Key Reservoirs and Transmission of Salmonella

Salmonella circulates through an unusually wide network of animal, environmental, and food-chain reservoirs, which is a big part of why it remains one of the most common causes of foodborne illness worldwide. Poultry and eggs sit at the center of that network, but cattle, pigs, reptiles, contaminated soil, irrigation water, and even household pets each play distinct roles in keeping the bacterium in circulation. Understanding where Salmonella hides and how it moves from one host or environment to the next helps explain why eliminating it from the food supply has proven so difficult.

Poultry and Eggs

Chickens are the single most important reservoir for the Salmonella strains that make people sick. The bacterium colonizes the chicken gut, particularly the ceca, often without causing any visible illness in the bird. A colonized hen can pass Salmonella to her eggs in two ways: the bacteria can contaminate the eggshell from the outside after it passes through the cloaca, or the reproductive organs themselves can be infected, depositing Salmonella directly inside the egg before the shell even forms.

1Oxford University Press (OUP). Mechanisms of egg contamination by Salmonella enteritidis

This internal contamination route is the more worrying one, because no amount of washing the shell will help. In experimentally infected laying hens housed indoors, roughly 3.4% of eggs tested positive for Salmonella Enteritidis inside the egg contents.

2PubMed Central. Research Note: Contamination of eggs by Salmonella Enteritidis and Salmonella Typhimurium in experimentally infected laying hens in indoor cage-free housing

A similar study in Bangladesh found about 5% of eggs from infected flocks carried Salmonella Enteritidis, and contact hens that had never been directly inoculated also produced contaminated eggs, confirming horizontal spread within a flock.

3PubMed Central. Assessment of foodborne transmission of Salmonella enteritidis in hens and eggs in Bangladesh

Reducing Salmonella carriage in poultry is an active area of research. Feed additives like coated butyric acid have been shown to lower fecal shedding, which reduces how much Salmonella a flock spreads to its environment, though the bacteria may still linger in the ceca at slaughter age.

4PubMed. Supplementation of coated butyric acid in the feed reduces colonization and shedding of Salmonella in poultry

Zinc-amino acid feed complexes have also shown promise in lowering both fecal shedding and cecal colonization in broilers.

5Journal of Applied Poultry Research. Reduction of cecal colonization and fecal shedding of Salmonella Typhimurium in broilers fed proprietary zinc- or manganese-amino acid complexes

Vaccination combined with competitive exclusion, where beneficial bacteria are introduced to chicks early in life to crowd out Salmonella, produces stronger protection than either approach alone, particularly in older birds where immune effects compound the exclusion effect.

6PubMed. Combination of vaccination and competitive exclusion to prevent Salmonella colonization in chickens: experimental studies

Spraying newly hatched broilers with an attenuated live Salmonella strain has even caused fecal shedding of a challenge strain to cease entirely during the experimental period.

7PubMed. Administration of a Salmonella Enteritidis ΔhilAssrAfliG strain by coarse spray to newly hatched broilers reduces colonization and shedding of a Salmonella Enteritidis challenge strain

Cattle, Pigs, and Slaughter Stress

Cattle carry Salmonella in their lymph nodes, and this matters because those lymph nodes can end up in ground beef. The peripheral lymph nodes embedded in the fat and lean trimmings used for grinding are not removed during processing. One large survey found an overall Salmonella prevalence of about 1.6% in bovine lymph nodes, with cull cattle showing higher rates than grain-finished animals.

8PubMed. Prevalence and characterization of Salmonella in bovine lymph nodes potentially destined for use in ground beef

That might sound low, but Salmonella hiding inside lymph nodes bypasses the surface-decontamination steps that processing plants rely on. A more detailed characterization found that during warmer months in certain U.S. regions, prevalence in peripheral lymph nodes of finished feedlot cattle reached about 31%, with some nodes containing Salmonella at concentrations high enough to make dilution in a ground-beef batch insufficient to eliminate risk.

