Causes of Salmonella

The causes of salmonellosis are a complex of etiological and pathogenetic factors that lead to infection of a specific person or bacterial carriers with salmonella infection. The disease affects people of all ages, as well as wild and domestic animals, cattle, poultry. The entrance gate of infection is mainly the mucous membrane of the small intestine. As a result of the pathogen entering the body, an acute infectious process develops in it, which is limited to the gastrointestinal tract or spreads to other tissues and organs.

Causative agent of the disease

For the first time, Salmonella was discovered in 1880, during the posthumous autopsy of a corpse that died from typhoid fever. The microorganism was present in the lymph nodes, spleen and Peyer's plaques. In 1884 and 1885, several varieties of pure bacterial culture were isolated, and by the beginning of the 20th century they were united in a separate genus in the family of enterobacteria. Since the 30s, Salmonella has been divided according to antigenic structure into various subspecies.

Microorganisms of the genus Salmonella - gram-negative motile bacilli. The Salmonella bacterium does not form capsules and spores, and is a facultative anaerobic organism of a conditionally pathogenic type. In fact, Salmonella does not have tropism only to certain tissues and organs. In total, there are about 2300 serovars in the genus, which are divided into 46 serogroups according to the somatic O-antigens, and 2500 serovars by the structure of the H-antigen. Human cases of salmonellosis are due to only 10-12 serovars.

The main two species that most often attack humans are S. enterica and S. bongori. They are divided into 7 subspecies: S. enterica (I), salamae (II), arizonae (III), diarizonae (IIIb), houtenae (IV), indica (V) and bongori (VI). This division is of epidemiological importance for the study and prevention of outbreaks, since the first subspecies of Salmonella lives in warm-blooded animals, and the reservoir for all the others is cold-blooded animals and the environment. Microorganisms grow on typical culture media.

Salmonella has a complex antigenic structure with two types of antigens: a flagellate thermolabile H-antigen, a surface Vi-antigen, and a thermostable somatic O-antigen. For humans, birds and animals, almost all subspecies of Salmonella are pathogenic, however, only some representatives of the genus are considered the most significant in terms of the spread of the epidemic. Almost 92% of all cases of salmonella infection cause:

  • S. typhimurium;
  • S. panama;
  • S. enteritidis;
  • S. newport;
  • S. derby;
  • S. infantis;
  • S. agona;
  • S. london.

At the same time, S. enteritidis and S. typhimurium are the most common pathogen types obtained in bacterial cultures from sick people.

Salmonellosis Contagiousness

Salmonellosis is an infectious disease of a predominantly acute course, which, even in isolated cases of the disease, can cause an epidemic outbreak. The disease is considered contagious, as it develops after the pathogen enters the human body in several ways, including through household contacts. However, the main source of infection, according to the observation of infectious disease doctors, is still insufficiently processed or raw eggs that are eaten.

The minimum concentration of microbial cells needed to infect a person is from 1.5 million to 1.5 billion.

Infectious period

The Salmonella bacterium is considered to be very tenacious, since it is not at all afraid of low temperatures, hibernating in frosts, and can live at temperatures up to 50-60 degrees for a long time. In addition, microorganisms persist for a long time in the environment, water, food. For example, salmonella lives in meat for 6 months, in frozen carcasses - up to a year. In water, the bacterium lasts for 5-6 months, in milk for about 20 days, in kefir for a month, and in butter for 4 months. Salmonella lives on the eggshell for 2 to 3 weeks, and in the soil for 18-20 months. In addition, during prolonged storage of salmonella eggs, which were originally on the shell, that is, the outside of the egg, can penetrate inside.

In a piece of meat, salmonella withstands some boiling time, and in other products it usually dies at a temperature of 70 degrees in 10-20 minutes. Microorganisms live in household dust and sea water for up to 500 days.

Being in products, microorganisms are able to live and multiply without changing the taste and appearance of food. Salting, drying and smoking do not affect salmonella in any way. The bacterium has a high sensitivity to conventional disinfectants, including those based on chlorine.

Both humans and animals, after suffering salmonellosis, continue to secrete a viable pathogen into the environment. The period of infectiousness, or bacteriocarrier, which lasts less than 3 months, is called acute, and if after 3 months or longer in the analyzes there are bacterium titers, we are talking about chronic bacterial carriage. Salmonella is excreted in feces and urine, may be present in the blood.

Sources and routes of transmission of bacteria

There are several routes of transmission of salmonella, but the main mechanism of infection is fecal-oral. You can get infected:

  • contact-household way;
  • water;
  • airborne or airborne dust;
  • through food products.

As with other intestinal infections, transmission of salmonellosis by contact-household means occurs through infected household items, towels, toys, pots, playpens, and mothers hands. The main reason contributing to such an infection is the lack of personal hygiene, that is, the transfer through the dirty hands of the patient with whom he touches the surrounding objects, leaving the pathogen on them. It should be noted that in this case, mass infection usually does not occur, only people with a susceptible organism, small children, people with weak immunity can get sick. The most typical example of contact-domestic salmonellosis infection is nosocomial outbreaks of salmonellosis, which are periodically observed in maternity wards. The fact is that newborn children are particularly sensitive to any microorganism due to weak immunity.

Waterborne infection is most commonly seen in birds and animals, on farms and in livestock farms.

The causative agent enters the water from the feces of infected people and the secretions of sick animals. It should be noted that in the aquatic environment, the reproduction of bacteria does not occur, therefore, its concentration in water is rarely significant. Accordingly, cases of infection through water are not common. In addition, the pathogen can enter the body during bathing in an infected reservoir if a person swallowed a large amount of water with microorganisms during bathing and diving.

