Salmonellosis, commonly known as a form of food poisoning, is a bacterial infection of the digestive tract caused by the organism Salmonella enterica. This bacteria is frequently transmitted to humans through contaminated food or water, leading to symptoms like diarrhea, fever, and abdominal cramps. The possibility of contracting Salmonella more than once is complex because the organism exists in a vast array of forms. Understanding the different types of this bacteria and how the body responds to them explains why recurrence is possible.
The Different Types of Salmonella
The Salmonella species is highly diverse, containing over 2,600 distinct types, referred to as serovars or serotypes. These serovars are broadly categorized into two major groups based on the diseases they cause in humans.
The first group is Non-Typhoidal Salmonella (NTS), responsible for the vast majority of foodborne illnesses and usually causing self-limiting gastroenteritis. Common NTS serotypes include S. Enteritidis and S. Typhimurium, which are generalist pathogens infecting a variety of hosts.
The second group consists of Typhoidal Salmonella, specifically S. Typhi and S. Paratyphi, which are largely restricted to human hosts. These typhoidal strains cause a severe, systemic illness known as enteric fever, or typhoid fever. The difference in their clinical presentation and host range influences the body’s immune response to each type.
Immunity After an Initial Infection
The likelihood of contracting Salmonella twice depends heavily on which serotype caused the first illness. After an infection with Non-Typhoidal Salmonella, the body mounts an immune response that is often short-lived and highly specific to the infecting serotype. If a person is exposed to the exact same serotype again, they may have temporary protection lasting months to a few years.
Because there are thousands of NTS serotypes, immunity generated by one strain (e.g., S. Typhimurium) offers little protection against a different strain (e.g., S. Enteritidis). Acquiring a second infection with a completely different serotype of NTS is therefore possible, as the immune system encounters a new bacterial presentation it has not been trained to fight.
In contrast, infection with S. Typhi generates a more robust, long-lasting immunity against that specific typhoidal serotype. The systemic nature of typhoid fever stimulates a strong cellular and humoral immune response, providing greater protection against subsequent exposure to the same strain.
When Sickness Returns: Relapse Versus Reinfection
When symptoms of salmonellosis return after recovery, the cause is classified as either a reinfection or a relapse, which are distinct events. Reinfection occurs when a person is exposed to and infected by a new Salmonella strain from an external source, often a different serotype than the first. This scenario is common with NTS, largely because serotype-specific immunity fails to protect against the new bacteria.
Relapse is the return of symptoms from the original infection, often occurring within weeks of discontinuing treatment because the bacteria were never fully cleared from the body, surviving in protected niches. For typhoidal strains, this persistence can lead to a chronic carrier state, where the bacteria, primarily S. Typhi, hide in the gallbladder or biliary tract.
In chronic carriage, the bacteria are shed intermittently in the stool, and the individual remains largely asymptomatic while posing a risk of transmission. A relapse is not a new infection but a reactivation of the initial one, often due to the organism re-emerging from sequestered sites. While NTS rarely leads to this carrier state, it is observed in immunocompromised individuals.