Table of Contents
Overview of Salmonella Infections
Salmonella are rod-shaped, Gram-negative bacteria that are among the most common causes of foodborne infections worldwide. In humans, they typically cause gastrointestinal disease (salmonellosis), but certain types can also cause serious systemic infections (enteric fevers).
Two medically important groups are distinguished:
- Non-typhoidal Salmonella (NTS) – e.g. Salmonella enteritidis, S. typhimurium
→ Mainly cause acute gastroenteritis. - Typhoidal Salmonella – Salmonella typhi and S. paratyphi
→ Cause typhoid and paratyphoid fever (systemic infections).
Most human infections are food-associated and transmitted via the fecal–oral route.
Types of Salmonella and Hosts
From a medical perspective, it is useful to distinguish Salmonella by their typical hosts and disease pattern:
- Human-adapted (typhoidal) Salmonella
- S. typhi – causes typhoid fever, humans are the only relevant reservoir.
- S. paratyphi A, B, C – cause paratyphoid fevers, also mainly human pathogens.
- Able to survive and multiply in macrophages and spread via the bloodstream.
- Zoonotic (non-typhoidal) Salmonella
- Many serovars (e.g. S. enteritidis, S. typhimurium) infect a wide range of animals.
- Reservoirs: poultry, pigs, cattle, reptiles, wild birds, pets.
- In humans, typically cause self-limiting intestinal infection; occasionally severe or invasive disease in infants, elderly, or immunocompromised people.
Both groups are facultatively intracellular: they can survive inside some host cells as well as outside in the environment.
Sources of Infection and Transmission
Salmonella are typically transmitted by the fecal–oral route, i.e. from feces to mouth, often via contaminated food, water, or surfaces.
Typical Sources
- Animal products
- Raw or undercooked eggs and foods containing raw eggs (e.g. tiramisu, mayonnaise, homemade ice cream).
- Poultry (chicken, turkey), especially if undercooked.
- Raw or insufficiently cooked meat (pork, beef).
- Unpasteurized milk and dairy products.
- Other food sources
- Contaminated fresh produce (salads, sprouts, herbs, fruits) via contaminated irrigation water or handling.
- Foods contaminated during processing or in the kitchen (e.g. cutting lettuce on a board that was used for raw chicken without cleaning).
- Water
- Contaminated drinking water or ice.
- Water in poorly maintained swimming pools or recreational waters.
- Animals and environment
- Reptiles and amphibians (turtles, snakes, lizards, frogs) often carry Salmonella asymptomatically.
- Pet birds, hedgehogs, rodents, and other pets can also be carriers.
- Contact with animal feces or contaminated bedding/litter.
Routes of Transmission
- Foodborne transmission
- The most common route. Bacteria multiply rapidly at room temperature in nutrient-rich foods.
- Direct contact
- Handling infected animals or their feces.
- Person-to-person spread
- Especially with poor hand hygiene after using the toilet or changing diapers.
- Chronic carriers (particularly in typhoid)
- Some infected individuals continue to excrete Salmonella for months or even years (e.g. from the gallbladder), although they may feel healthy.
- They can unknowingly spread the pathogen, especially if working with food.
Pathogenesis: What Happens in the Body?
Although the clinical picture differs between non-typhoidal and typhoidal Salmonella, some basic steps are similar.
Non-Typhoidal Salmonella (Gastroenteritis)
- Ingestion
- Large numbers of Salmonella are ingested with contaminated food or water.
- The infectious dose can vary; it is often high but may be lower in vulnerable individuals or with fat-rich foods that protect bacteria from stomach acid.
- Survival in the stomach
- Salmonella must pass the acidic environment of the stomach. Reduced stomach acid (e.g. due to medication) increases susceptibility.
- Colonization of the small intestine
- In the distal small intestine, Salmonella adhere to and invade the intestinal mucosa, especially via specialized M cells of the Peyer’s patches.
- Inflammatory response
- They trigger a strong local inflammatory reaction.
- This leads to:
- Damage to the intestinal epithelium.
- Secretion of fluid and electrolytes into the intestinal lumen.
- Increased motility (more frequent bowel movements).
- Clinical manifestation
- The result is acute inflammatory diarrhea, often with abdominal cramps and fever.
