Bacterial risks associated with ice cream are not rare events.

2014WashingtonListeria220Snoqualmie Ice Cream
2010-2015MultistateListeria10103Blue Bell
2008VermontE. coli O157630Homemade ice cream, unpasteurized milk
2008MinnesotaSalmonella 200Homemade ice cream
2008CaliforniaE. coli O157200Consumed at a restaurant; brand or pasteurization unknown
2007CaliforniaNorovirus1700Ice cream with berries; berries were likely the contaminated ingredient
2007PennsylvaniaCampylobacter300Homemade ice cream
2007MinnesotaHepatitis A1560Ice cream or yogurt at a restaurant
2007West VirginiaSalmonella 820Homemade ice cream
2006BelgiumE. coli O1451250Ice cream at a farm
2006CaliforniaSalmonella600Homemade ice cream
2005MultistateSalmonella 26110Cold Stone Creamery (cake batter was the contaminated ingredient)
1994MultistateSalmonella 1200Schwan’s Ice Cream – the estimated # of ill cases is 224,000!
1993FloridaSalmonella 593Homemade ice cream


As of June 29, 2022, a total of 23 people infected with the outbreak strain of Listeria monocytogenes have been reported from 10 states. Of the 22 people with information, 20 sick people reported living in or traveling to Florida in the month before they got sick, although the significance of this is still under investigation. Illnesses started on dates ranging from January 24, 2021, through June 12, 2022.

The CDC reported that sick people range in age from less than 1 to 92 years, with a median age of 72, and 52% are male. Twenty-two people (96%) have been hospitalized. Five people got sick during their pregnancy, and one illness resulted in a fetal loss. One death has been reported from Illinois.

The Florida Department of Health, CDC, public health and regulatory officials in several other states, and the U.S. Food and Drug Administration (FDA) are collecting different types of data to investigate a multistate outbreak of Listeria monocytogenes infections.

Public health officials continue to interview people about the foods they ate in the month before they got sick. Of the 17 people interviewed, 14 (82%) reported eating ice cream. Among 13 people who remembered details about the type of ice cream they ate, six reported eating Big Olaf Creamery brand ice cream or eating ice cream at locations that might have been supplied by Big Olaf Creamery.

As a result of this investigation, Big Olaf Creamery in Sarasota, FL, is voluntarily contacting retail locations to recommend against selling their ice cream products. Consumers who have Big Olaf Creamery brand ice cream at home should throw away any remaining product.

Listeria has been found on equipment in the ice cream processing facility and in 16 of 17 flavors. Big Olaf first refused to recognize that it was the cause of the outbreak and refused to stop production and stop ice cream sales. According to the Florida Department of Agriculture and Consumer Services (FDACS):

The results from product sampling taken from the Big Olaf production facility last week by FDACS found that 16 of the 17 flavors tested were positive for Listeria monocytogenes (L. mono). This includes Blueberry Cheesecake, Butter Pecan, Cherry Cordial, Chocolate, Chocolate Chip, Coconut, Coconut Almond Joy, Cookie Dough, Cookies & Cream, Kahlua Krunch, Mint Chip, Pistachio, Plantation Praline, Superman, Vanilla, and White Chocolate Raspberry. With these results, FDACS is currently issuing formal stop sales on the 16 products where L. mono was found, which were previously part of a voluntary recall. Our department continues to work closely with our state and federal partners on this investigation and enforcement of the stop sale.

Collected product samples yielded 16 Listeria-positive flavors. One outstanding environmental sample also came back positive, bringing the total positive environmental samples to 10.


Listeria is a gram-positive, rod-shaped bacterium that is ubiquitous and can grow under either anaerobic (without oxygen) or aerobic (with oxygen) conditions. 

A.        The Prevalence of Listeria in Food and the Environment

Listeriosis is one of the most important bacterial infections worldwide that arises mainly from the consumption of contaminated food.[1] The disease is caused by Listeria monocytogenes, which is considered an opportunistic pathogen that affects mainly those with underlying immune conditions, such as pregnant women, neonates, and elders, resulting in septicemia, meningitis, and/or meningoencephalitis. Of the six species of Listeria, only L. monocytogenes causes disease in humans. It thrives between 86-98.6oF (30-37oC), but Listeria can grow at temperatures as low as −0.4°C and survive in freezing conditions down to −18°C.[2] This unique quality allows thermal characteristics to be used as a means of differentiating Listeria from other possibly-contaminating bacteria. 

