Salmonella: Infection, Transmission & Diseases

Salmonella hides in food such as raw meat or eggs, in animal feces or even in public toilets. Often, a little hygienic carelessness is enough to get infected with the bacteria – the result is usually a classic food poisoning. But some types of salmonella are far more insidious; an infection can trigger the most serious illnesses such as typhoid or paratyphoid fever.

What is salmonella?

Salmonella is a genus of various bacteria that are members of the Enterobacteriaceae family and can cause disease in humans and animals. The bacterial genus Salmonella contains more than 2,500 variations, of which more than 500 are harmful to humans and can cause a condition known as salmonellosis. This contagious intestinal disease usually occurs after eating food contaminated with salmonella; the infection causes diarrhea, vomiting and nausea, but can also be fatal in individual cases. In addition to the infection confined to the gastrointestinal tract, salmonellae can also trigger other serious diseases such as typhoid fever and paratyphoid fever. However, not only humans but also animals can be infected by the pathogens. Although infection usually occurs via food or via human-to-human contact, transmission from animal to human cannot be ruled out. Therefore, the salmonella infection belongs to the so-called zoonoses, those diseases that can be transmitted between animals and humans. Because of the ease of transmission, there is a worldwide reporting requirement for all diseases caused by salmonella; this is especially true for outbreaks of typhoid and paratyphoid fever.

Occurrence, distribution, and characteristics

Salmonella are rod-shaped bacteria, which are distributed worldwide. They are about 0.7 to 1.5 µm in diameter and usually 2 to 5 µm in length. Salmonella belong to the group of gram-negative bacteria, they are actively motile and have an oxidative energy metabolism. Closely related to the genus Escherichia, Salmonella belong to the family Enterobacteriaceae. They are found in humans and animals, but also outside living organisms. Depending on the species, Salmonella populate a wide variety of habitats: For example, Salmonella enterica ssp. Arizonae is found mainly in mammals, in cold-blooded animals, and in poultry, while Salmonella Enteritidis is found mainly in the intestines of cattle, ducks, or rodents and can cause acute gastroenteritis in humans if infected. Salmonella Choleraesuis, on the other hand, is mainly found in the intestinal tract of pigs and is considered to cause salmonellosis in pigs. Humans can become infected with it through the consumption of contaminated meat. The species Salmonella Typhi occurs predominantly in subtropical climatic zones and is considered a typhoid pathogen. Transmission can occur through contaminated food and contact with infected persons, and water and fly droppings are also routes of transmission. In addition, up to 5 percent of all infected persons usually remain permanent carriers. Salmonella Typhimurium is mainly found in birds and mammals and leads to an infection of the gastrointestinal tract, which is usually fatal. In humans, the pathogen triggers the so-called salmonella enteritis, better known as food poisoning. Salmonella is one of the treacherous groups of pathogens, primarily due to its long outdoor survivability. Outside an animal or human body, the bacteria are viable for several weeks; in dried feces, they can even be detected for a period of 2.5 years. UV radiation, on the other hand, accelerates their death, as do temperatures above 75 degrees Celsius. Freezing does not kill the pathogens, but only inhibits their multiplication. In an acidic environment, on the other hand, they die relatively quickly; most disinfectants can kill salmonella within a few minutes.

Diseases and symptoms

Salmonella are pathogens that colonize the digestive tract of humans and can cause a wide variety of symptoms in the ill person. There are many ways of transmission: Contaminated food is probably the greatest risk of infection; the bacteria can multiply rapidly, especially on unrefrigerated food. Raw meat, raw sausages, raw eggs and egg products that have not been fully cooked, as well as raw cake or cookie dough are therefore frequently contaminated.However, purely plant-based foods can also be infected by salmonella. However, infection can also be caused by poor kitchen hygiene, for example when contaminated knives or cutting boards are used. Another important source of infection is from person to person. This can occur as part of a smear infection, as well as in the case of poor hygiene. In this case, the bacteria can be passed on from infected persons via minute stool residues through hand contact and then enter the mouth. Also possible, although rare, is infection from animal to human. Although this is unusual in domestic animals, reptile owners are at risk: snakes as well as turtles or bearded dragons not infrequently excrete salmonella – here the owner can easily become infected via the animal’s feces. A salmonella infection usually starts with sudden diarrhea, abdominal pain and vomiting, headache and mild fever are also common symptoms. In most cases, the symptoms disappear after a few days, but in some cases the disease can be severe, leading to blood poisoning and death. However, even if the ill person is already visibly on the road to recovery – the excretion of the pathogen can last between a month and half a year, depending on age; during this time, the affected person is still infectious.