Molecular Diagnosis of Salmonella Enterica
Mohammad Mahmoud Mohsen;
Abstract
Summary
T
yphoid fever is becoming an ever increasing threat in the developing countries. Typhoid fever is estimated to cause 21 million illnesses and 200,000 deaths annually worldwide and Ingestion of contaminated drinking water and food is the most common route of disease transmission.
In Egypt, current estimates of typhoid fever incidence are derived from case reports received from passive hospital-based surveillance, without laboratory confirmation of disease. In the year 2000, the estimated incidence of typhoid fever in Egypt was 15 cases per 100,000 persons per year (Egyptian national syndrome-based surveillance, unpublished data).
The existing modes of diagnosis are through the detection of antibodies against Salmonella bacteria by the Widal test and other serological tests like DOT enzyme immunoassay, dip stick assays, and semiquantitative tube agglutination test and PCR amplification of the bacterial DNA from blood. Apart from this, the bacteremia observed in typhoid fever around day 6 to 9 enables it to be detected through the blood culture test. Unfortunately, there are reports of a large number of false-positive cases especially in areas where typhoid fever is endemic and in patients exposed to typhoid fever earlier. The blood culture test has the major disadvantage of being a time-consuming test, which takes at least 2 to 3 days.
The ‘gold standard’ for identifying the cause of an infection is the isolation and identification of the causative agent of disease so the definitive diagnosis of typhoid fever depends on the isolation of S. typhi from blood, bone marrow or a specific anatomical lesion. The presence of clinical symptoms characteristic of typhoid fever or the detection of a specific antibody response is suggestive of typhoid fever but not definitive. Blood culture is the mainstay of the diagnosis of this disease. In the absence of a viable bacterium, antibody tests can give evidence of infection provided that suitable immunoassays, based on well-characterized antigens, are used.
A large proportion of typhoid patients excrete the pathogen in their urine and or stool so a nested polymerase chain reaction (PCR) specific for Salmonella enterica serovar Typhi was used for the detection of the pathogen in blood, urine, and stool. PCR has greatest diagnostic value for the detection of S. typhi among all the diagnostic tests used. PCR in urine has equal efficacy in detecting S. Typhi to PCR in blood.
The study was conducted on 50 patients suffering from fever less than one week and other clinical manifestations suspecting typhoid fever in addition to the living in endemic area. Blood cultures were received at the microbiology laboratory in Fowa Central Hospital-Kafr El-Shiekh. Complete culture and identification were conducted in the Microbiology Unit of the Central Laboratory of Ain Shams University Hospitals in Cairo.
The samples were collected from Fowa Central Hospital-Kafr El-Shiekh distributed as follows: 32 samples taken from tropical department, 8 from the gastroenterology Department, 10 from Pediatric department. Samples of patients were collected from 32 males and 18 females. Their ages ranged from 1.5 to 66 years, with a mean of 27.22 (+/- 16.86 years SD).
Fever was among all patients ranging from 38- 39.5 with mean of 38.85 ±0.41. The most prominent symptoms were constipation and colic among 26 (52%) and 21 (42%) of patients respectively.
Fifty samples from all patients were collected who were clinically suspected for typhoid fever, detection of Salmonella typhi by PCR in blood was positive for 42 (84%) patients, blood culture showed a positive results in 9 (18%). Stool culture had positive in 10 (34%) out of 29 typhoid fever patients from whom stool samples could be collected for culture. Negative test results for blood culture, stool culture and blood PCR were obtained for 8 (16%) patients. Seven of them had Widal titer < 320 and only one (2%) of them had titer ≥320.
In reference to the blood PCR, as a gold standard, the sensitivity of blood and stool culture was 21.43% and 38.46% respectively with 100% specificity. While Widal test at titer ≥320 had 57.14% sensitivity and 87.5% specificity and at titer ≥160 the sensitivity was higher 97.67% but with lower specificity 14.29%.
Among the demographic data and the clinical picture of the patients, only the positive previous history of salmonella infection was significantly associated with positive blood PCR for salmonella.