Is it possible to get airborne droplets?

On this occasion, competent experts have different points of view. It is believed that the bacterium is not transferred to other carriers directly in the air, as well as in microparticles of human saliva, therefore it is not possible to get an intestinal infection in this way. The possibility of an air-dusty path of Salmonella distribution under urban conditions with the participation of wild birds that pollute habitats and feeding with their droppings has been proved.

Is sexually transmitted infection possible?

Salmonellosis is an intestinal infection that cannot be transmitted through sexual contact through semen or vaginal secretions.

Infectious Products

The main way salmonella enters the body is through the use of contaminated food. It is important to understand that if the primary reservoir and source of infection is raw or improperly processed products of animal origin, then when they get into the refrigerator with vegetables, fruits and other food products that are stored in open containers, the pathogen from infected eggs or meat can migrate to any nearby food product. The fastest propagation and accumulation of salmonella occurs in protein foods, including meat products.

The bacterium can be found in pork, beef, chicken, turkey, veal, and most often ducks suffer from salmonellosis. Any food prepared from such meat, subject to insufficient heat treatment, becomes a source of infection.

The occurrence of salmonella can be associated with the use of dairy products, eggs and dishes from them, less often the bacterium enters the body with fish products.

In vegetables and fruits, the bacterium almost does not multiply, so they can become a cause of the disease only for people with weakened immunity. Heat treatment significantly reduces the amount of salmonella in food, so it is believed that the use of warm and hot dishes is safe for humans. The longer the infected food is stored unprocessed, the greater the concentration of the pathogen in it, and the likelihood of developing salmonellosis by eating it also increases.

Eggs as a source of salmonella

It is eggs that are considered the main source of salmonellosis among people, and the most dangerous of all are duck and goose. Quail and chicken eggs are not so often the cause of infection, but they cannot be completely excluded as a factor in the etiology of the disease.

It should be noted that the eggs themselves indirectly receive their dose of pathogenic microorganisms, and the bird that carries them is the direct carrier of salmonella. Initially, the infection can only be located outside the egg, on the shell. If the egg has been laid recently, and the shell is whole and without cracks, then contact with the egg itself is not a threat to humans, but unwashed hands after infected eggs have visited them can cause the development of an infectious process. Therefore, before cooking, be sure to wash the product with soap and warm water.

If the infection got inside, external treatment will not help, especially if there are cracks in the shell. Such eggs must be processed thermally. Boiled eggs in boiling water for 15 minutes can be considered safe. If an omelet or scrambled eggs is prepared from them, they need to be fried on both sides.

Duck and goose eggs are not popular because of the taste. In any case, they must be cooked for 15-20 minutes.

It is better to wash the eggs immediately before cooking, and not beforehand, before laying in the refrigerator, otherwise, having destroyed their natural protective barrier by washing, you can facilitate the penetration and multiplication of bacteria inside the egg.

Eggs with cracks cannot be laid on an omelet or omelet, as this heat treatment is not enough for such a state, they can only be cooked.

Causes of salmonellosis in children

Sources of infection of children are similar to adult factors of infection - pets, cats and dogs can be dangerous for a child if they are carriers of salmonella. Cattle and pigs are the source of infection either in direct contact with feces, for example, if the child lives in the house with the farm and has access to animals, or already as infected meat that fed the child.

Infection in young patients can occur as a result of communication with a sick person or an ill carrier of bacteria, since the child's body is more susceptible to salmonella.

However, the most common way of infection in children is through food, after eating meat, dairy products or eggs, as well as dishes from them that are insufficiently or improperly processed.

In addition, children can become sick after drinking large amounts of water infected with the pathogen, if it is drinking water, and after getting into their mouth and nose when bathing in water with salmonella.

In infants, the main route of infection is contact-household, after using a pacifier, toys, objects, through care items. The pathogen typically found in infants is a subspecies of S. Typhimurium. This pathogen often causes an epidemic in kindergartens, schools, hospitals, and children's hospitals.

Older children are more susceptible to the subspecies S. enteritidis.

Is transmitted by breastfeeding

Infection of a breast-fed baby is likely due to contact with the mother carrier if she does not wash her hands or breast well before feeding. Breast milk itself is not a source of infection.

Contributing and Developing Factors

The probability of a person becoming infected with salmonellosis depends, first of all, on how the infection got into the body. So, for example, with household contact, the probability of infection is significantly less, while with a salmonella ingestion, the disease will manifest itself with a probability of 99.5%. In addition, age also plays a role - children are more susceptible to bacteria, especially infants. Adults are mostly infected after eating contaminated foods.

The factors that contribute to the development of infection in humans, doctors call a low level of immunity, taking antibiotics that interfere with the normal microbiota of the intestine, the presence of chronic gastrointestinal diseases, improper nutrition and the purchase of products of animal origin and of dubious quality in the markets, from unverified sellers without documents to verify bacteriological products the laboratory.

The author of the article:
Medvedeva Larisa Anatolyevna

Specialty: therapist, nephrologist.

Total experience: 18 years.

Place of work: Novorossiysk, medical center "Nefros".

Education: 1994-2000 Stavropol State Medical Academy.


  1. 2014 - "Therapy" full-time continuing education courses on the basis of the Kuban State Medical University.
  2. 2014 - "Nephrology" full-time continuing education courses on the basis of GBOUVPO "Stavropol State Medical University".
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