotta salata.
“The fact that the outbreak strain was also isolated from wedges of l’Édel de Cléron at the store where the patient purchased the cheese (and where Marte Brand Frescolina Ricotta Salata was cut and repackaged) make it likely that this cross-contaminated cheese was the source of this patient’s illness.”
However, culturing of intact wheels did not yield the outbreak strain, making it unlikely that l’Édel de Cléron was the source of other patients’ illnesses.
“Second, when asked to list cheeses consumed in the past month, eight patients did not report eating ricotta salata. However, patients were not specifically asked if they had consumed ricotta salata.
“Although some may not have remembered eating ricotta salata, all eight reported eating other cheeses; three reported other cut and repackaged cheeses.
“Third, the same rare outbreak strain was isolated from open samples, but not intact wheels, of four other cheeses cut and repackaged at distributor C and the grocery chain A location where Marte Brand Frescolina Ricotta Salata was cut and repackaged, suggesting cross-contamination from the ricotta salata.
“By the investigation’s end, the outbreak strain had been isolated from five types of cheeses in six places.”
FDA isolated L. monocytogenes from all 10 samples of intact Marte Brand Frescolina Ricotta Salata from store B, with counts ranging from ∼9000 to ∼3 750 000 colony-forming units (cfu)/g.
These cheeses were sampled within seven days of sell-by date.
Source: Epidemiology and Infection
“Multistate outbreak of listeriosis caused by imported cheese and evidence of cross-contamination of other cheeses, USA, 2012”
Online ahead of print, DOI: 10.1017/S095026881500117X
Authors: K. E. Heiman, V. B. Garalde, M. Gronostaj, K.A Jackson, S. Beam, L. Joseph, A. Saupe, E. Ricotta, H. Waechter, A. Wellman, M. Adams-Cameron, G. Ray, A. Fields, Y. Chen, A. Datta, L. Burall, A. Sabol, Z. Kucerova, E. Trees, M. Metz, P. Leblanc, S. Lance, P.M. Griffin, R.V. Tauxe and B. J. Silk