Salmonella is a leading cause of foodborne illness worldwide, responsible for a significant portion of hospitalizations and deaths related to food poisoning. The bacterium Salmonella belongs to the Enterobacteriaceae family and encompasses more than 2,500 serotypes, but only a handful are responsible for the majority of human infections. Salmonella infections, or salmonellosis, generally occur when contaminated food or water is consumed. Two of the most common forms of salmonella that affect humans are Salmonella enterica serovar Typhimurium and Salmonella enterica serovar Enteritidis. Both strains are responsible for the bulk of human illnesses, though other serotypes also contribute to cases of food poisoning. Understanding the most common strains of Salmonella helps in both the prevention and management of salmonellosis outbreaks.
1. Salmonella Enteritidis
Salmonella enterica serovar Enteritidis (often simply referred to as Salmonella Enteritidis) is one of the most prevalent serotypes of Salmonella that cause human illness. In many countries, particularly the United States and Europe, it is responsible for a significant portion of salmonella infections each year.
Transmission:
Salmonella Enteritidis is commonly linked to poultry and eggs. The bacterium can infect the ovaries of hens, leading to the contamination of eggs before the shell forms. As a result, consuming raw or undercooked eggs is a major risk factor. Additionally, raw or undercooked poultry and cross-contaminated food products (such as vegetables and fruits that come into contact with contaminated meat or surfaces) are also sources of infection. Contaminated water can spread the bacterium to crops or other animals, extending the range of possible contamination.
Symptoms:
The symptoms of Salmonella Enteritidis infection typically include diarrhea, fever, abdominal cramps, and vomiting. These symptoms can develop between 6 hours and 6 days after exposure to the bacteria and typically last for about 4 to 7 days. In most cases, the infection is self-limiting, meaning it resolves without the need for antibiotics, but severe cases can occur, particularly in young children, the elderly, and immunocompromised individuals.
Outbreaks:
There have been numerous outbreaks of Salmonella Enteritidis over the years, often linked to contaminated eggs, raw chicken, and products that use raw eggs (like cookie dough or homemade ice cream). Contaminated eggs have been a persistent source of outbreaks, leading to extensive public health campaigns advocating for the thorough cooking of eggs and safe food-handling practices.
2. Salmonella Typhimurium
Salmonella enterica serovar Typhimurium (commonly referred to as Salmonella Typhimurium) is another widespread strain responsible for human illness. Alongside Salmonella Enteritidis, it accounts for a significant portion of salmonella infections worldwide.
Transmission:
Similar to Salmonella Enteritidis, Salmonella Typhimurium is often associated with contaminated food products, particularly raw or undercooked meat (especially pork and poultry), eggs, and dairy products. Cross-contamination of foods prepared on the same surfaces as contaminated meat is a significant risk factor. Additionally, infected animals, particularly reptiles like turtles and snakes, can carry Salmonella Typhimurium and transmit it to humans, particularly when proper hygiene practices are not followed after handling such animals.
Symptoms:
The symptoms of Salmonella Typhimurium infection are largely the same as those caused by other strains of Salmonella. Patients often experience diarrhea, fever, stomach cramps, nausea, and vomiting. These symptoms typically arise within a few days of exposure and last about a week. Severe cases can lead to dehydration or the spread of the bacteria to the bloodstream (bacteremia), which may require antibiotic treatment.
Outbreaks:
Salmonella Typhimurium has been involved in numerous outbreaks, often linked to contaminated meat, dairy products, and sometimes fresh produce. A notable outbreak occurred in the United States in 2008 and 2009, involving peanut butter contaminated with Salmonella Typhimurium, which resulted in over 700 illnesses and 9 deaths.
3. Salmonella Heidelberg
Transmission:
Salmonella enterica serovar Heidelberg is another common strain that frequently causes human illness, though it is less prevalent than Salmonella Enteritidis or Typhimurium. It is particularly known for its resistance to multiple antibiotics, which makes treating infections more challenging.
Like other Salmonella strains, Salmonella Heidelberg is commonly associated with poultry, including raw or undercooked chicken, eggs, and other animal products. Cross-contamination during food preparation can also spread the bacteria to other foods, like fruits and vegetables.
Symptoms:
Infection with Salmonella Heidelberg produces typical salmonellosis symptoms such as diarrhea, fever, and abdominal cramps. The illness can be more severe in vulnerable populations, and the emergence of antibiotic-resistant strains increases the risk of complications and prolonged illness.
Outbreaks:
Salmonella Heidelberg has been linked to outbreaks in the poultry industry. For instance, in 2013, an outbreak in the U.S. was associated with contaminated chicken from a specific processing plant, resulting in more than 600 cases of salmonellosis. The strain involved in this outbreak was resistant to multiple antibiotics, which complicated treatment and increased the severity of the outbreak.
4. Salmonella Newport
Salmonella enterica serovar Newport is another significant strain responsible for foodborne illnesses. While it is not as common as Enteritidis or Typhimurium, Salmonella Newport has been linked to serious outbreaks in both the U.S. and Europe.
Transmission:
Salmonella Newport is often associated with contaminated beef, raw milk, and produce such as lettuce or cucumbers. Contaminated irrigation water used in agricultural fields has been identified as a major source of Newport outbreaks in produce.
Symptoms:
Infection with Salmonella Newport causes the same general symptoms of salmonellosis, including diarrhea, stomach cramps, fever, and nausea. Symptoms usually appear within 6 to 72 hours after exposure and last for several days. In some cases, complications arise, especially if the bacteria spread beyond the intestines.
Outbreaks:
A well-known outbreak linked to Salmonella Newport occurred in 2020, when contaminated onions from a single supplier were distributed across North America, leading to thousands of reported cases. The outbreak underscored the challenges of tracking food contamination in a globalized food supply chain.
Conclusion
While Salmonella Enteritidis and Salmonella Typhimurium are the most common strains causing human illness, other serotypes such as Salmonella Heidelberg and Salmonella Newport also pose significant health risks. These strains are frequently associated with contaminated animal products, particularly poultry, eggs, and beef, as well as cross-contaminated fruits and vegetables. Public health efforts focus on improving food safety practices, such as proper cooking, hygiene, and the prevention of cross-contamination, to reduce the incidence of salmonellosis. Additionally, ongoing surveillance and rapid response to outbreaks help prevent widespread illness, but continued attention to food safety remains crucial in minimizing the risks posed by these dangerous pathogens.
