Overview

What is salmonella infection?

Salmonella infection is caused by Salmonella bacteria, which primarily affects the intestinal tract. The Salmonella bacteria live in your intestines and are excreted in faeces. Salmonella infection in your intestines results in acute bacterial gastroenteritis (diarrhoea, fever, and abdominal cramps).

Gastroenteritis is typically contracted through the consumption of animal-derived products, particularly pig and poultry meat. It is estimated that Salmonella causes 93.8 million cases of gastroenteritis worldwide each year, with approximately 155,000 of these cases sadly resulting in death.¹ 

Why is it important to understand the causes and symptoms of Salmonella?

In the United States alone, it has been estimated that Salmonella infects 1.45 million people annually, with 26,500 of these cases resulting in hospitalisation. Due to the significant number of illnesses that Salmonella causes, and the potential for severe outcomes, it is important to understand what can cause this infection, so that it can be prevented effectively. Additionally, it is important to recognise the symptoms so they can be identified, to allow for more efficient diagnosis. 

Causes of Salmonella

Source of infection

Contaminated food

Food can become contaminated with Salmonella through contact with faeces. Salmonella bacteria can be found in the following:

• Eggs: raw eggs may contain Salmonella, this includes products made with raw eggs such as mayonnaise
• Undercooked poultry: Meat from poultry, including chicken, if not cooked properly
• Raw meat: when meat is being butchered it may come into contact with faeces, which can result in Salmonella in the faeces being transferred to the food
• Unpasteurised milk: raw milk can be contaminated with bacteria such as Salmonella, and the pasteurisation process removes any bacteria that can cause illness

Ingesting Salmonella-contaminated food can result in a Salmonella infection. 

Contaminated water

Similar to food, water can also become contaminated with faeces, which can result in Salmonella infection when consumed.

Transmission methods

Person-to-person contact

Salmonella can pass from person to person via the faecal-oral route, where Salmonella is transferred from the faeces of the infected individual to the mouth of an individual susceptible to Salmonella infection.

Animal-to-person contact

Infected animals, including pets, can also pass on Salmonella to humans. Infected animals typically show no symptoms of being infected with Salmonella. 

Cross-contamination in food preparation

When preparing food, if cooked foods or foods that are ready to eat come into contact with raw meat or juices that contain Salmonella bacteria, it may pass from the raw meat to the cooked food. Consequently, this cross-contamination may result in the presence of Salmonella bacteria in the cooked food.²

Symptoms of Salmonella

Gastrointestinal symptoms

Diarrhoea

Symptoms of Salmonella infection include having diarrhoea, which may or may not be bloody.³

Abdominal cramps

Abdominal cramps are also another gastrointestinal symptom you may experience as a result of a Salmonella infection.³ Abdominal cramps are common, with 66% of those with Salmonella reporting having this symptom.⁴

Nausea and vomiting

Both nausea and vomiting are also common gastrointestinal symptoms of Salmonella infection. In one study looking at 225 individuals who became infected with Salmonella, 50.4% of these individuals reported feeling nauseous, and 62.7% of these individuals experienced vomiting.^3,5

Typically, gastrointestinal symptoms will resolve themselves within 5-7 days after the onset. However, it is important to note that in children, these symptoms may present for longer and be more severe.³

Systemic symptoms

Fever

Having a fever (where your internal body temperature is between 38°C to 39°C) is a common system of Salmonella infection. The fever typically lasts between 2 and 7 days. It is important to note that a severe systemic form of Salmonella infection, known as typhoid fever (also known as enteric fever), can occur, typically when you are infected with the Salmonella bacteria species known as Salmonella typhi. However, this infection can also occur if you are infected with other Salmonella bacterial species. Typhoid fever is life-threatening, so early identification of symptoms is crucial.⁴ 

The following are common symptoms of typhoid fever:

• Constipation
• Nausea
• Abdominal pain
• Having a headache
• High fever

Typically, enteric fevers and the symptoms occur before you experience gastrointestinal symptoms.⁴ 

Headache

Headaches are also a common symptom of Salmonella infection, with one study finding that 84.9% of those with Salmonella infection experienced headache as one of the symptoms.⁶

Muscle pain

Salmonella infections can cause myalgia, which is muscle aches and pains, which is another symptom to look out for.⁴

Duration and severity

Typical duration of illness

Typically, people recover from their symptoms of Salmonella infection within 4-7 days without the use of antibiotics. 