Listeria monocytogenes is omnipresent in nature; it is found widely in such places as water, soil, infected animals, human and animal feces, raw and treated sewage, leafy vegetables, effluent from poultry and meat processing facilities, decaying corn and soybeans, improperly fermented silage, and raw (unpasteurized)milk.[3]

Foodborne listeriosis is relatively rare but is a serious disease with high fatality rates (20%–30%) compared with other foodborne microbial pathogens. Severe L. monocytogenes infections are responsible for high hospitalization rates (91%) among the most common foodborne pathogens, may cause sporadic cases or large outbreaks, and can persist in food-processing environments and multiply at refrigeration temperatures, making L. monocytogenes a significant public health concern.[4]

Ready-to-eat foods are a notable and consistent source of Listeria. For example, a research study done by the Listeria Study Group found that L. monocytogenes grew from at least one food specimen in the refrigerators of 64% of persons with a confirmed Listeria infection (79 of 123 patients), and in 11% of more than 2,000 food specimens collected in the study. Moreover, 33% of refrigerators (26 of 79) contained foods that grew the same strain with which the individual had been infected, a frequency much higher than would be expected by chance. The danger posed by the risk of Listeria in ready-to-eat meats prompted the USDA to declare the bacterium an adulterant in these kinds of meat products and, as a result, to adopt a zero-tolerance policy for the presence of this deadly pathogen. The Code of Federal Regulations includes requirements for the post-lethality control of Listeria in meat and poultry products. This regulation is referred to as “The Listeria Rule” and was enacted in 2003. The rule outlines prevention and control measures that must be taken in processing facilities to reduce the risk of contamination of ready-to-eat products.[5]

B.        Transmission of and Infection with Listeria

Listeria typically spreads to people through contaminated food or water but can also be transmitted from mother to fetus.

Except for the transmission of mother to fetus, human-to-human transmission of Listeria is not known to occur. Infection is caused almost exclusively by the ingestion of the bacteria, most often through the consumption of contaminated food. The most widely accepted estimate of foodborne transmission is 85-95% of all Listeria cases. 

The infective dose—that is, the number of bacteria that must be ingested to cause illness—is not known but is suspected to vary based on the strain. In an otherwise healthy person, an extremely large number of Listeria bacteria must be ingested to cause illness—estimated to be somewhere between 10-100 million viable bacteria (or colony forming units “CFU”) in healthy individuals, and only 0.1-10 million CFU in people at high risk of infection. Even with such a dose, a healthy individual will suffer only a fever, diarrhea, and related gastrointestinal symptoms.

The amount of time from infection to the onset of symptoms—typically referred to as the incubation period—can vary to a significant degree.[6]

According to the CDC, symptoms of Listeria infection can develop at any time from the same day of exposure to 70 days after eating contaminated food. According to the FDA, gastroenteritis (or non-invasive illness) has an onset time of a few hours to 3 days, while invasive illness can have an onset varying from 3 days to 3 months. According to one authoritative text:

The incubation period for invasive illness is not well established, but evidence from a few cases related to specific ingestions points to 11 to 70 days, with a mean of 31 days. In one report, two pregnant women whose only common exposure was attendance at a party developed Listeria bacteremia with the same uncommon enzyme type; incubation periods for illness were 19 and 23 days.

Adults can get listeriosis by eating food contaminated with Listeria, but babies can be born with listeriosis if their mothers eat contaminated food during pregnancy. The mode of transmission of Listeria to the fetus is either transplacental via the maternal bloodstream or ascending from a colonized genital tract. Infections during pregnancy can cause premature delivery, miscarriage, stillbirth, or serious health problems for the newborn.

Pregnant women make up around 30% of all infection cases while accounting for 60% of cases involving the 10- to 40-year age group.

C.        Who is most susceptible to Listeria monocytogenes infection?

Several segments of the population are at increased risk and need to be informed so that proper precautions can be taken. The body’s defense against Listeria is called “cell-mediated immunity” because the success of defending against infection depends on our cells (as opposed to our antibodies), especially lymphocytes, otherwise known as “T-cells.” Therefore, individuals whose cell-mediated immunity is suppressed are more susceptible to the devastating effects of listeriosis, including HIV-infected individuals, who have been found to have Listeria-related mortality of 29%. 

The incidence of Listeria infection in HIV-positive individuals is higher than in the general population. One study found that:

The estimated incidence of listeriosis among HIV-infected patients in metropolitan Atlanta was 52 cases per 100,000 patients per year, and among patients with AIDS, it was 115 cases per 100,000 patients per year, rates 65-145 times higher than those among the general population. HIV-associated cases occurred in adults who were 29-62 years of age and in postnatal infants who were 2 and 6 months of age. 

Pregnant women naturally have a depressed cell-mediated immune system. While other systemic bacterial infections may result in adverse pregnancy outcomes at comparable frequencies, L. monocytogenes have notoriety because fetal complications largely occur in the absence of overt illness in the mother, delaying medical intervention. In addition, the immune systems of fetuses and newborns are very immature and are extremely susceptible to these types of infections. 

