PLOS ONE

RESEARCH ARTICLE

Antimicrobial agents for the treatment of

enteric fever chronic carriage: A systematic
review
Naina McCann ID1,2*, Peter Scott ID1,2, Christopher M. Parry ID3,4,5, Michael Brown ID2,6

1 UCL Faculty of Population Health Sciences, University College London (UCL), London, United Kingdom,
2 Hospital for Tropical Diseases, University College London Hospitals NHS Foundation Trust, London, United
Kingdom, 3 Clinical Sciences, Liverpool School of Tropical Medicine, Liverpool, United Kingdom, 4 Alder Hey
Children’s NHS Foundation Trust, Liverpool, United Kingdom, 5 Centre for Tropical Medicine and Global
Health, University of Oxford, Oxford, United Kingdom, 6 Clinical Research Dept, London School of Hygiene &
a1111111111 Tropical Medicine, London, United Kingdom
a1111111111 * Naina.mccann@nhs.net, zchaf30@ucl.ac.uk
a1111111111
a1111111111
a1111111111
Abstract

OPEN ACCESS Background

Citation: McCann N, Scott P, Parry CM, Brown M Chronic carriage of S. Typhi or S. Paratyphi is an important source of enteric fever transmis-
(2022) Antimicrobial agents for the treatment of sion. Existing guidance and treatment options for this condition are limited. This systematic
enteric fever chronic carriage: A systematic review.
PLoS ONE 17(7): e0272043. https://doi.org/
review aims to assess the evidence concerning the efficacy of different antimicrobials in
10.1371/journal.pone.0272043 treating enteric fever chronic carriage.
Editor: Praveen Rishi, Panjab University, INDIA

Received: November 15, 2021 Methods

Accepted: July 13, 2022 We searched major bibliographic databases using relevant keywords between 1946 and
Published: July 29, 2022 September 2021. We included all interventional studies that included patients with con-
Peer Review History: PLOS recognizes the firmed enteric fever chronic carriage and deployed an antimicrobial that remains in clinical
benefits of transparency in the peer review practice today. Case reports and case series of under 10 patients were excluded. Two
process; therefore, we enable the publication of reviewers screened abstracts, selected articles for final inclusion and quality-assessed the
all of the content of peer review and author
responses alongside final, published articles. The
included studies for risk of bias. Extracted data was analysed, with pooling of data and eradi-
editorial history of this article is available here: cation rates for each antimicrobial calculated. As only one randomised controlled trial was
https://doi.org/10.1371/journal.pone.0272043 identified, no meta-analysis was performed.
Copyright: © 2022 McCann et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which Results
permits unrestricted use, distribution, and
reproduction in any medium, provided the original Of the 593 papers identified by the initial search, a total of eight studies met the inclusion cri-
author and source are credited. teria and were included in the systematic review. Evidence was identified for the use of fluo-
Data Availability Statement: All relevant data are roquinolones and amoxicillin/ampicillin in the treatment for enteric fever chronic carriage.
within the paper and Supporting information files. Fluoroquinolones were superior to amoxicillin/ampicillin with 92% of patients achieving erad-
Funding: The author(s) received no specific ication after one antimicrobial course compared to 68% (p = 0.02). The quality of included
funding for this work. studies was poor, and all were carried out before 1990.

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PLOS ONE Antimicrobial agents for the treatment of enteric fever chronic carriage

Competing interests: The authors have declared Conclusion

that no competing interests exist.
This review identified fluoroquinolones and amoxicillin/ampicillin as treatment options for
enteric fever chronic carriage, with fluoroquinolones the more effective option. However,
this evidence pre-dates rises in antimicrobial resistance in enteric fever and therefore the
significance of these findings to today’s practice is unclear. Further research is needed to
investigate whether these antimicrobials remain appropriate treatment options or whether
alternative interventions are more effective.

