Etiology and Epidemiology of Salmonellosis

Salmonellosis is an acute zoonotic intestinal infection, characterized by the defeat of the digestive system with the development of the syndrome of intoxication and water-electrolyte disorders, more rarely – typhoid-like or septicopyemic course.

Brief Historical Information

The first representatives of the genus were discovered by K. Ebert (1880) in Peyer’s plaques, spleen and lymph nodes of a man who died of typhoid fever; G. Gaffki (1884) singled out the pure culture of the causative agent of the disease. Later D.E. Salmon and JT Smith (1885) during the outbreak of swine fever and A. Gertner (1888) from the beef and spleen of the deceased person isolated similar bacteria. At the beginning of the 20th century, a separate genus was formed for the pathogens in the Enterobacteriaceae family, which, in honor of Salmon, was named Salmonella. Salmonella is a large group of bacteria, the systematics of which has undergone significant changes as they improve knowledge of their antigenic structure and biochemical properties. In the early thirties, F. Kauffmann and P. White suggested dividing salmonella according to their antigenic structure; currently it is used to differentiate salmonella.


Salmonellosis pathogens are gram-negative mobile rods of the genus Salmonella of the Enterobacteriaceae family, which unite more than 2300 serovars, divided into a set of somatic O-antigens into 46 serogroups. By the structure of the H-antigen, about 2500 serovars are isolated. Despite the abundance of detectable serological variants of salmonella, the bulk of diseases of salmonella and cases of carriage in humans is due to a relatively small number of serovars (10-12). The last classification of salmonella (1992) distinguishes two species: S. enterica and S. bongori, in turn subdivided into 7 sub-species, designated by numbers or proper names – S. enterica (I), salamae (II), arizonae (III ), diarizonae (IIIb), houtenae (IV), indica (V), and bongori (VI). The main pathogens of salmonella are included in the I and II subgenera. Division into subspecies has a certain epidemiological significance, since the natural reservoir of Salmonella I subspecies is warm-blooded animals, and for representatives of other subspecies – cold-blooded animals and the environment. Bacteria grow on normal nutrient media, have a complex antigenic structure: they contain somatic thermostable O-antigen and flagellate thermolabile H-antigen. Many representatives reveal a superficial Vi-antigen. Some serotypes are amenable to phage typing. Most salmonella are pathogenic both for humans, for animals and birds, but only a few of them are the most important for the human epidemiologically. S. typhimurium, S. enteritidis, S. panama, S. infantis, S. newport, S. agona, S. derby, S. london and some others cause 85-91% of cases of salmonella. At the same time, the first two accounts for 75% of all isolates currently allocated from sick people. Salmonella persists for a long time in the environment: in water up to – 5 months, in meat – about 6 months (in carcasses of birds over a year), in milk – up to 20 days, kefir – up to 1 month, in butter – up to 4 months, in cheeses – up to 1 year, in egg powder – from 3 to 9 months, on the eggshell – from 17 to 24 days, in beer – up to 2 months, in the soil – up to 18 months. It has been experimentally established that with long storage (over a month) of chicken eggs in the refrigerator, S. enterica can penetrate into the eggs through intact shells and multiply in the yolk. At 70 ° C they die within 5-10 minutes, in the thickness of a piece of meat they can withstand boiling for some time, while in the process of cooking eggs they retain their vitality in protein and yolk for 4 minutes. In some products (milk, meat products) salmonella can not only be preserved, but also multiply, without changing the appearance and taste of foods. Pickling and smoking have a very weak effect on them, and freezing even increases the survival time of microorganisms in food. So-called resident (hospital) strains of salmonella are known, which are characterized by multiple resistance to antibiotics and disinfectants.

