The genus Salmonella was named after Daniel Elmer Salmon, an American veterinary pathologist. Salmon, along with Theobald Smith, discovered the organism that causes hog cholera, Salmonella enterica var. Choleraesius
-Discovery of the Typhoid Bacillus
At the beginning of the 19th century, typhoid was defined on the basis of clinical signs and symptoms and pathological (anatomical) changes. However, at this time, all sorts of enteric fevers were characterized as "typhoid".
In 1880s, the typhoid bacillus was first observed by Eberth in spleen sections and mesenteric lymph nodes from a patient who died from typhoid. Robert Koch confirmed a related finding by Gaffky and succeeded in cultivating the bacterium in 1881. But due to the lack of differential characters, separation of the typhoid bacillus from other enteric bacteria was uncertain.
In 1896, it was demonstrated that the serum from an animal immunized with the typhoid bacillus agglutinated (clumped) the typhoid bacterial cells, and it was shown that the serum of patients afflicted with typhoid likewise agglutinated the typhoid bacillus. Serodiagnosis of typhoid was thus made possible by 1896.
Salmonella species cause enterocolitis' enteric fevers' such as typhoid fever and septicemia with metastatic infections such as osteomyelitis important properties: Salmonella are gram negative rods don’t ferment lactose “produce H2S” their antigens (cell wall ' flagellar H' and capsullar vi(verulence) antigen. the O antigens wich are the polysacharides of the cell wall' are used to sub divide the Salmonellae in to group A-1. there are two forms of H antigens' phase 1 and 2. only one of the two H protiens is synthesized at any one time ' depending on wich gene sequence is in the correct alignment for transcription in to mRNA. the Vi antigens are anti phagocytic and important verulence facter for sallmonella typhi' the agent of typhoid fever. the Vi antigen also used for serotyping Salmonella typhi. There are three methods for naming the sallmonella: 1-Ewing divides the genus in to three species: Salmonella typhi, Salmonella cholereesuis, and salmonella enteritidis.in this scheme there is on serotype in each of the first two species and 1500 serotype s in the third . 2-Kaufman and white assign different species names to each serotype; there are roughly 1500 defferent species, usually named for the city in which they were isolated. 3- This approach is based on relatedness determined by DNA hybridization analysis, in this scheme, salmonella typhi is not distinct species but is classified as Salmonella enterica sub species typhi. All three of these naming systems are in current use. Clinically, the salmonella species Two types: - typhoidal (that cause typhoid fever) and non typhoidal (not cause typhoid fever) but cause diarrhea (entero colitis) and metastatic infection s, such as osteomelytis. The typhoidal species are typhi and para typhi. The non typhoidal are many strains of salmonella enteritidis.salmonella choleraesuis is the species most often involved in metastatic infections.
Pathogenesis and Epidemiology:- the three types of Salmonella species have different pathogenic features 1) enterocolitis is characterized by an invasion of the epithelial and sub epithelial tissue of small and large intestines .the strains that don’t invade don’t cause disease . The organisms penetrate both through and between the mucosal cells in to the lamina propria, with resulting inflammation and diarrhoea .Apoly morphonuclear leucocyte response limits the infection to the gut and the adjacent mesenteric lymph nodes; bacteremia is infrequent in enterocolitis. The dose of salmonella required is much higher, at least 100,000 organisms .the gastrectomy or use of anti acids lower the infectious dose significantly.
2) In typhoid and other enteric fevers, infection begins in the small intestine but few gastrointestinal symptoms occur. The organisms enter, multiply in the mononuclear phagocytes of payer’s patches and then spread to the phagocytes of the liver, gallbladder, and spleen. This lead to bacteremia, wich is associated with onset of fever and other symptoms, probably caused by endotoxin. Survival and growth of the organism within phagosomes in phagocytic cells are a striking feature of this disease, as is the predilection for invasion of the gall bladder wich can result in establishment of the carrier state and excretion of the bacteria in the feces for long periods.
Accounts for only about 5-10% of salmonella infections and occurs in one of two setting:-
A patient with an underling chronic disease such as sickle cell anemia or cancer or a child with enterocolitis. The septic course is more indolent than that seen with many other gram negative rods. Bacteremia results in the seeding of many organs, with osteomyelitis, pneumonia, and meningitis as the most common sequelae. Osteomyelitis in achild with sickle cell anemia is an important example of this type of Salmonella infection.
The epidemiology of Salmonella infections is related to the ingestion of food and water contaminated by human and animal wastes. S. typhi transmitted only by humans, but other species have a significant animal as well as human reservoir. Animal source is poultry and eggs, inadequately cooked is additional sources
After an incubation period of 12-48 hours, enterocolitis begins with nausea and vomiting and then progresses to abdominal pain and diarrhea with or without blood. Usually the disease lasts a few days, is self limited, causes non bloody diarrhea, and dose not require medical care except in the very young and very old. Much greter number of salmonella associate with HIV infections, including more sever diarrhea than those who not infected with HIV.