Introduction
Enteric fever is a systemic febrile illness caused by infection with the Gram-negative bacteria
Salmonella enterica subspecies serovars Typhi (S. Typhi) and Paratyphi A, B or C (S. Paraty-
phi). It causes significant morbidity and mortality globally, with approximately 14 million
cases and 135,000 deaths per year, with the highest number of reported cases from South and
South-East Asia [1].
Enteric fever is transmitted via the faecal-oral route via the ingestion of food or water con-
taminated with infected human faeces. Clinical illness usually occurs 7–14 days after exposure
with a non-specific febrile illness. The majority of patients recover from acute enteric fever fol-
lowing an appropriate course of antimicrobials. Around 10% of patients continue to excrete S.
Typhi or S. Paratyphi in their stool for a few weeks in the convalescent period following recov-
ery from acute infection with approximately 1–5% of patients continuing to excrete S. Typhi
or S. Paratyphi in their stool for more than one year [2, 3]. This latter group are known as
chronic carriers [4].
The pathogenesis of chronic carriage is not well understood. The gallbladder and biliary
system appear to be the site of primary persistence of S. Typhi and S. Paratyphi in chronic car-
riers and indeed those with gallstones are more likely to become carriers [2, 4, 5].
S. Typhi and S. Paratyphi are human-restricted pathogens and therefore chronic carriage
plays an important role in maintaining the reservoir of infection in humans. Asymptomatic
chronic carriers unknowingly transmit the disease to others by faecal contamination of food
and water. The first described, and perhaps most famous example of this was Mary Mallon
(‘typhoid Mary’) who worked as a chef in New York in the 1950s and infected at least 54 people
as an asymptomatic carrier [3]. Since then forensic epidemiology has demonstrated many
more such cases, for example Mr N the Folkestone milker who infected over 200 people over a
number of years via infected milk [6].
In addition to the public health risk of chronic carriage there is evidence that chronic car-
riage is associated with an increased individual risk of malignancy, particularly gallbladder
cancer [7–9]. A recent meta-analysis reported an overall odds ratio of gallbladder cancer in S.
Typhi carriers of 4.28 (95% CI: 1.84–9.96) [10].
The identification and treatment of chronic carriers therefore has both a significant public
health and arguably individual benefit. There is currently limited evidence or guidance on how
these chronic carriers should be treated. Furthermore, recent clinical reviews on the subject
give differing advice on whether antimicrobials are an effective treatment option for chronic
carriage treatment [11–13].
WHO enteric fever guidelines from 2003 suggest treatment options for enteric fever
chronic carriage of amoxicillin, co-trimoxazole or ciprofloxacin [14]. Over the last 30 years
there have been significant changes in antimicrobial resistance patterns of S. Typhi and S.

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PLOS ONE Antimicrobial agents for the treatment of enteric fever chronic carriage

Paratyphi, with decreased susceptibility to fluoroquinolones (Fq) now almost universal in

some areas of South Asia and increasing across sub-Saharan Africa [15–17]. Multi-drug resis-
tance (MDR), resistance to amoxicillin, co-trimoxazole, chloramphenicol, is also seen in
around 40% of patients worldwide [15]. A current outbreak of extensively-drug-resistant
(XDR) enteric fever in Pakistan has highlighted the limited antimicrobial treatment options
available for acute enteric fever in this setting. The options for antimicrobial treatment of
enteric fever chronic carriers in the era of drug-resistance are unknown.
The aim of this systematic review is to review the existing evidence of efficacy of antimicro-
bials in treating enteric fever chronic carriage. By doing so we hope to review existing knowl-
edge and highlight areas for ongoing research and review.

Methods
The reporting of this systematic review was guided by the standards of the Preferred Reporting
Items for Systematic reviews and Meta-Analyses (PRISMA) statement [18]. A search of PROS-
PERO database performed did not reveal any existing similar protocols. The protocol for this
review was not registered but can be found in supplementary material (S1 Protocol).

Search strategy and selection criteria

A systematic search through MEDLINE, EMBASE and Web of Science from 1946 was initially
performed on the 1st February 2021. The terms searched were (“typhoid” OR “paratyphoid”
OR “salmonella typhi” OR “salmonella paratyphi “OR “enteric fever”) AND (“chronic car-
riage” OR “disease carrier “OR “carrier state” OR “typhoid carrier” OR “paratyphoid carrier”
AND (“antibiotic” OR “antibacterial” OR “antibacterial treatment” OR “antibiotic treatment”
OR “antibacterial agent” OR “antibiotic agent” OR names of individual antibiotics). The full
list of search terms is listed in the supplementary information (S1 Checklist). No language
restrictions were included in the initial search. Reference lists and bibliographies of selected
articles were also searched. A search for unpublished literature was not performed.
A repeat search of the same terms was performed in September 2021 to check for any addi-
tional studies that could be included prior to publication. All new articles identified that been
published since the prior search date were reviewed by two independent reviewers.
We included studies that met the following criteria:
1. Baseline population of adults > 18 years with confirmed enteric fever chronic carriage
2. Intervention of an antimicrobial course
3. Outcomes of stool clearance measured as defined by authors of each included study
We did not include studies that assessed an antimicrobial that is no longer in clinical use
today, for example sulphathiazole. Case reports and case series of under 10 cases were excluded
to minimize study bias. If studies included participants with non-typhoidal salmonella (NTS)
chronic carriage in addition to patients with enteric fever chronic carriage these studies were
individually reviewed to assess whether they included 10 or more patients with enteric fever
chronic carriage and, if they met other inclusion criteria, they were included. Studies that com-
bined an antimicrobial treatment intervention with another intervention (e.g. cholecystec-
tomy) were only included if they had an intervention arm that included antimicrobials alone
and this was made up of at least 10 patients.
If journal articles were not available online, we requested print versions through store
requests in our academic institution. Articles were excluded if we could not access a full-text

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PLOS ONE Antimicrobial agents for the treatment of enteric fever chronic carriage

version for review. Two independent researchers screened titles and abstracts and reviewed
full text articles for inclusion with any disagreements resolved by consensus.

Quality of studies
Two independent reviewers evaluated the study quality with any disagreements resolved by
consensus. Quality assessment was performed using the National Heart, Lung and Blood Insti-
tute quality assessment tools [19]. Each study was given an overall rating of good, fair or poor.

Data extraction
Data extraction was performed using a standardized data extraction form. Data was collected
on publication year, study country, study design, participant characteristics including number
of patients with gallstones, number of participants given intervention, intervention type
including dose and duration, stool culture result at follow-up and side effects of intervention.

