Antimicrobial sensitivity patterns of Salmonella typhi in children

Background: Enteric fever is an important public health problem in developing countries including Bangladesh. A changing antibiotic sensitivity pattern of Salmonella typhi and emergence of resistance has increased to a great concern. Objective: Aim of the study was to investigate the antibiotic sensitivity pattern of Salmonella typhi. Methods: A total of 181 Salmonella typhi samples from 5 to 15 years age group were collected from blood culture during the period of October to December 2014 from IBN SINA Hospital, Dhaka, Bangladesh. Specimens from the blood culture were identified by standard procedures as needed. Antimicrobial susceptibility testing was performed by disk diffusion method according to ‘The Clinical Laboratory Standard Institute’ guidelines. Results: Among the tested antibiotics, S. typhi was susceptible to ceftriaxone 100%, followed by cefixime and gentamicin 99.4%, ciprofloxacin 98.6%, cotrimoxazole 88.9%, azithromycin 88.4 % and least susceptible antibiotic was Ampicillin 62.5% and nalidixic acid 5%. Conclusion: The antimicrobial sensitivity testing showed that the Salmonella typhi were highly sensitive (>88%) to most of the drugs used in this study, whereas nalidixic acid showed only 5% sensitivity. So this study indicates that ceftriaxone, cefixime, gentamicin and ciprofloxacin can be used as a first line therapy and nalidixic acid should be avoided for treatment.


Introduction:
Enteric fever is an important public health problem in many developing countries. 1Enteric fever includes typhoid fever caused by Salmonella typhi and paratyphoid fever by Salmonella paratyphi A B C. Typhoid fever affects roughly 21.6 million people annually, resulting 216,500 death. 2 More than 90% of this morbidity and mortality occurred in Asia. 3 World Health Organization in 2008 report on typhoid fever annual incidence in five Asian countries among 5-15 years age group.The incidence varied from 24.2 and 29.3 in Vietnam and China to 180.3 in Indonesia and to 412.9 and 493.5 in Pakistan and India respectively. 4nteric fever continues to be a major health problem despite the use of antibiotics and the development of newer antibacterial drugs. 5If not treated properly, enteric fever carries a mortality rate of 30%, whilst appropriate antimicrobial treatment reduces the mortality rate to as low as 0.5%. 6In cases of enteric fever, it is often necessary to commence treatment before the results of laboratory sensitivity testing. 7Fluoroquinolone ciprofloxacin has become the first-line drug for treatment, since the widespread emergence of S. typhi isolates that are multi drug resistant (MDR) to the more traditional antimicrobial agents comprising chloramphenicol, ampicillin and trimethoprim sulphamethoxazole (cotrimoxazole). 8Salmonella typhi has rapidly gained resistance to antibiotics like ampicillin, chloramphenicol and cotrimoxazole and also to previously efficacious drugs like ciprofloxacin. 9 The incidence of multidrug resistant (MDR) S. typhi was reported to be as high as 60 per cent. 10 US-based study noted an increase in the number of MDR strains and nalidixic acid resistant S. typhi (NARST), although overall, the isolates were sensitive to ciprofloxacin and ceftriaxone.11 Another study from Bangladesh reported a decrease in MDR isolates with no corresponding increase in sensitive strains.12 Due to the variation in the susceptibility patterns of S. Typhi, it is important to constantly monitor the susceptibility patterns of Salmonella typhi to commonly prescribed antibiotics and to prevent the emergence of multi drug resistance.

Material and methods:
The blood samples for culture were collected from patient's age group 0-15 years attending in IBN SINA Hospital, Dhanmondi, Dhaka, Bangladesh from March to June 2014.The specimens were Cultured and antibiotics susceptibility tests were performed according to standards microbiological techniques.Brain-Heart infusion (BHI) broth was used for culture.Subcultures into blood agar, Mac-Conkeys agar were done as per standard methods.Non-lactose fermenting colonies were identified by biochemical reactions and confirmed by group and type specific Salmonella antisera.Antimicrobial susceptibility was performed by Kirby-Bauer Disc-diffusion methods on Muller-Hinton agar plates.Antibiotics disc used in this study were Ceftriaxone, Chloramphenicol, cefixime, Co-trimoxazole, Nalidixic acid, Ampicillin, Azithromycin, Ciprofloxacin and Gentamicin.Interpretation was done in accordance with the National Committee for Clinical Laboratory Standardards (NCCL).This Study was approved by ethics Committee of Ibn Sina Hospital.

Results:
Culture positive samples were collected from the IBN SINA Hospital from the period of March to June 2014.A total of 181 S. typhi were obtained by blood culture from the suspected cases of enteric fever.Isolates were from 5-15 years age group (Table 1) and both male (60%) and female (40%) children (Table 2) included.Salmonella typhi was 100% susceptible to ceftriaxone followed by cefixime and gentamicin 99.4%, ciprofloxacin 98.6%, cotrimoxazole 88.9%, azithromycin 88.4 %; and least susceptible antibiotic was Ampicillin 62.5% and nalidixic acid 5% (Table 3).Discussion: Enteric fever is a major public health problem in our country. 1The highest prevalence rate was observed in adult whose age was between 21 to 30 years whereas the lowest incidence of typhoid fever was in between 51 to 60 years age. 13Prompt institution of appropriate antimicrobial therapy can reduce the mortality and morbidity of this illness. 2Since 1948, chloramphenicol had been the mainstay of treatment of enteric fever until 1972. 14A study done by Sharma et al who showed that 92.7% isolates were susceptible to chloramphenicol. 15This finding was near about similar to our finding where 88% isolates were susceptible to chloramphenicol.Ceftriaxone was 95.7 % sensitive to Salmonella typhi reported from Pakistan and Bangladesh. 16Another study showed 100% sensitivity against ceftriaxone and gentamicin. 17Above two findings were in agreement with the findings of our study where ceftriaxone showed 100% sensitive to S. typhi.This underlines the importance of this drug for treating MDR and ciprofloxacin resistance cases in this region. 13mphasis is laid on the sparing use of this drug.It should be used only if the first and second line antibiotics fail to evoke a satisfactory response or if the isolate is resistant to nalidixic acid.Krishnan et al observed that S. typhi was highly sensitive to chloramphenicol (86%), ampicillin (84%), and cotrimoxazole (88%). 18In our study sensitivity pattern to chloramphenicol (88%), ampicillin (62.5%), cotrimoxazole (88.9).The treatment of typhoid fever was relatively simple till 1989 in Bangladesh like many other countries because all S. typhi were uniformly  susceptible to ciprofloxacin and the clinician advised to switch over to another antibiotic. 5

Conclusion:
The findings of the present study indicated that the first line antibiotics like chloramphenicol, ampicillin, cotrimoxazole may still have a role in the treatment of typhoid fever.Also the usefulness of nalidixic acid as a screening test for detecting reduced susceptibility to ciprofloxacin.Thus susceptibility pattern of causative organisms must be studied especially of their hospital when choosing a treatment regimen.

Conflict of interest:
None susceptible to first line antibiotics like ampicillin, chloramphenicol and co-trimoxazole. 19A few sporadic cases of typhoid fever caused by antibiotic resistance S. typhi were reported.All of the drugs are more than 60% sensitive except nalidixic acid which shows lowest sensitivity about 5%. 20Nalidixic acid susceptibility has been validated as a screening test for reduced susceptibility to ciprofloxacin and nalidixic acid resistance is associated with a high MIC of ciprofloxacin which in turn is associated with treatment failure. 21Any isolate that shows nalidixic acid resistance should be reported as intermediately

Table 2 :
Sex distribution of patients