However, the outbreak did show it is possible for highly susceptible people to get sick from products with low-level contamination.
In 2015, an outbreak of invasive listeriosis linked to Blue Bell brand ice cream products in the US hospitalised 10 people from four states.
Because ice cream has a long shelf life and Listeria monocytogenes does not grow but survives for long periods in frozen items, the level in implicated products manufactured during the outbreak, although collected afterwards, was likely to be representative of that in products eaten by exposed people.
Knowledge on dose-response relationship
Researchers assessed outbreak data to gain insight into contamination levels among products from an implicated factory, the number of L. monocytogenes cells ingested by general and susceptible populations and the dose-response relationship.
The number of L. monocytogenes cells ingested by the population was estimated by multiplying the average number of organisms per serving by the number of servings distributed in the various subpopulations. This number was estimated from distribution records for the Texas factory.
The study only includes the four hospitalized Kansas case-patients whose illnesses were from products manufactured in Texas. Illness onset dates were from January 2014 to January 2015.
They were between 67 and 84 years of age and had underlying medical conditions that contributed to compromised immune function before exposure to L. monocytogenes in milkshakes made from ice cream.
All of them ate a product from the Texas factory through milkshakes and strains of L. monocytogenes isolated were indistinguishable by pulsed-field gel electrophoresis to strains from the product.
The researcher said if there was no initial contamination of the milkshake machines and no growth of the pathogen in milkshakes, the mean contamination level of L. monocytogenes in them (8 cells/g of ice cream) was relatively low compared with levels in other outbreaks.
However, as there were no leftovers from implicated milkshakes, it cannot be ruled out that the four susceptible patients received some of the highest contaminated products from the factory line, triggering infection, they added.
The factory production line linked to the Kansas cases made eight different ice cream products but it is not known when contamination began.
“In the low-exposure scenario and medium-exposure scenario, we assumed the date at which contamination began at factory 1 was December 1, 2013, that is, a few weeks before hospitalization of the first case-patient whose illness was linked to ice cream produced at this facility.
“In the high-exposure scenario, we assumed contamination began 2.5 years before the outbreak was recognized, that is, midway between 2010 and the date the outbreak was recognized.”
The dose-response relationship between the number of ingested L. monocytogenes cells in food and the likelihood of developing listeriosis is not well understood, according to the study.
This is due to the lack of an appropriate animal model, relative rarity of outbreaks, long incubation periods that impede collection of well-preserved implicated food samples and heterogeneity of the initial contamination level.
The team estimated that the general population ingested 1.5 × 109 (low-exposure scenario) to 1.4 × 1010 (high-exposure scenario) L. monocytogenes cells and the highly susceptible population ingested 7.2 × 106 (low-exposure) to 3.3 × 107 (high-exposure) of cells.
In microbial dose-response frameworks, it is generally assumed that as few as 1 independently acting cell that survives host defense measures can initiate infection. This minimal infective dose of 1 cell is associated with a probability (r) of infection.
R can be estimated by the ratio of the number of invasive listeriosis cases in a subpopulation (Xp), by the estimated number of L. monocytogenes cells ingested by the subpopulation Dp.
“Infectious Dose of Listeria monocytogenes in Outbreak Linked to Ice Cream, United States, 2015”
Authors: Régis Pouillot , Karl C. Klontz, Yi Chen, Laurel S. Burall, Dumitru Macarisin, Matthew Doyle, Kären M. Bally, Errol Strain, Atin R. Datta, Thomas S. Hammack, and Jane M. Van Doren