Data analysis
For each included study the proportion of those chronic carriers that eradicated the pathogen
(eradication proportion) was calculated (i.e., the number of participants culture negative for S.
Typhi or S. Paratyphi after an antimicrobial intervention divided by the total number of partic-
ipants in the study).
For those that had a control population we calculated the eradication proportion in this
population. Data were synthesised for studies using the same antimicrobial intervention and
overall eradication proportions presented. The main outcome measure was eradication pro-
portion after one course of antimicrobials. As some studies re-treated their patients’ multiple
times, we also calculated eradication proportion per antimicrobial course. Other data synthe-
sised for each antimicrobial intervention included intervention characterises and side effects
of antimicrobial use. Results were presented in tabular format comparing antimicrobial inter-
vention. Categorical variables were compared using Fishers exact test. Data was collated in
Microsoft Excel and analysed in RStudio version 1.4.1103.

Results
Flow of included studies
The initial search strategy identified 579 papers. The abstracts and titles were reviewed and
after applying inclusion and exclusion criteria 26 papers were identified for full-text evalua-
tion. Of these a further 18 were excluded for reasons shown in the PRISMA flow-chart in Fig
1, leaving 8 articles which met our inclusion criteria. Full-text manuscripts could not be
accessed in 7 cases; all but one of these were published in non-English language journals (3
German, 1 Polish, 1 Russian, 1 Japanese). Of these 3 were published before 1960 and the
remaining 4 were published before 1980. A further search using the same terms was performed
in September 2021 which identified a further 14 papers, none of which were deemed eligible
for inclusion.

Study characteristics
Study characteristics of all included studies are summarised in Table 1. Only one used a
blinded randomised-control trial (RCT) design. The remaining seven studies used an open,
pre- post design method with no controls, comparing stool culture (outcome measure) in par-
ticipants before and after treatment.

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PLOS ONE Antimicrobial agents for the treatment of enteric fever chronic carriage

Fig 1. Preferred Reporting Items for Systematic Reviews (PRISMA) flow diagram of the study selection process. From: Moher D, Liberati A,
Tetzlaff J, Altman DG, The PRISMA Group (2009). Preferred Reporting Items for Systematic Reviews and Meta-Analyses: The PRISMA Statement.
PLoS Med 6(7): e1000097. doi:10.1371/joumal.pmed1000097 For more information, visit www.prisma-statement.org.
https://doi.org/10.1371/journal.pone.0272043.g001

Included studies were carried out between 1966 and 1988, with no studies included from
the last 30 years. The majority of studies were carried out in the USA (n = 4) with the remain-
ing papers conducted in Chile (n = 1), Peru (n = 1), Italy (n = 1) and Israel (n = 1).
Inclusion criteria for almost all the studies was presence of S. Typhi or S. Paratyphi in the
stool or bile for greater than 12 months. One study necessitated prior history of acute enteric

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Table 1. Summary of included studies.

Study author Country Year Journal published Planned intervention, Study design Outcome measure Quality
and references drug, dose and duration assessment
Ferreccio et al. Chile 1988 Journal of Ciprofloxacin PO 750mg Open pre- and post- trial, 3 stool specimens cultured at 3,6,9 and Fair
[20] Infectious Diseases BD, 28 days no control 12 months post treatment
Gotuzzo et al Peru 1988 Journal of Norfloxacin PO 400mg Double-blind Stool culture at months 1,2,3,6,9,12 Good
[21] Infectious Diseases BD, 28 days randomised-controlled post treatment
trial, followed by open
trial
Phillips et al. USA 1971 Journal of Ampicillin PO 1g QDS, Open pre- and post- trial, Stool culture monthly to a minimum Fair
[22] American Medical 90 days no control of 6 months post treatment
Association
Simon et al. USA 1966 New England Ampicillin PO 75- Open pre- and post- trial, Stool cultures monthly for first 3 Poor
[23] Journal of 100mg/kg/day for 28 no control months, 3 monthly for following year,
Medicine days then 2–3 times/year to a minimum of
7 months
Nolan et al. USA 1978 Journal of Amoxicillin PO 2g TDS Open pre- and post- trial, Stool culture at 3,6,12 months post Fair
[24] American Medical for 28 days no control treatment
Association
Dinbar et al. Israel 1969 American Journal Ampicillin PO 5.25mg/ Open, multi-arm pre- Stool cultures monthly for minimum Poor
[25] of Medicine day for 10–40 days and post- trial, no control of 12 months post treatment
Kaye et al. [26] USA 1967 Annals of the New Ampicillin PO 1.5mg Open pre- and post- trial, Stool cultures (regularity not defined) Poor
York Academy of QDS + probenecid 0.5g no control to a minimum of 12 months post
Science QDS, 42 days treatment
Scioli et al. Italy 1970 Journal of Ampicillin IV 1g TDS, 15 Open pre- and post- trial, Stool cultures twice/week for 9 weeks, Poor
[27] Infectious Diseases days no control then once/month for a minimum of
16 months
https://doi.org/10.1371/journal.pone.0272043.t001