Other adults, especially transplant recipients and lymphoma patients, are given necessary therapies with the specific intent of depressing T-cells, and these individuals become especially susceptible to Listeria as well.

Other adults, especially transplant recipients and lymphoma patients, are given necessary therapies with the specific intent of depressing T-cells, and these individuals become especially susceptible to Listeria as well.

According to the FDA, CDC, and other public health organizations, individuals at increased risk for being infected and becoming seriously ill with Listeria include the following groups:

D.        Symptoms of Listeriosis

Only a small percentage of persons who ingest Listeria fall ill or develop symptoms. For those who do develop symptoms because of their infection, the resulting illness is either mild or quite severe, in what is sometimes referred to as a “bimodal distribution of severity.”[7] Listeria can cause two different types of disease syndromes with differing severity. Non-invasive Listeria infection causes gastroenteritis with symptoms such as diarrhea, nausea, and vomiting that resolve on their own. Healthy adults without any immunocompromising conditions typically experience this milder version of the disease. The more severe type of disease caused by Listeria monocytogenes is called listeriosis and is referred to as an invasive illness. 

On the mild end of the spectrum, listeriosis usually consists of the sudden onset of fever, chills, severe headache, vomiting, and other influenza-type symptoms. Along these same lines, the CDC notes that infected individuals may develop fever, muscle aches, and sometimes gastrointestinal symptoms such as nausea or diarrhea. When present, the diarrhea usually lasts 1-4 days (with 42 hours being average), with 12 bowel movements per day at its worst.

The more severe form of the illness occurs when the bacteria infect parts of the body that are typically sterile, such as the blood, brain, liver, and cerebral spinal fluid. The presence of the bacteria in these areas triggers the immune response and can lead to those more severe symptoms. L. monocytogenes has a specific affinity for the central nervous system (CNS), especially in cell-mediated immunodeficient individuals.[8]

E.        Complications of Listeria Infection

For those persons who suffer a Listeria infection that does not resolve on its own, the complications can be numerous and possibly severe. The most common complication is septicemia (bacterial infection in the blood), with meningitis being the second most common. Other complications can include inflammation of the brain or brain stem (encephalitis), brain abscess, inflammation of the heart-membrane (endocarditis), septic arthritis, osteomyelitis (infection in the bone), and localized infection, either internally or of the skin. 

Death is the most severe consequence of listeriosis, and it is tragically common. The CDC has estimated that L. monocytogenes is the third leading cause of death from foodborne illness, with approximately 260 of 1,600 people diagnosed dying from their infections. For example, based on 2018 FoodNet surveillance data, 96% of 126 Listeria cases ended up in the hospital, the highest hospitalization rate for pathogenic bacterial infection. This data showed a fatality rate of 21%. According to the FDA, the case-fatality rate increases substantially based on complications, possibly reaching rates of 70% in cases with Listeria meningitis, 50% in septicemia cases, and over 80% for perinatal/neonatal infections. In one U.S. study, L. monocytogenes was reportedly the cause of nearly 4% of all cases of bacterial meningitis.19

[1]           Reda, W. W., Abdel-Moein, K., Hegazi, A., Mohamed, Y., & Abdel-Razik, K. (2016). Listeria monocytogenes: An emerging food-borne pathogen and its public health implications. The Journal of Infection in Developing Countries10(02), 149-154.

[2]           Santos, T., Viala, D., Chambon, C., Esbelin, J., & Hébraud, M. (2019, May 24). Listeria monocytogenes Biofilm Adaptation to Different Temperatures Seen Through Shotgun Proteomics. 

[3]           Manning, A. (2019). Microbial Food Spoilage and Food Borne Diseases. In Food microbiology and food processing (pp. 125–130). Chapter 2. ED-TECH PRESS. 

[4]           Arslan, F., Meynet, E., Sunbul, M. et al. The clinical features, diagnosis, treatment, and prognosis of neuroinvasive listeriosis: a multinational study. Eur J Clin Microbiol Infect Dis 34,1213–1221 (2015).

[5]           USDA Staff. (2014, January 1). Controlling Listeria monocytogenes in Post-lethality Exposed Ready-to-Eat Meat and Poultry Products.

[6]           Goulet V, King LA, Vaillant V, de Valk H. What is the incubation period for listeriosis? BMC Infect Dis. 2013;13:11. Published 2013 Jan 10. doi:10.1186/1471-2334-13-11

[7]           Waldron, C. M. (2017, September 15). The Recovery and Transfer of Aerosolized Listeria Innocua. 

[8]           Arslan, F., Meynet, E., Sunbul, M., Sipahi, O. R., Kurtaran, B., Kaya, S., … Mert, A. (2015, June). The clinical features, diagnosis, treatment, and prognosis of neuroinvasive listeriosis: a multinational study. European journal of clinical microbiology & infectious diseases: official publication of the European Society of Clinical Microbiology.