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What is Salmonella Typhi O and H & Which Diseases are Caused by Salmonella Typhi?

Salmonella typhi is a gram negative bacterium which is responsible for causing diseases such as typhoid fever in humans (also known as enteric fever). These types of bacteria are rod shaped and flagellated. Humans are sole reservoirs for Salmonella typhi bacterium. Salmonella is responsible for death in most of the developing countries mainly due to poor hygiene and sanitation, and also due to ingestion of contaminated water and undercooked food. It is difficult to completely eradicate the strains of Salmonella due to emergence of multi drug resistance strains; because of which the newer strains are resistant to older antibiotics. The first strain of Salmonella typhi was isolated from a patient in Vietnam.

What is Salmonella typhi O and H?

What is Salmonella typhi O and H?

Patients affected by typhoid fever produce antibodies against the antigens of Salmonella bacteria. These antigens which are found on the cell surface of Salmonella are known as O and H antigens. The O antigen (also known as somatic antigen) is found on the cell wall of the bacteria, whereas the H antigen is produced in the flagellated portion. The flagella found on the surface of these bacteria helps in movement of the organism. The presence of these antigens and production of antibodies by the affected individual, due to the response of their immune system forms the basis of a diagnostic test called as Widal test.

Widal test is a specific laboratory test, used for diagnosis of typhoid fever. It was developed by Georges Ferdinand Widal in the year 1896. It is a serological test, where the titre of antibodies produced in the patients serum (in response to O and H antigens found on the surface of the Salmonella bacteria) is measured by a process called as haemagglutiation. The presence of O and H antigen are clinically very significant as the antibodies produced in response to O antigen (or IgM) is noted in the early stages of the diseases and it reaches a peak level by the second week of the disease. Antibodies produced in response to H antigen (or flagellar antigen) are noted during later stage of the disease and it reaches a peak during the third week of the disease. Antibodies produced in response to H antigens are generally present for a couple of months whereas the antibodies produced in response to O antigen generally reduce after 2 weeks of the disease. This helps us in understanding the severity and extent of the condition and thus helps us in rightful treatment planning.

Although, Salmonella strains can be isolated from blood samples, stool samples and urine samples, the most reliable diagnostic test is the Widal test, which works on the principle of production of antibodies against antigen O and H of Salmonella bacterium. Widal test is considered to be positive if the O antigen titre is more than the ratio 1:160 (in an active infection); or if the titre of H antigen was more than 1:160 in past infection or in immunized persons.

Which Diseases are Caused by Salmonella Typhi?

Salmonella typhi causes a serious condition called as Typhoid or enteric fever. It is a contagious disease and it is usually caused by eating or drinking food or water contaminated by Salmonella bacteria. These bacteria enter the human blood stream and begin to multiply. The condition is associated with symptoms such as increase in body temperature (often up to 104 degree Fahrenheit) along with generalised weakness, fatigue, loss of appetite, lack of energy, and/or diarrhoea. Typhoid or enteric fever usually last for about 3 to 4 weeks with an incubation period of 1 – 2 weeks. Even after complete recovery, there may be reoccurrence in future or the affected individual may act as a carrier for a considerable period of time. The treatment depends on the strain of bacteria that affects the person. Appropriate antibiotics along with supportive care generally help the patient. Adequate hydration and adequate rest is recommended for the patient for a speedy recovery. Vaccines are available, which are recommended to be taken in case of travel plans to areas prone to typhoid fever. Besides being in contact with contaminated water or food, occupations such as clinical microbiologist are also at a risk of developing typhoid and thus, should take necessary preventive measures.


Salmonella is a rod shaped flagellated bacteria which cause typhoid or enteric fever in humans. They are gram negative in nature and can survive for weeks in contaminated water or even in dried sewage areas. In the past, Salmonella infection was considered as highly fatal; however, at present the number of deaths caused by salmonella has reduced a lot due to advancement in medicine and also due to improvement in sanitation and hygiene. The diagnosis of salmonella infection is done by a serological testing called as Widal test. It works on the principle of production of antibodies against the O and H antigens found on the cell surface of Salmonella bacteria. The treatment plan includes antibiotic therapy along with other supportive care.


  1. Centers for Disease Control and Prevention (CDC) – Typhoid Fever: https://www.cdc.gov/typhoid-fever/index.html
  2. Mayo Clinic – Typhoid Fever: https://www.mayoclinic.org/diseases-conditions/typhoid-fever/symptoms-causes/syc-20378661
  3. National Health Service (NHS) – Typhoid Fever: https://www.nhs.uk/conditions/typhoid-fever/
  4. Widal Test – A Useful Laboratory Tool for the Diagnosis of Typhoid Fever: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3675882/

Also Read:

Pramod Kerkar, M.D., FFARCSI, DA
Pramod Kerkar, M.D., FFARCSI, DA
Written, Edited or Reviewed By: Pramod Kerkar, M.D., FFARCSI, DA Pain Assist Inc. This article does not provide medical advice. See disclaimer
Last Modified On:August 3, 2023

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