- In healthy individuals, the infection is usually confined to the intestine.
- Occasional systemic spread
- In young children, the elderly, or people with weakened immune systems, bacteria may enter the bloodstream (bacteremia) and infect other organs (bones, meninges, joints).
Typhoidal Salmonella (Typhoid and Paratyphoid Fever)
Typhoidal Salmonella have additional properties that enable them to cause systemic disease:
- Invasion beyond the gut
- After crossing the intestinal mucosa, they survive and multiply within phagocytic cells (macrophages).
- Dissemination
- Via the lymphatic system and bloodstream, they reach organs of the reticuloendothelial system (liver, spleen, bone marrow, gallbladder).
- Systemic infection
- The result is a generalized infection with:
- High, often stepwise rising fever.
- Affected intestinal lymphoid tissue, especially Peyer’s patches.
- Complications
- Necrosis of intestinal lymphoid tissue can lead to intestinal bleeding or perforation.
- Bacteria in the gallbladder can lead to a chronic carrier state.
Clinical Manifestations
Non-Typhoidal Salmonellosis (Gastroenteritis)
Typical course:
- Incubation period
- Usually 6–72 hours after ingestion, often 12–36 hours.
- Symptoms
- Sudden onset of:
- Nausea, sometimes vomiting.
- Abdominal cramps.
- Diarrhea (watery to mushy, sometimes mucus or small amounts of blood).
- Fever, chills, malaise.
- Headache, muscle aches.
- Duration
- Symptoms usually last 1–7 days; fatigue may persist longer.
- Severity
- Often self-limiting in otherwise healthy individuals.
- Severe dehydration may occur, particularly in infants and the elderly.
- Complicated infections
- Bacteremia (bacteria in the blood).
- Focal infections (e.g. osteomyelitis, endocarditis, meningitis) especially in patients with immunodeficiency or pre-existing conditions (e.g. sickle cell disease).
Typhoid and Paratyphoid Fever
Typical clinical features (classic untreated course):
- Incubation period
- About 7–21 days (sometimes shorter or longer).
- Early phase
- Gradual increase in fever.
- Headache, malaise, muscle aches.
- Loss of appetite, abdominal discomfort.
- Sometimes constipation initially, later diarrhea.
- Progression
- High continuous fever.
- Dullness, slowed reactions (“typhoid state”).
- Rose-colored skin spots on the trunk (rose spots), often few and transient.
- Enlarged spleen and liver.
- Possible complications
- Intestinal bleeding, intestinal perforation.
- Sepsis and multi-organ involvement.
- Without treatment, typhoid can be life-threatening.
Diagnosis
Diagnosis of Salmonella infections uses both clinical assessment and microbiological tests.
For Non-Typhoidal Salmonella
- Clinical suspicion
- Acute gastroenteritis following ingestion of potentially risky food or contact with known sources.
- Stool culture
- Detection and identification of Salmonella from stool samples is the standard.
- Serotyping and, if needed, molecular typing help trace outbreaks and determine epidemiological links.
- Blood cultures
- Indicated if systemic infection is suspected (high fever, signs of sepsis).
- Other specimens
- From normally sterile sites in focal infections (e.g. blood, cerebrospinal fluid, joint fluid).
For Typhoid and Paratyphoid
- Blood cultures
- Particularly important early in the disease, as typhoidal Salmonella are often present in the bloodstream.
- Stool and urine cultures
- Become positive in later phases or in chronic carriers.
- Bone marrow culture
- Potentially more sensitive than blood cultures in some cases.
- Serological tests
- Antibody-based tests exist but have limited specificity and are interpreted with caution; they are supplementary, not definitive.
Treatment
Management depends on the type and severity of Salmonella infection and the patient’s risk factors.
Non-Typhoidal Salmonellosis
- Supportive therapy is central
- Replacement of fluid and electrolytes (oral rehydration solutions, in severe cases intravenous fluids).
- Light diet as tolerated.
- Use of antibiotics
- In uncomplicated gastroenteritis in healthy individuals, antibiotics are often not recommended because:
- They usually do not shorten the disease duration.
- They may prolong excretion of the pathogen.
- They promote antibiotic resistance.
- Indications for antibiotics:
- Severe disease (high fever, systemic symptoms).