Salmonella typhimurium is most common species of salmonella to cause enterocolitis in the United States. In typhoid fever caused by salmonella typhi, and in enteric fever caused by Salmonella typhi A, B, and C. The onset of illness is slow, with fever and constipation rather than vomiting and diarrhea predominating. After the first week, as the bacteremia becomes sustained, high fever, delirium, tender abdomen, and enlarged spleen occur, Rose spots, rose colored macules on the abdomen, are associate with typhoid fever but rarely.
In enterocolitis, the organism is most easily isolated from a stool sample. How ever, in the enteric fevers, a blood culture is the procedure most likely to reveal the organism during the first 2 weeks of illness. The most commonly used media selective for Salmonella are SS agar, bismuth sulfite agar, Hektoen enteric (HE) medium, brilliant green agar and xylose-lisine-deoxycholate (XLD) agar. All these media contain both selective and differential ingredients and they are commercially available.(13) Salmonella form non lactose fermenting, produce gas and H2S.S .typhi is the major exception (not form gas) but only produce H2S. Urease negative.
Enterocolitis caused by salmonella is usually a self limited that resolve without treatment. Fluid and electrolyte replacement may be required. Antimicrobial agents are indicated only for neonates or person with chronic diseases who at risk for septicemia. Antibiotic sensitivity tests should be done. Enteric fever treat by ciprofloxacin, Ampiclillin and ciprofloxacin use for chronic carrier state. Cholecystectomy may be necessary for chronic carrier state.
-By public health and personal hygiene.
-Proper sewage treatment, a chlorinated water supply, hand washing, pasteurization of milk, and proper cooking eggs and meat.
Three vaccines are available, but they confer limited (50-80%) protection against S. typhi. one contains the Vi capsular polysaccharides of vaccine give fewer side effects than the than the third vaccine which contains killed organism.
Vaccination against Typhoid Fever
Three types of typhoid vaccines are currently available for use in the United States: (1) an oral live-attenuated vaccine; (2) a parenteral heat-phenol-inactivated vaccine; (3) a newly licensed capsular polysaccharide vaccine for parenteral use. A fourth vaccine, an acetone-inactivated parenteral vaccine, is currently available only to the armed forces.
1. Live oral vaccines. Although oral killed vaccines are without efficacy, vaccines using living avirulent bacteria have shown promise. A galactose-epimeraseless mutant of Typhi has given very good results in a field trials. Mutants of Typhimurium that have given a good protection in animals include mutants lacking adenylate-cyclase and AMP receptor protein, and mutants auxotrophic for p-aminobenzoate and adenine.Typhi with the same mutations does not cause adverse reactions and is immunogenic in human.
The Live Oral Typhoid Vaccine should not be given to children younger than 6 years of age. It is given in four doses, 2 days apart, as needed for protection. The last dose should be given at least 1 week before travel to allow the vaccine time to work. A booster dose is needed every 5 years for people who remain at risk.
2. The parenteral heat-phenol-inactivated vaccine has been widely used for many years. In field trials involving a primary series of two doses of heat-phenol- inactivated typhoid vaccine, efficacy over the 2- to 3-year follow-up periods ranged from 51% to 77% . Efficacy for the acetone- inactivated parenteral vaccine, available only to the armed forces, ranges from 75% to 94%.
Since the inactivated vaccines contain the O antigen (endotoxin), local and general reactions occur. Vi antigen extracted following the methodology used for the meningococcal vaccine seems to avoid reactions to endotoxin.
The inactivated Typhoid Vaccine should not be given to children younger than 2 years of age. One dose provides protection. It should be given at least 2 weeks before travel to allow the vaccine time to work. A booster dose is needed every 2 years for people who remain at risk.
3. The newly licensed parenteral vaccine [Vi capsular polysaccharide (ViCPS)] is composed of purified Vi ("virulence") antigen, the capsular polysaccharide elaborated by S.Typhi isolated from blood cultures. In recent studies, one 25-ug injection of purified ViCPS produced seroconversion (i.e., at least a fourfold rise in antibody titers) in 93% of healthy U.S. adults. Two field trials in disease-endemic areas have demonstrated the efficacy of ViCPS in preventing typhoid fever. In one trial in Nepal, in which vaccine recipients were observed for 20 months, one dose of ViCPS among persons 5-44 years of age resulted in 74% fewer cases of typhoid fever. ViCPS has not been tested among children less than 1 year of age.
NOTE: No typhoid vaccine is 100% effective and is not a substitute for being careful about what you eat or drink.
Routine typhoid vaccination is not recommended in the United States, but typhoid vaccine is recommended for travellers to parts of the world where typhoid is common, people in close contact with a typhoid carriers, and laboratory workers who work with Salmonella Typhi bacteria. (13)