fever [21] and one study included two patients who had positive stool cultures with an elevated
Vi antibody level, without need for repeated positive cultures of > 1 year [20]. One study men-
tioned that included patients had been previously treated for chronic carriage [27] but others
did not specify this. Length of chronic carriage varied between participants within studies,
with the longest carrier state being 39 years. Two studies excluded those with serious underly-
ing health conditions and patients with penicillin allergies [21, 24]. Exclusion criteria were not
well defined in the other studies.
The age range of included participants was broad, including adults from 18–81 years. One
study included one child aged 7 years old with the results not separated from the 14 adults in
the study [23]. After discussion between reviewers this study was included. The majority of
participants in the studies were female which correlates with prior knowledge that females are
more likely to be carriers [28].
Most of the studies investigated participants for underlying gallstones before enrolment in
the study using ultrasound or cholangiography or X-ray. However, in some patients the biliary
tract was not well visualised and two studies did not investigate for underlying gallstones prior
to enrolment [20, 22]. These patients were categorised in a third category ‘gallstone status
unknown’.

Study intervention
The intervention drug was a fluoroquinolone (Fq) in two studies: norfloxacin was used in one
and ciprofloxacin in the other. Four of the studies used ampicillin alone as the intervention,
one study used amoxicillin and one used ampicillin with probenecid. One of the ampicillin
studies used intravenous administration but the rest of the studies used oral administration.
Dosing varied between the different studies (see Table 1). One study also included separate

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arms of the study with patients undergoing cholecystectomy and cholecystectomy in combina-
tion with ampicillin treatment [25]. In this study six of the patients undergoing antimicrobial
treatment alone had previously undergone cholecystectomy in the other arm of the study.
These were included as they were confirmed to still be chronic carriers 1 year post cholecystec-
tomy and prior to enrolment in the antimicrobial arm.

Quality assessment
Overall methodological quality of included studies was poor (see Table 1). There was only one
randomised and blinded study, with the majority of studies designed as pre- vs post-studies
without a control.
In the RCT study, the outcomes were inconsistently reported with the summary table
showing no cases of eradication in the placebo group but the text reporting that one patient
this arm did have negative stool cultures at follow-up (indicating spontaneous cure). Fur-
thermore, 13 patients were enrolled in the norfloxacin arm but only 12 patients included in
the final analysis with no intention-to-treat analysis performed. No relative risk or odds ratio
was calculated.
The remainder of the studies were open, pre- vs post- studies without controls. Overall
inclusion criteria were well defined, but studies did not clearly state how patients were
enrolled.
Interventions were generally well defined but often not consistent across participants, with
some studies changing dosing regimens during the study period and one study using multiple
interventions on participants [25]. All studies clearly defined the outcome measure, and this
was measured before and after intervention and at repeated intervals for adequate follow-up
periods after intervention. Loss to follow-up in the included studies was small (less than 20%
in all). None of the studies mentioned a power calculation and numbers of participants were
small in all studies. Minimal statistical analysis was performed.
A meta-analysis was not performed as there was only one RCT identified.

Outcomes by antimicrobial group

Fluoroquinolones. Two studies assessed Fq as a treatment option for S. Typhi chronic
carriage, and no studies assessed Fq as a treatment option for S. Paratyphi chronic carriage. In
the S. Typhi studies a total of 25 patients received an intervention course [20, 21]. The norflox-
acin study was an RCT, comparing 28-days of 400mg BD norfloxacin to placebo. The cipro-
floxacin study was a pre-post study investigating the effect of a 28-day course of 750mg BD
ciprofloxacin. All patients had S. Typhi chronic carriage, 44% of whom were known to have
gallstones (although this was not specifically looked for in one of the studies). Both Fq studies
showed an eradication proportion of 92% following a single 28-day treatment course. These
two studies are summarised in Table 2.
Effect of gallstones on eradication. Patients in the ciprofloxacin study were not routinely
screened for biliary disease before enrolment in the study, however two patients were known
to have gallstones. In the norfloxacin study patients underwent oral cholecystogram, IV chol-
angiography or gallbladder ultrasound before enrolment.
Eradication proportions were 100% in those without gallstones, 89% in those with
unknown gallstone status and 82% in those with known gallstones.
Side effects. Side effects were seen in a quarter of patients. In the ciprofloxacin study two
patients stopped treatment early due to side effects (haemoglobin drop and urticarial rash). Of
note, three other patients in this study had a haemoglobin drop that were attributed to

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PLOS ONE Antimicrobial agents for the treatment of enteric fever chronic carriage