9PubMed Central. Salmonella in Peripheral Lymph Nodes of Healthy Cattle at Slaughter

Pigs present a different challenge. Porcine Salmonella falls into two broad groups: a host-adapted serovar, Choleraesuis, which causes severe bloodstream infections in young pigs, and a wide collection of broad-host-range serovars like Typhimurium that cause brief intestinal illness and are the ones most likely to reach humans.

10PubMed Central. Salmonella enterica: Survival, Colonization, and Virulence Differences among Serovars

Stress during transport to slaughter measurably increases Salmonella shedding in finishing pigs. Fecal loads rise after transport, and pigs that are transported and not given feed withdrawal show more Salmonella in their gut contents at slaughter than pigs that stayed on the farm.

11PubMed Central. Transport to the Slaughterhouse Affects the Salmonella Shedding and Modifies the Fecal Microbiota of Finishing Pigs12PubMed. Effect of transportation and feed withdrawal on shedding of Salmonella typhimurium among experimentally infected pigs

The practical upshot is that a pig carrying Salmonella asymptomatically on the farm may suddenly shed far more bacteria right when it enters the processing environment, contaminating carcasses and equipment at the worst possible moment.

How Salmonella Colonizes the Gut

Once swallowed, Salmonella moves quickly. In mice, researchers found that within three hours of an oral dose, Salmonella had established footholds in the Peyer’s patches, the small immune-surveillance structures lining the small intestine.

13PubMed Central. Intestinal colonization and virulence of Salmonella in mice

Large numbers also appear in the cecum, though initially those bacteria are floating free in the gut contents rather than firmly attached to the intestinal wall. The Peyer’s patches turn out to be remarkably poor at killing even small numbers of Salmonella in the first several days, though by about a week they develop the ability to clear less virulent strains.

In the intestinal lining itself, Salmonella invades epithelial cells and replicates inside membrane-bound compartments. The host fights back by detecting bacterial components and triggering the infected cells to eject themselves into the gut lumen, a process that slows bacterial growth but does not stop it. In a small fraction of cells, the bacteria escape from those compartments into the cell’s interior, where they multiply rapidly and re-arm themselves for a fresh round of invasion.

14PubMed Central. Mechanisms of Salmonella pathogenesis in animal models

When the molecular machinery that detects and ejects infected cells is knocked out experimentally, bacterial loads in the gut wall increase a hundredfold to a thousandfold, underscoring how important this defense is.

Broad Versus Narrow Host Range

Not all Salmonella serovars are equally comfortable in every host. Some, like Enteritidis and Typhimurium, are “generalists” that infect a wide range of mammals and birds and are responsible for the vast majority of human food-poisoning cases. Others are restricted to a single host species. Salmonella Typhi, for example, causes typhoid fever only in humans; Salmonella Gallinarum infects chickens but is essentially harmless to mammals.

Mouse experiments illustrate the differences starkly. A broad-host-range serovar like Enteritidis causes severe gut inflammation and spreads systemically. The host-restricted Gallinarum, by contrast, produces no intestinal inflammation at all.

15PubMed Central. Virulence of Broad- and Narrow-Host-Range Salmonella enterica Serovars in the Streptomycin-Pretreated Mouse Model

Genomic analyses show that host adaptation typically involves a trade-off: the pathogen picks up new genes that help it evade a specific host’s immune system while losing genes it no longer needs. Salmonella Typhi, for instance, acquired a large cluster of genes that help it hide from human innate immune defenses, a key step in its evolution toward causing chronic typhoid carriage.

16PubMed Central. Evolution of Salmonella within Hosts17PubMed Central. Patterns of genome evolution that have accompanied host adaptation in Salmonella

Environmental Persistence

Salmonella does not need a living host to survive for long stretches. In soil amended with contaminated compost, Salmonella Typhimurium persisted for up to 231 days. On parsley grown in that soil, the bacteria survived equally long; on lettuce, up to 63 days. Whether the original contamination came from manure-based compost or from contaminated irrigation water made little difference to how long the bacteria lasted.

18PubMed. Persistence of Salmonella enterica serovar typhimurium on lettuce and parsley and in soils on which they were grown in fields treated with contaminated manure composts or irrigation water

Adding poultry litter to soil extended Salmonella survival by roughly 89 days compared to unamended soil, consistent with the idea that organic matter gives the bacteria nutrients and a hospitable microenvironment.