Epidemiology of Salmonellosis

Reservoir and sources of infection – many species of agricultural and wild animals and birds; they can have the disease in the form of pronounced forms, as well as asymptomatic carriage. The most important source of infection for salmonellosis is cattle, as well as pigs, whose infection can reach 50%. Carrier animals are most dangerous to humans. In healthy animals, salmonella does not cause disease, with the weakening of the organism, salmonella penetrates from the intestine into tissues and organs, resulting in septic diseases. Infection of a person occurs when caring for animals, forced to slaughter them in meat-packing plants and the use of lifetime or posthumously infected meat, as well as milk and dairy products. The carrier of salmonella was noted in cats and dogs (10%), as well as among the synanthropic rodents (up to 40%). Salmonellosis is widespread among wild birds (pigeons, starlings, sparrows, seagulls, etc.). In this case, birds can pollute the litter and thereby contaminate the external environment and food products. In the last 30 years, in most countries of the world, there has been a sharp increase in the number of positive Salmonella finds in agricultural birds and, primarily, in chickens.

A person can be the source of some species of salmonella (S. typhimurium and S. haifa), especially in hospital settings. The greatest danger to a person (a patient or a carrier) is for children of the first year of life, who are particularly susceptible to salmonella. The duration of the infectious period in patients determines the duration and nature of the disease; it can last in animals for months, and for a person – from several days to 3 weeks. Convalescence in humans can sometimes last up to a year.

The mechanism of transmission is fecal-oral, the main mode of transmission is food, mainly through products of animal origin. The most significant are meat dishes, prepared from minced meat, and meat salads; less important are fish and vegetable products. The waterway of transmission plays a role in the infection of animals in livestock complexes and poultry farms. The contact-household way of transfer (via contaminated household items, towels, toys, pots, changing tables, arenas, hands of medical personnel and mothers) plays the largest role in hospitals, especially in maternity hospitals, pediatric and geriatric departments. Transmission factors may include medical instruments, equipment (catheters, endoscopes, etc.) in violation of their sterilization regime.

The possibility of an air-dust path of salmonella distribution in urban conditions with the participation of wild birds polluting their litter with habitat and feeding is proved.

The natural susceptibility of people is high, depends on the set of many known and unknown factors that determine the outcome of a person’s encounter with the pathogen: the dose of the causative agent, its antigenic structure, the characteristics of biological properties; individual characteristics of a person, his immune status, etc. The most sensitive to salmonella are children in the first months of life (especially premature ones), people of advanced age and persons with an unfavorable premorbid background. Postinfectious immunity persists for less than a year.

The main epidemiological signs. Salmonella is considered to be ubiquitous (ubiquitous) infections; in recent years, there has been a trend towards a further increase in the incidence rate. Unlike most intestinal infections, salmonellosis is most widespread in large, well-maintained cities, in countries characterized by a high level of economic development, which allows them to be classified as a “disease of civilization” group. The increase in the spread of salmonella on a global scale is associated with a number of reasons, the main of which are the intensification of livestock production on an industrial basis, the centralization of food production and the changed ways of their realization, in particular, the increase in the production of semi-finished products. An important role is played by the processes of urbanization, the activation of migration processes, the expansion of exports and imports of food and feed, intensive pollution of the environment, etc. Salmonella are recorded as epidemic outbreaks and sporadic cases; quite often even with a thorough epidemiological investigation, outbreaks remain undeciphered. Their detection is difficult in large cities due to the highly developed infrastructure, intensive intra-urban migration of the population, a wide network of shops and public catering establishments. The manifestation of the epidemic process in salmonellosis is largely determined by the serovar of the causative agent that caused it. In recent years, there has been a significant increase in the incidence of bacteria associated with the spread of bacteria (S. enterica) through poultry meat and eggs, as well as products made from them. When bacteria are introduced into large poultry farms, they quickly infect part of the livestock due to the ability to transovarial transmission. Among the sick people, adults predominate (60-70%), although the highest rate of incidence is found among young children. High sensitivity of young children to small doses of the pathogen predetermines the possibility of transferring it among them not only food, but also the so-called household way. Outbreaks are, as a rule, explosive. Morbidity of salmonella increases in the warm season.

A peculiar feature of salmonellosis was their anthroponous character. The most common nosocomial distribution of salmonella is associated with the contact-household transmission of antibiotic-resistant strains of S. typhimurium or S. haifa. Hospital strains of bacteria are distinguished by multiple resistance to antibiotics and disinfectants. Foci (outbreaks) of intrahospital salmonella arise mainly in children’s hospitals (somatic and infectious hospitals, departments for premature babies, newborns, etc.). Outbreaks often have a high mortality rate among young children and can last for a long time.