Diseases

Prevention and Control of Typhoid

Theoretically typhoid is an eradicable (as smallpox) disease as Salmonella that cause enteric fever survive only in human hosts and are spread by contaminated food and water. But high prevalence of typhoid in developing countries and lack of adequate sewage disposal and water treatment make the goal of eradication of typhoid impracticable at present. So travelers from developed countries to developing country should monitor their food and water intake carefully and also get vaccinated for typhoid.

Typhoid vaccines:

At present 2 types of typhoid vaccines are commercially available: (1) Ty21a, an oral live attenuated vaccine of Salmonella Typhi (given on days 1, 3, 5, and 7, with a booster every 5 years) and (2) Vi CPS, a parenteral vaccine (given intramuscularly) consisting of purified Vi polysaccharide from the bacterial capsule (given in 1 dose, with a booster every 2 years). The older vaccine of whole cell typhoid/paratyphoid A and B vaccine (given intramuscularly) is no longer licensed due to its side effects. The minimum age for vaccination is 6 years for oral type or Ty21a and 2 years for Vi CPS. At present there is no vaccine licensed for paratyphoid. An acetone-killed whole-cell vaccine is available which is only used by the U.S. military.

Vi CPS typhoid vaccine is poorly immunogenic in children of less than 5 years of age because of T cell–independent properties. More than three new live vaccines are in clinical development and experts hope these will prove to be more efficacious and longer-lasting than previous live vaccines.

Ty21a, whole cell vaccines, and Vi CPS are all equally effective for the first year, the 3-year cumulative efficacy of the whole-cell vaccine (73%) exceeds that of both Ty21a (51%) and Vi CPS (55%). The heat killed whole cell vaccine has efficacy for 5 years, but Ty21a and Vi CPS has efficacy for 4 and 2 years, respectively. But the whole cell vaccine also has more side effects than the other two.

Data on typhoid vaccines in travelers are limited and some evidence suggests that they may be less efficacious in travelers than those for local populations in endemic areas. WHO and CDC recommend typhoid vaccination for travelers to typhoid-endemic countries like in Indian subcontinent. An analysis in the CDC found that 16% of travel associated cases occurred among persons who stayed at their travel destination for 2 weeks or more. So vaccination should be done even if the duration of stay is short in typhoid endemic areas.

Typhoid can be prevented by providing good quality drinking water to the population and sewerage disposal in a hygienic way. But these two things are difficult to achieve in developing countries and these countries harbor the disease in the community.

Diagnosis of Typhoid Fever

The clinical features of typhoid fever are nonspecific. So diagnosis of typhoid fever should be considered if a traveler returning from developing country with fever, especially from Indian subcontinent, and Latin America or in developing countries any patient with fever. Other diagnosis to be considered in case of fever and international travel are malaria, hepatitis, bacterial enteritis, dengue fever, rickettsial infections, leptospirosis, amebic liver abscesses, and acute HIV infection etc.

A positive blood culture is the only confirmatory test for diagnosis of typhoid fever and other laboratory tests are not diagnostic. There may be non specific laboratory changes like leukopenia and neutropenia in 15% to 25% of the cases of typhoid fever and also moderately elevated liver function tests and muscle enzyme levels.

The definitive diagnosis of enteric fever is done by isolation of Salmonella Typhi or Salmonella Paratyphi from blood, bone marrow, other sterile sites, rose spots, stool, or intestinal secretions. The positive blood culture is 90% during the first week of infection and decreases to 50% by the third week. A low yield of positive result in infected patients is related to low numbers of salmonellae (less than 15 organisms/ml) and/or to recent antibiotic treatment. Bone marrow culture yield is 90% despite approximately 5 days of antibiotic therapy. Even if bone marrow culture is negative, culture of intestinal secretions (best obtained by a noninvasive duodenal string test) can give a positive result for typhoid fever. If bone marrow and intestinal secretions are cultured together, it can give more than 90% positive result anytime during Salmonella infection.

There are other serologic tests also like classic Widal test which is for “febrile agglutinins”, but none of these tests are sufficiently sensitive or specific to replace culture methods especially in developed countries. Some new tests are being developed like PCR (Polymerase chain reaction) and DNA probe assays to detect S. Typhi in blood.

Typhoid Fever: Clinical Features

Typhoid Fever: an Understanding