19PubMed Central. Strain, Soil-Type, Irrigation Regimen, and Poultry Litter Influence Salmonella Survival and Die-off in Agricultural Soils

On dry surfaces, Salmonella forms biofilms that are physiologically distinct from those grown in wet conditions. The cells become encased in large capsules and compress together. Although over 60% of cells in these dry-surface biofilms are metabolically dormant, the remaining fraction retains virulence capabilities, posing a persistent risk in low-moisture food manufacturing environments.

20PubMed Central. Salmonella dry surface biofilm: morphology, single-cell landscape, and sanitization

Mixed-species biofilms can actually boost Salmonella’s numbers. When grown alongside common environmental bacteria like Staphylococcus or Pseudomonas on feed-industry surfaces, Salmonella biofilm volume increased roughly threefold compared to pure Salmonella biofilms.

21PubMed Central. Micro ecosystems from feed industry surfaces: a survival and biofilm study of Salmonella versus host resident flora strains

Produce as a Vehicle

Fresh vegetables are not just passive carriers of surface contamination. Salmonella can actually internalize inside leafy greens, slipping through the stomata, the tiny pores on a leaf’s surface, and settling into the tissue beneath. Microscopy has confirmed fluorescently labeled Salmonella underneath stomata and in the intercellular spaces of iceberg lettuce, reaching depths of over 50 micrometers.

22PubMed Central. Salmonella enterica Growth Conditions Influence Lettuce Leaf Internalization23PubMed Central. Internalization of Salmonella in Leafy Vegetables during Postharvest Conditions

The degree of internalization varies a lot by plant species. Iceberg lettuce, basil, and certain other herbs are more susceptible, while other leafy greens show much lower rates.

24PubMed. Salmonella Typhimurium internalization is variable in leafy vegetables and fresh herbs

This matters because once bacteria are inside the leaf tissue, commercial washing and sanitizing steps cannot reach them. It helps explain why leafy greens are so frequently implicated in Salmonella outbreaks and why the contamination events are hard to trace and harder to prevent once the produce is already grown in contaminated soil or irrigated with tainted water.

Reptiles and Exotic Pets

Reptiles naturally carry Salmonella as part of their gut flora. They typically shed it in their feces without showing any symptoms, and the bacteria readily spread to the animal’s skin, mouth, and terrarium surfaces.

25PubMed Central. Pet Reptiles: A Potential Source of Transmission of Multidrug-Resistant Salmonella

Reptiles harbor a wide variety of Salmonella serotypes that can infect humans, and the risk is not limited to handling the animal itself. Simply cleaning a terrarium, touching cage furnishings, or allowing a reptile to roam in a kitchen can introduce the bacteria into a household.

26PubMed. Risk for zoonotic Salmonella transmission from pet reptiles: A survey on knowledge, attitudes and practices of reptile-owners related to reptile husbandry

This is one reason public health agencies consistently advise against keeping reptiles in homes with young children or immunocompromised individuals. Eliminating Salmonella from a pet reptile is essentially impossible. Some experimental work has explored phage therapy to reduce bacterial loads on reptile skin, but the approach is still in early stages.

27PubMed Central. Salmonella in reptiles: a review of occurrence, interactions, shedding and risk factors for human infections

Wild Birds and the Question of Spillover

The role of wild birds in Salmonella transmission is real but easy to overstate. On Ohio dairy farms where the resident cows shed Salmonella at extremely high rates (71 to 94% prevalence), the wild birds living nearby carried the bacterium at a prevalence of just 1.16%, suggesting that birds are not efficiently picking it up from contaminated farm environments.

28The Ohio State University. Wild Birds as Vectors for Salmonella on Ohio Dairies

A California study of wild birds near agricultural land reached a similar conclusion: most species rarely carry foodborne pathogens, though large aggregating flocks can still represent contamination risk through sheer volume of fecal output.