- Very young children, elderly, or immunocompromised patients.
- Invasive infections (bacteremia, focal organ infections).
- Choice of antibiotics
- Depends on local resistance patterns; commonly used are e.g. fluoroquinolones, third-generation cephalosporins, or azithromycin in selected cases.
- Antibiotic resistance, including multi-resistant strains, is increasingly problematic and requires careful selection.
Typhoid and Paratyphoid Fever
- Antibiotic therapy is mandatory
- Early and adequate antibiotic treatment greatly reduces complications and mortality.
- Common options (depending on resistance): fluoroquinolones, third-generation cephalosporins, azithromycin.
- Treatment duration is typically longer than for non-typhoidal infections.
- Supportive measures
- Fluid and electrolyte management.
- Monitoring for complications (e.g. intestinal bleeding or perforation).
- Chronic carriers
- May require prolonged antibiotic therapy; in some cases, removal of the gallbladder is considered if it serves as a persistent reservoir.
Prevention and Control
Preventing Salmonella infections focuses on breaking the fecal–oral transmission chain, especially in food production and handling, and on vaccination against typhoid.
Food Hygiene
- Proper cooking
- Heat foods thoroughly:
- Poultry and minced meat should be cooked until the core temperature reaches at least about 70 °C for several minutes.
- Avoid consuming raw or undercooked eggs and poultry, especially in risk groups.
- Kitchen hygiene
- Separate raw and cooked foods.
- Use different cutting boards and knives for raw meat and ready-to-eat food.
- Wash hands, utensils, and surfaces with hot water and detergent after contact with raw meat or eggs.
- Refrigeration
- Keep perishable foods refrigerated (≤ 4–7 °C).
- Do not leave prepared dishes at room temperature for long periods (Salmonella multiply quickly between about 10–47 °C).
- Safe water and raw foods
- Use safe drinking water; in areas with questionable water quality, boil or use bottled water.
- Wash fresh fruits and vegetables thoroughly; in high-risk settings, peeling or cooking may be advisable.
Personal and Environmental Hygiene
- Hand hygiene
- Careful handwashing with soap after using the toilet, changing diapers, handling raw meat, or cleaning animal cages.
- Handling of animals
- Awareness that reptiles and some other pets can carry Salmonella.
- Wash hands after contact; avoid close contact of such animals with infants or immunocompromised persons.
- Isolation and exclusion from food handling
- People with acute salmonellosis or who excrete large amounts of Salmonella should temporarily not work in food preparation or in institutions with vulnerable individuals (e.g. hospitals, daycare centers), according to local regulations.
Vaccination
- Typhoid vaccination
- Available as:
- Injectable polysaccharide vaccine.
- Oral live attenuated vaccine (in some regions).
- Recommended, for example:
- For travelers to areas with high typhoid prevalence and poor sanitation.
- For certain occupational or outbreak-related exposures.
- Vaccination reduces risk and severity but does not guarantee complete protection; hygiene measures remain important.
- No routine vaccine is currently used in humans against non-typhoidal Salmonella, although research is ongoing.
Control in Food Production
- Surveillance and control programs
- Monitoring of Salmonella in poultry flocks, pigs, and other livestock.
- Testing of eggs and meat products.
- Hygienic measures in slaughterhouses and food processing plants.
- Consumer education
- Public health campaigns on safe food handling and risks of raw animal products.
Public Health Aspects
Salmonella are important notifiable pathogens in many countries:
- Outbreak detection
- Increases in cases with identical or related Salmonella strains can indicate common sources (e.g. a particular food batch, restaurant, or farm).
- Rapid identification allows targeted recalls and other control measures.
- Antibiotic resistance monitoring
- Surveillance helps to track the emergence and spread of resistant Salmonella strains.
- Information feeds into treatment guidelines and policies on antibiotic use in human and veterinary medicine.
- Global relevance
- Non-typhoidal Salmonella are among the leading causes of bacterial foodborne illness worldwide.
- Typhoid fever remains a major problem in areas with poor sanitation and limited access to clean water and healthcare.
Understanding Salmonella as pathogens makes clear how closely human health, food production, hygiene, and antibiotic use are interconnected, and why coordinated measures at individual, medical, and societal levels are necessary to control these infections.