Foodborne Pathogens: The Unusual Suspects

Many years ago, I posited this definition for food safety: Food safety is the chemical, physical, or biological status of a food that allows its consumption without incurring an excessive risk of injury, morbidity, or mortality. The fulcrum of this definition is the notion of risk. As much as we would like the limits of operation to approach zero, they seldom do. Simply put, risks are inherent with the consumption of food. There are regulatory risks, reputational risks for the enterprise, and most important of all, risks to public health. It is the latter that we strive most ardently to control or eliminate. Of the three risk categories—chemical, physical, and biological—perhaps the most daunting in terms of control are foodborne pathogens.

Foodborne pathogenic agents comprise an immensely diverse assortment of life forms, including viruses, fungi, algae, protozoa, and bacteria. The United States, a country with a well-developed public health infrastructure and an abundance of regulations and regulatory oversight of its food system, reports 48 million cases of foodborne illness and about 3,000 deaths annually attributed to the consumption of tainted food. The usual suspects implicated as the etiological agents in these outbreaks are Salmonella spp., Campylobacter, Listeria, and E. coli O157:H7. And then there are the other disease-causing agents that contribute mightily to the public health burden. Let’s call them the “not-so-popular but deadly few.”

The U.S. Centers for Disease Control and Prevention (CDC) estimates that 31 of the most important known agents of foodborne disease found in foods consumed in the United States each year cause 9.4 million illnesses, 55,961 hospitalizations, and 1,351 deaths. CDC also reports that “not all agents of foodborne disease are known or can be counted as a ‘known agent of foodborne disease.’” These other “unspecified agents,” as the agency calls them, include:

• Known agents with insufficient data to estimate the agent-specific public health burden
• Known agents not yet recognized as causing foodborne illness (emerging pathogens)
• Microbes, chemicals, or other substances known to be in food whose ability to cause illness is unproven
• Agents not yet described

CDC reports suggest that unspecified agents in food consumed in the United States cause an additional 38.4 million gastroenteritis illnesses, 71,878 hospitalizations, and 1,686 deaths each year. After combining the estimates for the major known pathogens and the unspecified agents, the overall annual estimate of the total burden of disease due to contaminated food is 47.8 million illnesses, 127,839 hospitalizations, and 3,037 deaths. From these data it is clear that the “not-so-popular but deadly few” pathogenic agents are responsible for causing about 80% of foodborne illnesses and more than 50% of associated mortalities annually in the United States.

The Unusual Suspect Lineup

In the United States, the top five pathogens contributing to acquired foodborne illnesses in descending order of magnitude are: norovirus, Salmonella (nontyphoidal), Clostridium perfringens, Campylobacter spp., and Staphylococcus aureus. However, when it comes to deaths caused by foodborne illness, the top five are a somewhat different cast of characters and include: Salmonella (nontyphoidal), Toxoplasma gondii, Listeria monocytogenes, norovirus, and Campylobacter spp. Parasites, as noted in the above rankings, are important foodborne pathogens. Their complex lifecycles, varied transmission routes, and prolonged periods between infection and symptoms mean that the public health burden and relative importance of different transmission routes are often difficult to assess. Furthermore, there are major challenges in detection and diagnostics and variations in reporting.

Among the “not-so-popular but deadly few” foodborne pathogens worthy of increased attention, according to food safety experts and scientists, are two parasites and one bacterial microorganism.

Toxoplasma gondii. T. gondii is an interesting organism in the context of foodborne disease and food safety. Public health agencies report that as much as one-third of the world’s population are infected with this parasite, and it often goes unrecognized, as most patients are asymptomatic. It is important, from a perspective of risk, to consider that the source of this deadly disease-causing parasite is inadvertent consumption of feline feces. The lifecycle of T. gondii is dependent on its definitive host, domestic or wild cats. Humans are susceptible to infection by its oocyst that are shed in the cat’s feces. Likewise, herbivores can become infected by eating grass or other feedstuffs contaminated by cat feces. The parasite then becomes established in the muscle tissues of those animals and remains for the balance of their life.

Although foodborne infections in humans are rare, T. gondii infection could occur through eating raw vegetables from a home garden contaminated with cat excrement. Raw or undercooked meat derived from both pork and lamb also are reported as foodborne sources of the parasite. Toxoplasmosis is typically asymptomatic or associated with mild flu-like illness, but the infection can be serious or life threating in immunocompromised individuals. The unborn fetus also is at risk due to the parasite’s ability to cross the placenta. Control of T. gondii involves thorough cooking of foods that are at risk of contamination. Washing fresh fruit and vegetables has not been shown effective in removing the parasite. Keeping cats away from all food sources and the strictest hygienic practices should be observed when handling and cleaning cat boxes. Hand hygiene is an imperative in preventing the spread of this parasite into humans.