Factors influencing severity 

There are different factors that can influence the severity of Salmonella infection. For example, according to the FDA, the following groups have a higher risk of developing Salmonella infection and experiencing more severe symptoms:

• Those who are pregnant
• Elderly individuals
• Those who are immunocompromised
• Children under the age of 5

Risk factors

High-risk groups

Infants and young children

Infants under 1 year old, particularly those who are not breastfed, have a higher risk of becoming infected by Salmonella. Additionally, children under the age of 5 are also at a higher risk. 

Elderly individuals

Adults aged 65 and over are more susceptible to Salmonella infection. 

Immunocompromised individuals

Being immunocompromised and having a weakened immune system as a result, can make you more susceptible to Salmonella infection. This is due to your body having a reduced ability to fight off infection from Salmonella bacteria.

Behavioural and environmental risks

Improper food handling

Inadequately cooking food, such as poultry, can result in Salmonella infection in humans. Additionally, cross-contaminating raw foods, which may carry Salmonella bacteria, with ready-to-cook foods can also result in Salmonella infection. 

International travel

Travelling to developing countries with poor sanitation can increase your risk of contracting Salmonella infection. However, whilst there are frequent reports of Salmonella infection in developing countries, they can occur all over the world. Therefore it is important to remain vigilant wherever you travel.⁷

Contact with certain animals 

Salmonella bacteria are part of the normal gut microbiota (bacteria and microbes that naturally occur and live in the gut) of reptiles. However, these naturally occurring Salmonella bacteria can cause Salmonella infection in reptiles, and be passed onto humans, causing infection.⁸ Salmonella bacteria can also be passed from birds to humans via direct contact with the bird or their faeces.

Prevention and control

Safe food practices

Proper cooking and storage temperatures

To prevent Salmonella growth and infection, ensure that chilled products are stored below 4.4°C and that frozen products are stored below -17.8°C. When foods are taken outside of their ideal temperature range this can create conditions for Salmonella and other bacteria to grow. In order to prevent Salmonella growth it is important to refrigerate any perishable foods promptly. Cooked leftovers should be refrigerated within 2 hours, or 1 hour if the outside temperature is above 32.2°C. 

Avoiding cross-contamination

Different methods can be implemented when you prepare food to prevent cross-contamination. For example, you can use a different chopping board and utensils when preparing the following foods:

• Raw meat- including fish and poultry
• Ready-to-eat foods, for example, cheese
• Fruits and vegetables

If you do not have access to different chopping boards and utensils, in order to minimise cross-contamination you should prepare fruits and vegetables first, and any raw meat last.

In addition, it is important to wash any fruits and vegetables prior to food preparation to wash off any harmful bacteria, including Salmonella. However, you should not wash raw meat, as this will not remove bacteria from raw meat, but can facilitate their spread instead.

Ensure that you are thoroughly washing your hands before and after preparing different foods.

Personal hygiene

Handwashing

Ensure that proper handwashing practices are implemented to prevent the spread of Salmonella. This includes washing your hands in the following instances:

• After you have been in direct contact with animals, including pets
• After you have been to the toilet
• After you have changed a nappy
• After you have helped someone with diarrhoea
• After you have touched a pet's possessions, including their toys and their bed

Safe handling of pets

Be sure to wash your hands properly after touching your pets and their possessions. You should also avoid kissing your pets. 

Summary

• Salmonella infection is caused by Salmonella bacteria, and affects the intestinal tract, causing gastroenteritis
• You can become infected with Salmonella by consuming contaminated water and food
• Salmonella can result in gastrointestinal symptoms, such as diarrhoea, nausea, vomiting, and abdominal cramps
• Salmonella can result in systemic symptoms including headaches, fever, and muscle pain
• High-risk groups of contracting Salmonella include those who are immunocompromised, adults over the age of 65, children under the age of 5, and infants under the age of 1 who are not being breastfed