Table 2. Summary of results from studies using fluoroquinolone antibiotics to treat S. Typhi chronic carriage.
Gotuzzo Ferreccio Combined interventions
Intervention given Norfloxacin Placebo Ciprofloxacin
Participant characteristics
Total no of participants, n 13 12 12 25
With gallstones, n (%) 9 (69) 9 (75) 2 (17) 11 (44)
Without gallstones, n (%) 4 (31) 3 (25) 0 4 (16)
Unknown gallstone status, n (%) 0 (0) 0 10 (75) 10 (36)
Demographics
Age, mean 36 35 31 34
Age, range 18–58 12–67 20–51 18–67
Female, n (%) 6 (46) 11 (92) 10 (83) 16 (64)
Microbiological characteristics
S. Typhi, n (%) 13 (100) 12 (100) 12 (100) 25 (100)
MIC to intervention drug, range (ug/mL) 0.06–0.5 0.06–0.5 0.0156–0.0078 0.0078–0.5
Intervention
Administration method PO PO PO
Total daily dose, mean (mg) 800 NA 1500
Total duration, mean (days) 28 28 25
Duration range, (days) 28 28 10–28
Outcomes—eradication rates
Minimum follow-up (months) 3 3 12 3
Total no of patients included in analysis1, n 12 12 12 24
Total no patients eradicated at end of follow-up, n (%) 11 (92) 1 (8) 11 (92) 22 (92)
Eradication rate in those with gallstones, n (%) 7 (88) 1 (8) 2 (100) 9 (82)
Eradication rate in those without gallstones, n (%) 4 (100) 0 (0) 0 (0) 4 (100)
Eradication rate in those with unknown gallstone status, n (%) 0 (0) 0 (0) 9 (90) 9 (90)
Outcomes—side effects
Total side effects, n (%) 1 (8) 3 (25) 5 (41) 6 (25)
Diarrhoea, n (%) 0 0 0 0
Rash, n (%) 0 0 1 (8) 1 (4)
Other, n (%) 1 (8) 3 (25) 4 (33) 5 (21)
Total no of antibiotic courses given2, n 23 12 12 35
Cure rate per no of courses given, n (%) 18 (78) 1 (8) 11 (90) 29 (83)
1
—One patient was excluded from norfloxacin analysis in Gottuzo paper due poor adherence
2
—In Gotuzzo paper 10 placebo patients were re-treated with norfloxacin openly taking the total number of treatment courses given in this paper to 23
In the RCT the norfloxacin group had a higher proportion of S. Typhi eradication than the control group (92% vs 8%, p <0.001).

https://doi.org/10.1371/journal.pone.0272043.t002

ciprofloxacin therapy. Norfloxacin was relatively well-tolerated in the other study with only
one patient reporting a rash, although these are not further explored in this paper.
Effect of repeated courses. In the norfloxacin study patients initially treated with placebo
were then retreated with norfloxacin. This group had a slightly lower eradication proportion
of 70%, lowering the overall eradication proportion to 83% per antibiotic course overall.
Ampicillin/amoxicillin. The remaining six studies identified assessed amoxicillin or
ampicillin (amox/amp) use. The results of these are summarized in Table 3.
Overall, 101 patients were given a total of 115 courses of amox/amp (either PO or IV). Most
of these patients were female and 28% had confirmed gallstones. Only one patient had S. Para-
typhi. The dosing regimens were relatively high with the mean daily dose of amoxicillin 4.5g/

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PLOS ONE Antimicrobial agents for the treatment of enteric fever chronic carriage

Table 3. Summary of results from studies using amoxicillin or ampicillin to treat enteric fever chronic carriage.
Phillips Simon Nolan Dinbar Kaye Scioli Overall
Participant characteristics
Total no of participants 12 15 15 16 24 19 101
With gallstones, n (%) 0 5 (33) 4 (27) 3 (12) 12 (50) 5 (26) 29 (29)
Without gallstones, n (%) 0 10 (66) 11 (73) 7 (87) 7 (25) 8 (42) 43 (42)
Unknown gallstone status, n (%) 12 (100) 0 (0) 0 (0) 6 (38) 5 (21) 6 (32) 29 (29)
Known to have had cholecystectomy, n (%) 0 (0) 1 (7) 0 7 (44) 1 (4) 1 (5) 10 (10)
Demographics
Age, mean 57 44 65 53 59 45 54
Age, range 23–81 7–62 48–77 36–67 33–83 21–64 7–81
Female, n (%) 7 (58) 10 (66) 13 (87) 7 (44) 18 (75) 16 (84) 71 (70)
Microbiological characteristics
S. Typhi, n (%) 12 (100) 15 (100) 12 (100) 15 (94) 24 (100) 19 (100) 100 (99)
MIC to intervention drug, range (ug/mL) NA <1–2 <1 0.5–2.5 NA NA
Intervention characteristics
Intervention name Ampicillin Ampicillin Amoxicillin Ampicillin Ampicillin Ampicillin
Administration method PO PO PO PO PO IV
Intended daily regimen 1g QDS 75-100mg/kg 2g TDS 1.25g QDS 1.5g QDS 1g TDS
Intended duration (day) 90 28 28 10–40 42 15
Addition of probenecid No No No No Yes No
Dosing details of initial course given
Total daily dose, mean (g) 4 4 5.2 5.1 5.5 3 4.5
Total course duration, mean (days) 90 28 28 25 36 15 37
Total dose taken during course, mean (g) 360 112 146 128 198 45 165
Outcomes—eradication
Minimum follow-up (months) 18 7 12 12 12 16
Total no patients eradicated after initial course, n (%) 9 (75) 13 (87) 11 (73) 7 (44) 9 (38) 19 (100) 68 (67)
Eradicated in those with gallstones, n (%) 0 (0) 4 (80) 3 (75) 0 (0) 4 (33) 5 (100) 16 (55)
Eradicated in those without gallstones, n (%) 0 (0) 8 (89) 9 (82) 4 (57) 5 (71) 8 (100) 34 (79)
Eradicated in those with unknown gallstone status, n (%) 9 (75) 0 (0) 0 (0) 3 (50) 0 (0) 6 (100) 18 (62)
Eradicated in those with known cholecystectomy, n (%) 0 (0) 1 (100) 0 (0) 3 (43) 0 (0) 1 (100) 5 (50)
Total no of patients eradicated at end of follow-up, n (%) 9 (75) 13 (87) 11 (92) 9 (56) 12 (50) 19 (100) 73 (73)
Total no of antibiotic courses given2, n 12 17 15 21 31 19 115
No of antibiotic courses resulting in successful eradication (%) 9 (75) 13 (76) 11 (73) 9 (43) 12 (39) 19 (100) 73 (63)
Outcomes—side effects
Total side effects, n (%) 8 (67) 10 (67) 7 (47) NA 5 (21) 3 (15) 33 (33)
Diarrhoea, n (%) 5 (42) 8 (53) 3 (20) 1 (4) 2 (10) 19 (19)
Rash, n (%) 5 (42) 6 (40) 2 (13) 5 (21) 1 (5) 19 (19)
Anaphylaxis 1 (9) 1 (1)
Other, n (%) 0 (0) 2 (20) 3 (20) 5 (5)
https://doi.org/10.1371/journal.pone.0272043.t003

