Salmonella sps (S. typhimurium and S. paratyphi): Notes on Typhoid fever, the diagnosis, treatment, and prevention of Typhoid

           Notes on Salmonella - Enteric or Typhoid fever, the diagnosis, treatment, and prevention                 of  Typhoid 

      Salmonella:

• Main types of Salmonella related to human and animal infections are Salmonella serotype Typhi and Salmonella serotypes Paratyphi A, B, or C typhi that cause typhoid fever and paratyphoid fever respectively.
• Salmonella are found as a gut flora/normal flora in many vertebrate species. 
• The bacteria are often associated with contaminated food, particularly undercooked or raw eggs, poultry, meat, and unpasteurized dairy products. 
• Salmonellosis is a common gastrointestinal infection caused by the bacteria, symptoms include diarrhea, abdominal cramps, fever, and sometimes vomiting.
• People at any age are susceptible to the infection, children and elderly age people are at higher risk.
• Typhoid fever or enteric fever is a more severe and potentially life-threatening illness caused by the bacterium Salmonella typhi. The common symptoms typhoid infection include high-grade and prolonged fever, diarrhea (gastroenteritis /food poisoning), in severe condition septicemia may occur leading to the death when treatment is not available.
• The infection is more prevalent in the subtropical and tropical parts of the world, common in countries with inadequate water treatment and sanitation facilities.

Morphology:
- Gram-negative bacilli (rods), motile, facultative anaerobic bacteria
- Non-spore-forming
- S. typhi is capsulated and only some strains of S. paratyphi are capsulated

Cultural characteristics of the bacteria:
- Gram-negative bacilli, aerobic and facultative anaerobic, most of the species produce hydrogen sulfite gas (H2S)
- Form large translucent, colorless colonies on Mac Conkey agar and deoxycholate citrate agar
- Black colonies appear on selective media Wilson and Blair Bismuth Sulphite medium
- Brilliant green agar is used for the isolation of Salmonella spp
- Selenite F broth is an enrichment medium for the growth of Salmonella spp
- The optimal temperature for the growth is 37 degrees Celcius

Biochemical properties:
- S. typhi is anaerogenic  (that do not produce gas/air)
- Catalase positive

- Oxidase negative
- Non-lactose fermenter (NLF), ferment glucose, mannitol, and maltose with acid
- Produces hydrogen sulphide (H2S), S. paratyphi A does not produce H2S gas
- MR positive VP negative, Indole negative, citrate positive, urease negative

Virulence Factors:
- Salmonella typhi and S. paratyphi possess flagellar antigen (H), somatic antigen (O) and capsular antigen (Vi)
- S. typhi consists of AB enzymatic toxin known as typhoid toxin/endotoxin 

Pathogenesis:
 The bacteria reach the gut through ingestion of contaminated food or water, the incubation period is two to three weeks
- Attachment to the microvilli and penetration to the ileal mucosa of the intestine
- Multiplication of the bacteria via the thoracic duct and invades the bloodstream causing bacteremia during the first week of the infection
- Infections occur in the liver, gall bladder, kidney during the second week
- Enter the gall bladder and involves the Peyer's patches of the ileum
- Ulceration of the ileum also called “typhoid ulcers” may occur in serious and chronic conditions
- During the infection period a person develops high-grade fever, nausea, vomiting, abdominal pain, some may experience diarrhea. Some patients may develop rose spots around the abdomen

Epidemiology:
- Endemic infections occur in countries with poor sanitation and hygiene, 

- Fewer cases are observed in the developed countries
- Typhoid fever is more prevalent than paratyphoid fever
- S. paratyphoid B and C are rare
- S. paratyphii A is more prevalent in Asian countries, South America, and Eastern Europe 

- The bacteria infects all age groups but is mostly seen in the 5- 20 years age group 
- The patient may shed the bacteria in the feces from three weeks to three months after the cure, these people are called the convalescent carriers and are the source of the infection
- Typhoid fever can occur as an endemic or epidemic type

Symptoms (Typhoid fever and paratyphoid):

• High-grade fever (102 to 104 degrees Celsius) and body aches are very common. Chills, stomach pain and abdominal discomfort, diarrhea.
• Loss of appetite are other symptoms that usually appear within a week after exposure to the bacteria.
• Muscle weakness, headache, sweating, fatigue can last up to weeks after the onset of the disease
• Skin rashes, a rose-colored rash can sometimes appear.

Laboratory Diagnosis:

Specimens:

i) Blood culture

• Blood specimen is the standard and most commonly used specimen for the diagnosis of the infection.
• 5- 10 ml of blood is collected during the first to second weeks of infection, inoculated in bile broth, and incubated overnight. 
• Mac Conkey agar is the most commonly used media for the subculture method, in the identification and differentiation of enteric gram-negative bacteria. 
• The optimal incubation period is 37-degree Celcius for 24 hours. 
• Salmonella typhi and S. paratyphi are non lactose fermenters, round colorless colonies appear on the Mac Conkey agar usually after 24 hours of the incubation period. Bacteria usually show non-hemolytic properties when cultured on blood agar.

ii) Stool culture

• Specimens are collected during the first to second weeks of the infection, directly plated on Mac Conkey, XLD (xylose lysine deoxycholate) selective for Salmonella. Salmonella-Shigella agar (SS) selective for Salmonella and Shigella
• Incubation at 37 degree Celsius for 24 hours 
• Colonies appear usually after 24 hours of incubation

iii) Urine culture

• Urine sample is centrifuged and the deposit is plated in the Wilson-Blair media, inoculated in selenite and tetrathionate broth.
• Biochemical tests are performed for the proper identification of the enteric bacilli, catalase, and oxidase tests are done.

Microscopy:
Detected by gram staining method, S. typhi and S. paratyphi appear as gram-negative bacilli /rods 

Widal test (Serological test)

- Usually positive after the first week of infection 

- Detection of the antibodies against O and H antigens 

- Confirmed by the presence of O and H agglutinins for typhoid and H for paratyphoid (A and B) bacilli in the patient’s serum 

Molecular diagnosis:
- Nucleic acid-based test including commercially available PCR test

Treatment:

Drugs/ Medications
- Chloramphenicol was effective for typhoid fever until it became resistant during the 1970s
- Ampicillin, Amoxycillin, Cotrimoxazole. Fluoroquinolones and Cephalosporins are effective and in use.

Typhoid fever vaccine:

 Typhoid vaccines are recommended while traveling to the different parts of the world where the infection is highly prevalent.

Two types of vaccines are available for the prevention of typhoid infection. 

i) Typhoid injections (inactivated typhoid vaccine) for 2 years and older people. A vaccine dose is recommended every 2 years

ii) Oral vaccine capsules ( live typhoid vaccine) for 6 years and older people. A vaccine dose is recommended every 5 years

Prevention:
Avoid consuming contaminated food and water, particularly in regions with poor sanitation. Drink bottled or boiled water and consume thoroughly cooked foods.

Wash your hands frequently, especially before eating or preparing food, and after using the restroom.
Vaccines are recommended for travelers to regions where the disease is endemic and for individuals at high risk of exposure. Tab vaccine which was first introduced during the first World war

Divalent typhoid- paratyphoid A vaccine or the monovalent typhoid vaccine (preferred in Asia)