Salmonella infections, or salmonelloses, are diseases caused by Salmonella bacteria. These
bacteria are Gram-negative, rod-shaped, flagellated, and can grow in both presence and absence
of oxygen. There are 2 species of Salmonella: S. enterica which further divides into 6 subspecies,
and S. bongori. Salmonella bacteria are differentiated as
serotypes, or serovars, depending on the variations of antigens they have on their surface. Major
antigenic determinants are O antigens of the outer bacterial membrane, and H antigens of
the flagella. Over 2500 serotypes have been identified so far; the majority of them belong
to S. enterica subspecies enterica, which is responsible for 99% of all salmonella infections
in humans. Salmonella bacteria are transmitted through
the oral-fecal route. Infection is typically acquired via consumption of food or water
that has been contaminated with human or animal feces. Common culprits include raw or undercooked
meat, poultry, seafood, eggs, dairy products, and fresh fruits and vegetables.
The bacteria must be ingested in large amounts to survive the acidic environment of the stomach.
Once in the intestine, they induce their own phagocytosis, but without the lytic step,
to enter the cells of the intestinal mucosa. This invasion induces inflammatory response,
causing diarrhea, and may lead to mucosal ulceration and destruction. Some bacteria
can enter the bloodstream, resulting in bacteremia. From there, they may also spread to various
organs, producing focal infections. Salmonella bacteria are responsible for two
distinct syndromes: - gastroenteritis - a typically uncomplicated
condition known as stomach flu - caused by non-typhoidal serotypes;
- and potentially life-threatening enteric fevers, such as typhoid fever, caused by typhoidal
serotypes. Non-typhoidal salmonellosis, NTS, is usually
self-limiting, meaning symptoms, such as diarrhea, vomiting, nausea, headache, abdominal cramps
and muscle ache, typically resolve without antibiotics. However, serious complications
may occur in immunocompromised patients, very young children or the elderly. Multidrug-resistant
strains are a common cause of bacteremia in children in developing countries. NTS has
a short incubation period, from 6 hours to 2 days, and symptoms usually last for 10 days
or less. NTS can be transferred from animals to humans, or humans to humans, with animals
being the main reservoir of the bacteria. NTS is a worldwide disease of humans and animals
and the most common form of salmonella infection. Enteric fevers include typhoid fever and the
similar but somewhat less severe paratyphoid fever. These conditions are characterized
by a gradually increasing fever that rises from low-grade to high-grade over the course
of several days, and can persist for a month or more. Other symptoms include headache,
abdominal pain, diarrhea or constipation, enlarged liver and spleen, and occasionally
a rash of rose-colored spots on the trunk. The incubation period is 1 to 3 weeks. Typhoid
fever has high mortality rates, especially when left untreated.
Typhoidal serotypes can only be transmitted among humans. Asymptomatic chronic human carriers,
colloquially known as “typhoid Marys” may spread the disease. A chronic carrier
is someone who sheds bacteria in stools for more than a year after the acute infection,
while having no obvious signs of disease. About 4% of patients with enteric fever become
chronic carriers. Chronic carriers are rare in non-typhoidal infections, because the primary
reservoir for these infections is animals, and not humans. Typhoid fever is uncommon
in developed countries, but still a serious health threat in the developing world, especially
for children. Salmonellosis should be considered in any
acute illness with diarrhea or fever without obvious cause. Diagnosis is confirmed by isolating
and serotyping the bacteria from stool or blood samples. Because antibiotic resistance
is common, susceptibility testing is essential. There are vaccines for typhoid fever, but
not for paratyphoid fever or non-typhoidal salmonellosis. Prevention is by safe food
and water practices and frequent handwashing. Enteric fevers must be treated with antibiotics.