day but this varied significantly between studies (range 3–5.5g/day). The mean duration of
treatment was 37 days but again this was highly variable between regimens (range 15–90 days).
The overall eradication proportion after a single course of amox/amp was 68% but again,
this was highly variable between studies (range 38–100%).
Effect of administration method on eradication. There was only one study looking at
the effect of IV ampicillin on chronic carriage eradication proportion [27]. The mean daily
dose and total cumulative dose of IV ampicillin used in this study were much lower than those

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PLOS ONE Antimicrobial agents for the treatment of enteric fever chronic carriage

used in the oral studies (3g vs a mean of 4.7g for the oral studies). However, the eradication
proportion in this study was notably higher when compared to the oral ampicillin studies
(100% vs 60%, p <0.01) with no failures described in the IV arm.
Effect of dosing regimen and duration on eradication. The total daily dose of amox/
amp varied between 4 and 5.5g for the studies using oral intervention, with the total drug
given over the duration of the course between 112g to 360g.
Overall comparisons between dosing regimen and duration are not possible as different
studies used widely variable dosing regimens and durations, often not standardised within
their own study population. Therefore, individual regimens within studies were examined
more closely to investigate for possible associations of dosing or duration with outcome.
In the Nolan et al study [24] two different dosing regimens of amoxicillin were given. For
those that tolerated it (n = 10) a dosing regimen of 2g of oral ampicillin was administered
three times daily (total daily dose of 6g) which had a 90% success at 12 months after one course
of 28 days. In those who did not tolerate the higher dose (n = 5) a lower dosing regimen of 1g
oral ampicillin three times a day (total daily dose of 3 grams) was started. This regimen only
had a 40% eradication proportion at 12 months after 28 days, but the numbers were very
small.
Similarly, in Phillips et al. [22] those who completed the full dosing schedule of 1g QDS for
90 days had a 90% eradication proportion whereas those who were unable to complete it had a
0% eradication proportion, although these numbers are small and the exact dosing regimens
are not fully described in the paper.
In Dinbar et al. [25] the initial course length was 10 days with a total of 52g of ampicillin
given over this time. This regimen had an eradication proportion of 38%. Over the period of
the study the dose of ampicillin was increased to 200g given over 40 days. The eradication pro-
portion of this regimen was 57%. There was no evidence to support a difference in eradication
proportions between dosing regimens (p = 0.61).
The Kaye study, which have a mean daily dose of ampicillin of 5.5g for a mean duration of
36 days, was the only study to use probenecid in addition to ampicillin. Eradication propor-
tions were lower in this study when compared to the other studies where probenecid was not
used (38% vs 76%).
Overall, we are unable to draw any firm conclusions from this data whether dose or dura-
tion of this intervention effects eradication proportion although there is a slight suggestion
that higher daily dosing and intravenous dosing may be slightly more effective.
Effect of gallstones on eradication. All studies investigated for prior presence of gall-
stones except Phillips et al.
The eradication proportion was lower in those with gallstones compared to those without
gallstones (55% vs 79%, p = 0.039). Those with unknown gallstone status had an eradication
proportion of 62%, whereas those with a past cholecystectomy only had an eradication propor-
tion of 50%, although the numbers in this group are small.
Effect of repeated courses on eradication. After failing an initial course of therapy some
patients were re-treated with a second or third course. In Kaye et al six patients who failed an
initial course were re-treated with further courses of ampicillin. Of these only two were suc-
cessfully eradicated [26]. In Dinbar et al four patients who initially failed therapy were success-
fully treated with higher dosing regimens of ampicillin [25]. In Simon et al one patient was re-
treated twice with prolonged six-week courses of ampicillin which did not result in eradication
[23]. In total, of the 11 patients that were re-treated, the eradication proportion was only 36%.
Side effects. A third of patients in the ampicillin studies suffered side effects from their
therapy. Rash and diarrhoea were the most common side effects and in two studies this

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PLOS ONE Antimicrobial agents for the treatment of enteric fever chronic carriage

resulted in a modification to therapy [22, 24]. In the IV amoxicillin trial side effects were mini-
mal. However, side effects were not routinely reported across the studies.
Comparing antimicrobial groups. Fluoroquinolones have a higher overall eradication
proportion than amox/amp (92% vs 68%, p = 0.02). In those with gallstones the eradication
proportion remains higher in those taking a Fq compared to those taking amox/amp but this
is not significant (82% vs 55%, p = 0.16).