29PubMed Central. Carriage and Subtypes of Foodborne Pathogens Identified in Wild Birds Residing near Agricultural Lands in California: a Repeated Cross-Sectional Study

There is an interesting twist, though. On an outdoor pig farm in the UK, after pigs left the fields, the proportion of Salmonella-positive wild bird droppings climbed from about 7% to over 44% across successive sampling visits. The bacteria in those droppings were the same pig-associated strain, and some droppings contained Salmonella at unusually high concentrations, indicating active replication rather than passive carriage. The pigs were the original source, but the birds became amplifiers, potentially re-seeding the environment before new animals arrived.

30PubMed. Role of wild birds and environmental contamination in the epidemiology of Salmonella infection in an outdoor pig farm

Insects as Mechanical Vectors

House flies and litter beetles do not get much attention in popular discussions of Salmonella, but they are efficient shuttles between contaminated environments and clean ones. On poultry farms, flies collected both inside and outside broiler houses have tested positive for Salmonella serogroups matching those found in chicken feces.

31Journal of Applied Poultry Research. Incidence of bacterial pathogens in flying insects collected near poultry farms

Flies pick up Salmonella from manure, feed, and waste, and can transmit it to other flies and even to their offspring.

32PubMed Central. House Flies (Musca domestica) from Swine and Poultry Farms Carrying Antimicrobial Resistant Enterobacteriaceae and Salmonella

Lesser mealworms, the small beetles that infest the litter of poultry houses, present a dual threat. They carry Salmonella both externally on their bodies and internally in their guts, and chicks eat them. Whether the beetle is a true biological reservoir or simply a mechanical carrier is still unclear, but either way it introduces Salmonella directly into the bird’s digestive tract.

33PubMed Central. Detection of Antimicrobial Resistant Salmonella enterica Strains in Larval and Adult Forms of Lesser Mealworm (Alphitobius diaperinus) From Industrial Poultry Farms

Animal Feed as a Quiet Entry Point

A scoping review covering hundreds of studies found that Salmonella contamination in animal feed has been documented at every stage from ingredient sourcing to on-farm delivery. The most commonly sampled and frequently contaminated ingredient categories were animal by-products and grains and oilseeds, which together accounted for the largest share of studies. Contamination was documented on farms, in feed mills, during transportation, and even at the retail level.

34PubMed Central. Salmonella in Animal Feeds: A Scoping Review

Feed contamination is a particularly insidious pathway because it introduces Salmonella into flocks and herds before any other control measure has a chance to act. Heat-treated feed can be recontaminated during cooling, storage, or transport, which is why the problem has persisted despite decades of awareness.

Raw Pet Food and Companion Animals

Dogs fed commercially available raw meat diets shed Salmonella in their feces at rates that track closely with how contaminated the food itself is. In one study, seven dogs began shedding Salmonella within one to seven days of eating contaminated raw food, with five shedding the exact same serotypes found in the food. None showed any signs of illness.

35PubMed Central. The risk of salmonellae shedding by dogs fed Salmonella-contaminated commercial raw food diets

Raw feeding has been identified as a major risk factor for Salmonella shedding in dogs, and the frequency of shedding appears equal to or greater than the frequency of contamination in the food itself, suggesting that the bacteria multiply in the dog’s gut rather than just passing through.

36PubMed Central. Raw diets for dogs and cats: a review, with particular reference to microbiological hazards

Surveillance data from Great Britain confirmed that Salmonella strains isolated from raw pet food and from dogs overlap, though the same strains also circulate in the wider food chain, making definitive source attribution tricky.

37PubMed Central. Salmonella surveillance in raw pet food and dogs in Great Britain, 2013–2022

The practical concern is not the dog getting sick. It is the asymptomatic shedding that contaminates floors, food bowls, backyard soil, and the hands of anyone cleaning up after the pet.

Kitchen Cross-Contamination

Even when Salmonella arrives in a kitchen on a piece of raw chicken, the way people handle it determines whether anyone gets sick. A direct observational study found that 100% of participants engaged in practices that led to cross-contamination of hands, utensils, countertops, and even their own cell phones while preparing raw poultry.

38Food Protection Trends. Direct Observational Study of the Risk of Cross-contamination during Raw Poultry Handling: Practices in Private Homes

Washing raw chicken, a widespread kitchen habit, creates additional splashing risk that can spread pathogens to nearby surfaces.