T
yphoid fever is becoming an ever increasing threat in the developing countries. Typhoid fever is estimated to cause 21 million illnesses and 200,000 deaths annually worldwide and Ingestion of contaminated drinking water and food is the most common route of disease transmission.
In Egypt, current estimates of typhoid fever incidence are derived from case reports received from passive hospital-based surveillance, without laboratory confirmation of disease. In the year 2000, the estimated incidence of typhoid fever in Egypt was 15 cases per 100,000 persons per year (Egyptian national syndrome-based surveillance, unpublished data).
The existing modes of diagnosis are through the detection of antibodies against Salmonella bacteria by the Widal test and other serological tests like DOT enzyme immunoassay, dip stick assays, and semiquantitative tube agglutination test and PCR amplification of the bacterial DNA from blood. Apart from this, the bacteremia observed in typhoid fever around day 6 to 9 enables it to be detected through the blood culture test. Unfortunately, there are reports of a large number of false-positive cases especially in areas where typhoid fever is endemic and in patients exposed to typhoid fever earlier. The blood culture test has the major disadvantage of being a time-consuming test, which takes at least 2 to 3 days.
The ‘gold standard’ for identifying the cause of an infection is the isolation and identification of the causative agent of disease so the definitive diagnosis of typhoid fever depends on the isolation of S. typhi from blood, bone marrow or a specific anatomical lesion. The presence of clinical symptoms characteristic of typhoid fever or the detection of a specific antibody response is suggestive of typhoid fever but not definitive. Blood culture is the mainstay of the diagnosis of this disease. In the absence of a viable bacterium, antibody tests can give evidence of infection provided that suitable immunoassays, based on well-characterized antigens, are used.
A large proportion of typhoid patients excrete the pathogen in their urine and or stool so a nested polymerase chain reaction (PCR) specific for Salmonella enterica serovar Typhi was used for the detection of the pathogen in blood, urine, and stool. PCR has greatest diagnostic value for the detection of S. typhi among all the diagnostic tests used. PCR in urine has equal efficacy in detecting S. Typhi to PCR in blood.
The study was conducted on 50 patients suffering from fever less than one week and other clinical manifestations suspecting typhoid fever in addition to the living in endemic area. Blood cultures were received at the microbiology laboratory in Fowa Central Hospital-Kafr El-Shiekh. Complete culture and identification were conducted in the Microbiology Unit of the Central Laboratory of Ain Shams University Hospitals in Cairo.
The samples were collected from Fowa Central Hospital-Kafr El-Shiekh distributed as follows: 32 samples taken from tropical department, 8 from the gastroenterology Department, 10 from Pediatric department. Samples of patients were collected from 32 males and 18 females. Their ages ranged from 1.5 to 66 years, with a mean of 27.22 (+/- 16.86 years SD).
Fever was among all patients ranging from 38- 39.5 with mean of 38.85 ±0.41. The most prominent symptoms were constipation and colic among 26 (52%) and 21 (42%) of patients respectively.
Fifty samples from all patients were collected who were clinically suspected for typhoid fever, detection of Salmonella typhi by PCR in blood was positive for 42 (84%) patients, blood culture showed a positive results in 9 (18%). Stool culture had positive in 10 (34%) out of 29 typhoid fever patients from whom stool samples could be collected for culture. Negative test results for blood culture, stool culture and blood PCR were obtained for 8 (16%) patients. Seven of them had Widal titer < 320 and only one (2%) of them had titer ≥320.
In reference to the blood PCR, as a gold standard, the sensitivity of blood and stool culture was 21.43% and 38.46% respectively with 100% specificity. While Widal test at titer ≥320 had 57.14% sensitivity and 87.5% specificity and at titer ≥160 the sensitivity was higher 97.67% but with lower specificity 14.29%.
Among the demographic data and the clinical picture of the patients, only the positive previous history of salmonella infection was significantly associated with positive blood PCR for salmonella.
Other data
| Title | Molecular Diagnosis of Salmonella Enterica | Other Titles | التشخيص الجزيئي للسالمونيلا المعوية | Authors | Mohammad Mahmoud Mohsen | Issue Date | 2014 |
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