Cyclospora cayetanensis. This is a type of protozoa, which is a tiny, spherical single-celled organism. It is transmitted when people ingest contaminated feces, typically through exposure to contaminated food or water. It can be spread only through human excreta, unlike E. coli and Salmonella, which also can be spread from animal fecal matter. Cyclosporiasis is caused by ingestion of oocysts of Cyclospora cayetanensis, a parasite that was first recognized in the late 1970s. The clinical picture of cyclosporiasis can be indistinguishable from that of giardiasis, cryptosporidiosis, and isosporiasis. Cyclospora infection can cause severe abdominal pain, watery diarrhea, nausea, vomiting, body aches, and fatigue. Symptoms can develop between two and 14 days after exposure. Though symptoms can be severe enough to send people to the hospital, it’s rare for people to die from Cyclospora infections. Cyclospora infections generally increase in the summer months but can occur at any time.

According to the U.S. Food and Drug Administration (FDA) and CDC, identifying the particular food item or other ingredient that has caused an outbreak of cyclosporiasis can be very challenging. CDC and other agencies are working to develop and validate molecular typing methods that could distinguish among different strains of the parasite Cyclospora cayetanensis. In the past, infections have been traced to fresh produce, such as cilantro, fresh basil, bagged salads, and lettuce. This year, in the United States, there has been a dramatic increase in the confirmed cases of cyclosporiasis. According to the CDC, as of August 29, 2023, 40 jurisdictions, including 39 states and New York City, have reported a total of 1,957 laboratory-confirmed cases of cyclosporiasis.

Prevention and control of the spread of this parasite, according to CDC guidelines, is thorough hand washing, washing utensils used in food preparation, and thoroughly washing all fresh fruits and vegetables before cooking and/or refrigerated storage. Other food safety experts say there’s no evidence that washing produce will effectively remove the parasite. On the farm, good agricultural practices are critical for preventing food crops from coming into contact with human waste. Light-based technologies, such as high intensity pulsed light, also may be an effective control intervention. In the processing of fresh fruits and vegetables, due to the resistant nature of these complex organisms, it is difficult to define and validate chemical-aided washing as a critical control point targeting foodborne parasites. Ultimately, and because Cyclospora are associated exclusively with human waste, a key in preventing and controlling its spread is good hand hygiene. Effective hand washing will break the fecal to oral route of transmission.

Vibrio vulnificus. Of the more than 70 species of Vibrio, about a dozen species of Vibrio are pathogenic to humans and cause human illness known as vibriosis. CDC estimates that annually in the United States, 80,000 individuals become sick with vibriosis, and 100 die from their infection. Of its strains, V. parahaemolyticus causes the most infections in the United States, accounting for about 40% of all reported cases of vibriosis. Most people with Vibrio infection have diarrhea, and some might experience stomach cramping, nausea, vomiting, fever, and chills. V. vulnificus is known to cause life-threatening infections. About 150–200 V. vulnificus infections are reported to CDC each year and about one in five people with this infection die, sometimes within 1–2 days of becoming ill.

V. vulnificus bacteria thrive in warmer coastal waters and are present in higher concentrations between May and October when water temperatures are warmest. While Vibrio bacteria can enter the body through a break in the skin, it can also come from consuming contaminated seafood. V. vulnificus infections in the eastern United States increased eight-fold from 1988–2018. During July–August 2023, the United States has experienced above-average coastal sea surface temperatures and widespread heat waves. During the same period, several East Coast states, including Connecticut, New York, and North Carolina, have reported severe and fatal V. vulnificus infections. Several of these reported cases of vibriosis were associated with the consumption of raw or undercooked seafood or had unclear etiology. In late September 2023, it was reported by multiple sources that a California woman, after consuming undercooked fish, succumbed to a severe case of vibriosis that resulted in the amputation of her arms and legs. Public health officials in the state continue investigating this incident.

Preventing such infections is highly dependent upon adhering to the advice of public health agencies and avoiding seafood that may have been harvested from sites known to be contaminated with the bacteria. Ensuring that all seafood purchases are from safe sources will help in preventing the spread of the pathogen. Using high hydrostatic pressure processing technology to treat raw oysters also has been confirmed as an effective preventive control technique. Thoroughly cooking seafood and shellfish also is effective for mitigating the threat of food-acquired vibriosis.

Prevention Is Key

Food safety is dynamic. We learn new things about new pathogens or new threats to the food supply almost daily. There are the usual suspects—Salmonella, Campylobacter, Listeria, E. coli and Staphylococcus—and they must be accounted for and then taken seriously. Who knew that Toxoplasma gondii was a killer, or that one-third of the people on the planet are infected with this parasite? Then there are the organisms of unknown status: Do they or do they not cause human illness, or are they capable of transmission via foods intended for human consumption? For example, we have clinical confirmation in ill patients that they have been exposed to Cyclospora, but the FDA and CDC cannot pinpoint the food or beverage sources that have caused the infection.

It also appears that climate change may be changing the geographic distribution of certain pathogenic microorganisms, both bacterial and parasitic, resulting in infections and food safety failures in areas where prior assessments of risk would not have been predicted. Vibrio vulnificus outbreaks in New York and Connecticut are good examples. The list of unusual suspects is growing, and we must therefore be ever vigilant in our effort to identify and control these new threats to the food supply and public health (see sidebar below). But in the end, good hand hygiene is proven to be the most effective intervention in preventing the spread of foodborne disease.ft