39PubMed Central. Chickensplash! Exploring the health concerns of washing raw chicken

Surprisingly, an observational study found that even people who did not wash their chicken still contaminated their salad lettuce at comparable rates: about 31% of non-washers in the control group produced contaminated salads, compared with 26% of washers. The sink and hands turned out to be larger contamination vectors than splashing alone.

40PubMed. Observational Study of the Impact of a Food Safety Intervention on Consumer Poultry Washing

The lesson is that the real danger is not the act of rinsing but the chain of hand-to-surface-to-food contact that happens around raw poultry, regardless of washing habits.

Chronic Human Carriers and the Gallbladder

In areas where typhoid is common, an estimated 1 to 4% of people infected with Salmonella Typhi become chronic asymptomatic carriers, shedding viable bacteria for months or years.

41PubMed Central. Biofilm Producing Salmonella Typhi: Chronic Colonization and Development of Gallbladder Cancer

The gallbladder is the main hiding place. Salmonella reaches it either through the bloodstream during systemic infection or by ascending directly from the small intestine through the bile ducts.

42PubMed Central. Salmonella chronic carriage: epidemiology, diagnosis and gallbladder persistence

Gallstones make the situation worse. In a study conducted in typhoid-endemic Mexico City, about 5% of patients undergoing gallstone removal carried Salmonella Typhi, and the bacteria had formed visible biofilms on the gallstone surfaces. Mouse experiments confirmed that cholesterol gallstones support biofilm formation and lead to both heavier bacterial colonization of gallbladder tissue and increased fecal shedding.

43PubMed Central. Gallstones play a significant role in Salmonella spp. gallbladder colonization and carriage

This is why cholecystectomy, the surgical removal of the gallbladder, has historically been part of the treatment for chronic typhoid carriers.

Seasonality and Temperature

Salmonella infections follow a clear seasonal pattern. In the United States, seasonality explains roughly 64% of the variability in Salmonella case counts, the strongest seasonal signal among the major foodborne pathogens tracked nationally.

44PubMed Central. Seasonal synchronization of foodborne outbreaks in the United States, 1996–2017

Cases rise in warmer months, and the relationship between temperature and infection rates is strong. Modeling in three southern U.S. states showed a tight positive correlation between high temperatures and Salmonella infections.

45PubMed Central. Effects of climate change on Salmonella infections

An Australian study added nuance by looking at specific Salmonella subtypes. Daily case counts rose by about 1.3% per degree Celsius during warm-season months, with some subtypes showing increases of 3 to 4% per degree. Extreme heat days, those exceeding 39°C (about 102°F), were associated with spikes in certain Typhimurium phage types.

46Epidemiology & Infection. The effect of temperature on different Salmonella serotypes during warm seasons in a Mediterranean climate city, Adelaide, Australia

The mechanisms are probably a mix of faster bacterial growth on improperly stored food, more outdoor cooking and picnics, and higher Salmonella shedding by farm animals during heat stress. The seasonal link also means that climate change is expected to increase the global Salmonella burden, particularly in regions already experiencing long, hot summers.

Aquaculture and Farmed Shrimp

Aquaculture ponds are another reservoir that does not get enough attention. A study of shrimp farms in northeastern Brazil identified Salmonella in about 16% of isolates from pond water and sediment, across multiple serovars.

47PubMed Central. Antibiotic Resistance of Salmonella spp. Isolated from Shrimp Farming Freshwater Environment in Northeast Region of Brazil

A significant relationship has been demonstrated between fecal indicator bacteria in grow-out pond water and the presence of Salmonella in harvested shrimp, confirming that bacteria in the pond environment become attached to or internalized within the shellfish.

48PubMed Central. Salmonella risks due to consumption of aquaculture-produced shrimp

Runoff from nearby livestock operations, use of animal manure as pond fertilizer, and contaminated water sources all introduce Salmonella into aquaculture systems. The antibiotics routinely used in some shrimp-farming regions add another layer of concern, because they select for drug-resistant Salmonella strains that are harder to treat when they do reach a human host.