Discussion
This is the first systematic review on the efficacy of antimicrobial agents for treatment of
enteric fever chronic carriage. A total of 126 participants in 8 studies were included in this
review and two intervention groups were identified: Fq and amox/amp. Overall eradication
proportion was higher in the Fq compared to the amox/amp group (92% vs 68%, p = 0.02), but
eradication proportions varied highly across the included studies.
These findings are consistent with the limited existing guidelines available on treatment of
chronic carriage. As previously stated, the only available guidance for treatment of chronic car-
riers is from the WHO 2003 guidelines which suggests a quinolone, amoxicillin or co-trimoxa-
zole [11, 14]. In this review we have not identified any specific evidence for use of co-
trimoxazole. During the screening process we did identify literature investigating the use of
co-trimoxazole in addition to other antimicrobials such as chloramphenicol, kanamycin and
penicillin [29–33]. None of these studies met the criteria for inclusion in this study, again
highlighting the poor quality of available evidence on this subject. Most of these studies were
case reports, small case series or did not report on necessary outcome data.
The findings of this systematic review should be interpreted with caution. Firstly, the overall
quality of the included studies was poor, with only one RCT included. The remainder of the
studies were case series or pre- and post- studies with no control group and no blinding,
which allowed for significant bias. Furthermore, in most studies it was unclear how patients
had been identified for participation. Interventions were non-consistent within and across
studies and the total number of included patients is small, limiting statistical analysis and over-
all conclusions.
Secondly, almost all the patients included in this study had S. Typhi chronic carriage and
therefore limited conclusions can be made about the effectiveness of these antimicrobials on S.
Paratyphi chronic carriage. With the rollout of the typhoid conjugate vaccine and the likely
associated decrease in incidence of S. Typhi, the incidence of S. Paratyphi may start to rise
[34]. Given there is currently no licensed vaccine for S. Paratyphi the role of treatment of carri-
ers of this condition may become increasingly important in reducing enteric fever transmis-
sion and disease. This review identifies current large gaps in the evidence base for the
treatment of this neglected condition.
Thirdly, and perhaps most importantly, the included trials were all carried out prior to
widespread antimicrobial resistance of enteric fever. There are no studies that have been car-
ried out to assess treatment options for enteric fever chronic carriage in patients with multi-
drug resistant (MDR), Fq-resistant, or indeed extensively-drug-resistant enteric fever. This
data is therefore inadequate to inform decisions regarding antimicrobial treatment of chronic
carriage in the modern-day era of drug-resistant enteric fever.
Further research needs be done to understand and improve treatment options for enteric
fever chronic carriers today. The first barrier to investigating treatment interventions on
chronic carriers is the accurate identification of chronic carriers from the population. In the
trials included in this study, regular stool cultures were used to identify carriers over many
years. This is logistically challenging and not suitable for large-scale screening today,

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PLOS ONE Antimicrobial agents for the treatment of enteric fever chronic carriage

particularly in low-resource endemic areas. Furthermore, as stool shedding is intermittent, the

