Introduction

Although food is essential for the survival of all living organisms, it can also be a safety hazard, posing potential risks to our health and even our lives. This is due to the presence of invisible microorganisms to the human eye which can lead to foodborne illness when unknowingly consumed. 

Foodborne illnesses (also known as foodborne diseases) are infections caused by entering harmful pathogens into our bodies through ingesting contaminated food. These pathogens could be bacteria, viruses, parasites, or chemical substances.¹

More than 200 foodborne diseases arise from contamination, meaning contracting a foodborne illness is fairly common.² One of the most well-known foodborne illnesses is Salmonella infection, a bacterial disease affecting the intestinal tract. 

On the other hand, Listeriosis might be more unfamiliar to most people. It is a rare foodborne disease caused by the Listeria monocytogenes bacteria (referred to as L. monocytogenes from here on), occurring in approximately 0.1-10 cases per 1 million people worldwide.

Importance of the topic

Although foodborne listeriosis occurs relatively infrequently, its impact can vary widely depending on the individual infected. Healthy individuals might experience only mild symptoms while high-risk groups tend to face more severe, potentially fatal outcomes. This makes listeriosis a significant threat to public health, not to mention its considerable economic burden on the livestock industry.³

L. monocytogenes is a zoonotic agent, meaning it is capable of infecting both humans and animals. L. monocytogenes has been isolated from multiple animal species, primarily livestock animals, humans and the environment (e.g., soil, rivers).³ 

Its environmental distribution results in the contamination of vegetation and fields, which are the primary food sources for livestock.^{3 4} This contamination negatively impacts livestock health, leading to economic repercussions for the livestock and food processing industries.³

Listeriosis in animals

Affected animal species and their transmission

Due to its wide dissemination in soils and fields, ruminants such as cattle, sheep, and goats that primarily feed on grass are susceptible to the invasion of L. monocytogenes. Once ingested, L. monocytogenes predominantly reside in the gastrointestinal tract, medulla oblongata, placenta, and buccal mucosa (lining of the cheeks).

One of the most recognised manifestations of listeriosis in adult ruminants is encephalitis (brain inflammation). The bacteria invade the oropharyngeal membrane of ruminants (covering areas at the back of the mouth), often through wounds in the oral cavity. 

The bacteria then invades the trigeminal nerves, a pair of nerves connecting the brain to the face. This results in severe neurological symptoms, including head tilt, depression, seizures, and paralysis of facial muscles. However, a significant problem arises when ruminants are asymptomatic, as this can lead to the widespread dissemination of the pathogen via faecal shedding into the environment, enhancing animal-to-animal transmission via the faecal-oral route.³

Besides encephalitis, another common manifestation of listeriosis is septicemia, typically seen in monogastric mammals such as dogs, cats and pigs.⁵ In comparison to ruminants, monogastric mammals are less likely to contract listeriosis. 

In the event of an infection, they tend to exhibit mild gastrointestinal symptoms, such as diarrhoea and vomiting. Due to their nearly asymptomatic response to the disease, domestic animals can significantly contribute to animal-to-human transmission via faecal shedding in domestic environments, potentially infecting human hosts residing under the same roof.³

Zoonotic aspects of listeriosis

Transmission from animals to humans 

The zoonotic potential of the disease has led to several outbreaks in the human population over the years. The two routes of transmission are through direct and indirect contact.

Direct contact

The transmission often involves farm workers, veterinarians, and professionals working in the livestock industry. Their jobs often require direct contact with animals at high risk of contracting or carrying listeriosis, which includes but is not restricted to the removal of faeces, feeding animals, or administering treatment for various conditions. All of these tasks put them at a high risk of becoming infected by the bacteria.⁶

Indirect contact

L. monocytogenes is isolated from a huge variety of animal species, demonstrating its ability to contaminate a wide range of human food sources.⁴ Humans primarily acquire the disease via the intake of contaminated food such as processed meat (sausage, ham, salami), ready-to-eat and raw seafood (smoked fish, sashimi), unpasteurised milk, raw eggs and soft cheeses.⁷ 

In addition, the bacteria can populate and survive under harsh conditions, including a range of pH levels, temperatures from -2 to -42 degrees Celsius, and high salt concentrations. 

This resilience has made L. monocytogenes a significant challenge for the food processing industry, as they are forced to come up with countless solutions to eliminate its presence before the products enter the consumer market.⁷ 

Implications for human health

Most cases of listeriosis are reported sporadically, but zoonotic outbreaks of the disease have also been documented over the years. As previously mentioned, although listeriosis is rare, it is treatable. However, regardless of available treatments, it reports a shockingly high mortality rate of approximately 20-30% each year.

L. monocytogenes enters the circulatory system and invades various parts of the body, such as the brain, placenta, and gastrointestinal tract, causing inflammation. There are two primary forms of listeriosis: non-invasive and invasive. 

The non-invasive form generally affects healthy individuals, portraying only mild symptoms of the disease that don’t usually require hospitalisation.

Mild symptoms

• Fever
• Muscle and joint aches
• Gastrointestinal issues (diarrhoea, vomiting) 
• Headache 

Contrastingly, high-risk groups are more susceptible to the invasive form of listeriosis, contributing to the disease’s high mortality rate and hospitalisation rate (>95%). 

According to statistics, approximately 16-27% of invasive listeriosis is associated with pregnant patients, with a fetal mortality rate ranging from 16-45% among infected women.⁵ Due to its severity, a zero-tolerance policy towards L. monocytogenes is enforced by the FSIS in the United States to fully eliminate the bacteria in processed food.⁴ 

High-risk groups

• Pregnant mothers 
• Infants 
• Immunocompromised individuals
• Elderly 

Severe symptoms

• Septicemia 
• Meningitis 
• Encephalitis 
• Perinatal infections 
• Stillbirth
• Abortion 
• Premature labour
• Meningoencephalitis

Treatment

Listeriosis is a curable disease. Depending on the severity, patients from high-risk groups are usually prescribed antibiotics (sulfamethoxazole or ampicillin) to eliminate the infection. Appropriate medications can also be given to manage symptoms of the disease, such as nausea and diarrhoea. 

Control and prevention strategies

As listeriosis mainly arises from food contamination, it is necessary to create public awareness regarding hygiene practices and safety measures when consuming raw or processed food that could potentially lead to listeriosis infection. 

Food industries should strictly adhere to hygienic SOPs (standard operating procedures) when producing processed food to prevent food contamination.

The public should practise the following safety measures to reduce the risk of contracting listeriosis: 

• Cook food thoroughly
• Avoid eating raw food 
• Practise sanitation and always maintain hand hygiene
• Store food according to the instructions given

Summary

Listeriosis might sound like a devastating disease due to its alarming statistics. Still, it is entirely preventable if strict hygiene practices and safety precautions are put in place when handling food. Individuals at high risk of getting infected should be extra vigilant with their diet plans and food choices, avoiding high-risk foods such as deli meat, raw seafood and ready-to-eat products. 

If you experience any of the listed symptoms after consuming potentially contaminated food, seek medical advice promptly. If you have more severe symptoms, such as a high fever or a stiff neck, seek emergency care immediately, as these could be signs of meningitis, which could be life-threatening if left untreated.