sensitivity is low [35]. Molecular and serological strategies to identify carriers have been
explored but do not perform well, particularly in endemic settings [36, 37]. New methods to
identify carriers such as antigen-specific antibodies and biomarkers show promise but need
further exploration [38, 39].
Once carriers can be accurately identified, further work is needed to investigate the effect of
different interventions on this population. There is limited data on the antimicrobial suscepti-
bility of chronic carriage isolates and whether they harbour similar resistance profiles to acute
enteric fever strains. A study from 2012 in Nepal looking at S. Typhi and S. Paratyphi isolates
identified from cholecystectomies found lower rates of antimicrobial resistance in these
strains, compared to acute enteric fever isolates in the same area [40]. This suggests that
chronic carriage strains may originate from older, drug-susceptible strains for which Fq and
Amp may still be effective. Previous work looking at the genetic diversity of S. Typhi has sug-
gested this may be due to two different evolutionary methods within the population structure
of S. Typhi [41].
Even if chronic carriage isolates do show reduced susceptibility to ciprofloxacin, the excel-
lent bile penetration of this antimicrobial (reaching 2500–4500% of plasma concentrations in
the bile [42]), may be sufficient to overcome the relatively low minimum inhibitory concentra-
tion (MIC) (e.g. with MIC � 1) isolates, although there is no patient outcome data to support
this.
Alternative antimicrobials that have not been identified in this review may also be effective
in treating chronic carriage. Azithromycin is currently used to treat uncomplicated enteric
fever worldwide with good efficacy [43] but no studies have examined the efficacy of azithro-
mycin on enteric fever chronic carriage. Azithromycin has good bile penetration [42] and a
single case report suggests it may eradicate non-typhoidal Salmonella carriage [44]. It therefore
may be a good intervention for enteric fever chronic carriage treatment and is currently the
preferred treatment for Fq resistant chronic carriage eradication in the UK; post-treatment
monitoring will be used to assess outcome.
It is well understood that the gallbladder is an important niche for the persistence of S.
Typhi and S. Paratyphi and the development of carriage [3, 11]. Accordingly, cholecystectomy
has been used as treatment for chronic carriage in the past [25, 45, 46]. In addition to carrying
a significant anaesthetic and surgical risk this intervention does not guarantee elimination of
the carrier state, with success rates reported between 70–90%. Studies in this review also iden-
tify multiple patients identified as carriers who had already undergone a prior cholecystec-
tomy. Hence, it is likely that there are other foci of infection outside the gallbladder that also
play an important role in S. Typhi and S. Paratyphi persistence and carriage, for example the
biliary tree, liver or lymph nodes [47].
Recently, murine models have shown that biofilm formation, on gallbladder epithelium
and cholesterol-rich gallstones, may play an important role in the development of carriage and
protect against antimicrobial treatment [48–51]. Prior guidance similarly states that carriers
with underlying gallstones are unlikely to be eradicated with antimicrobials alone [11, 14]. In
contrast, this review shows relatively good eradication proportions in those with gallstones,
particularly when Fq treatment was used, albeit lower than those without gallstones. Further-
more, many of the identified carriers in this review were not found to have gallstones, despite
thorough investigation. It is evident that further work is required to better understand the
pathophysiology of chronic carriage to allow the development of targeted interventions.
This review was limited by the high risk of bias in all included studies. Many studies showed
selection bias with undefined selection of participants and exclusion of certain groups. The
participants may not be a true representative of the carrier population. Minimal data were

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PLOS ONE Antimicrobial agents for the treatment of enteric fever chronic carriage

reported from all studies with very little statistical analysis performed. Particularly, the pres-
ence of gallstones was not investigated or reported in all studies meaning conclusions on
whether gallstones affect the effect of antimicrobial eradication are uncertain.
Side effects of medications were also poorly reported across studies. Some adverse events
were linked to the intervention without obvious causality e.g. ciprofloxacin causing haemoglo-
bin drop, when this is not a known common side effect. Furthermore, associations of Fq ther-
apy that were not recognised when these studies took place, for example tendinopathy and
aneurysm rupture, may have been under-reported. The recent realisation of potential signifi-
cant side effects of Fq use has also led to restrictions in their use and they may no longer be
suitable to treat patients with chronic carriage, again highlighting the need for alternative treat-
ment strategies [52, 53].
In this review we excluded case reports or case series of less than 10 patients in attempt to
reduce bias but in doing so may have excluded studies with additional interventions. Further-
more, the evidence relating to this topic is almost all more than 30 years old and some full-text
articles were not able to be accessed, some of which may have otherwise been included in the
final review. The strengths of our study include a comprehensive search strategy, the use of
two independent reviewers throughout the process and the use of well-defined data extraction
and quality assessment tools.
In conclusion, this review identifies evidence for fluoroquinolones and amoxicillin/ampicil-
lin as antimicrobial interventions for treatment of enteric fever chronic carriage. Fluoroquino-
lones are the most effective antimicrobial intervention and should be recommended for first-
line treatment of chronic carriage where the isolate is susceptible, given the lack of evidence
for any other interventions. There is insufficient evidence to recommend empiric use of fluo-
roquinolones or amoxicillin/ampicillin for enteric fever chronic carriage today, due to signifi-
cant changes in antimicrobial resistance over the last 30 years. Updated evidence on this
neglected topic is required and should be a vital component of global eradication strategies for
enteric fever going forward.

Supporting information
S1 Checklist. PRISMA 2020 checklist.
(PDF)
S1 File. Search strategy.
(PDF)
S1 Protocol. Systematic review protocol.
(PDF)
S2 File. Comparing categorical variables, R file.
(PDF)

Acknowledgments
The authors would like to thank Dr Daniel Davis for providing advice during the systematic
review process and Dr Robert Shaw for proof-reading the manuscript.

Author Contributions
Conceptualization: Naina McCann, Christopher M. Parry, Michael Brown.
Data curation: Naina McCann, Peter Scott.

PLOS ONE | https://doi.org/10.1371/journal.pone.0272043 July 29, 2022 13 / 16

PLOS ONE Antimicrobial agents for the treatment of enteric fever chronic carriage

Formal analysis: Naina McCann, Peter Scott.

Investigation: Naina McCann, Peter Scott.
Methodology: Naina McCann, Peter Scott.
Supervision: Christopher M. Parry, Michael Brown.
Validation: Christopher M. Parry, Michael Brown.
Visualization: Naina McCann.
Writing – original draft: Naina McCann.
Writing – review & editing: Naina McCann, Peter Scott, Christopher M. Parry, Michael
Brown.

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