Salmonella enteritidis diarrhoea in Harare, Zimbabwe

Tropical Medicine and International Health

volume 5 no 7 pp 503–506 july 2000

Salmonella enteritidis diarrhoea in Harare, Zimbabwe

Clifford Simango and Charity Mbewe

Department of Medical Laboratory Sciences, University of Zimbabwe Medical School, Harare, Zimbabwe

Summary The study was conducted to determine the prevalence of Salmonella enteritidis diarrhoea in an urban area inZimbabwe. Stool specimens from people of all ages presenting at primary level health centres in Harare wereinvestigated for S. enteritidis and other bacterial and parasitic enteric pathogens. The first 46 S. enteritidisisolates were phage-typed, and all isolates were tested for susceptibility to ampicillin (10 �g), chloram-phenicol (30 �g), cotrimoxazole (25 �g), tetracycline (30 �g), gentamicin (10 �g), nalidixic acid (30 �g),ciprofloxacin (5 �g) and ceftriaxone (30 �g). S. enteritidis was isolated in 74 (1.8%) of 4155 stool specimenswhich represented 30.7% of all Salmonella species isolated. The most common S. enteritidis phage type was4 (78.3%) followed by 7, 9 and 23 (8.7%, 2.2%, 2.2%, respectively) All S. enteritidis isolates were sensitive togentamicin, ciprofloxacin and ceftriaxone. Less than 10% of the isolates were resistant to the other anti-microbials except ampicillin, to which 13.5% were resistant. One isolate was resistant to ampicillin,chloramphenicol, cotrimoxazole and nalidixic acid.

keywords Salmonella enteritidis, enteric pathogens, resistance, Zimbabwe

correspondence Dr Clifford Simango, Department of Medical Laboratory Sciences, University ofZimbabwe Medical School, PO Box A178, Avondale, Harare, Zimbabwe

Introduction

Human infections with Salmonella enteritidis have beenincreasing wordwide since 1980. S. enteritidis is an importantserotype of Salmonella comprising 37% of Salmonella iso-lates in Maryland since 1990 (Morris et al. 1992) and 28% inthe north-eastern United States in 1986 (St Louis et al. 1988).It is normally found in the intestinal tracts of humans and ofanimals such as chickens and domestic livestock and remainsan important cause of gastroenteritis in humans worldwide.It is usually acquired by ingestion of contaminated water andfood, mainly poultry, eggs and egg products (Humphrey et al.1988, 1989; St Louis et al. 1988; Cowden et al. 1989; Hedberget al. 1993). Eggs are the most probable primary source ofcontamination in many outbreaks of S. enteritidis infectionin developed countries (St Louis et al. 1988).

Phage-typing of S. enteritidis in the United States showedthat type 8 strains are the most common (Hickman-Brenneret al. 1991; Usera et al. 1994). In Europe the most commonphage type is 4 (Ward et al. 1987; Cowden et al. 1989; Stanelyet al. 1992; Schroeter et al. 1994). Data on the prevalence of S.enteritidis as a causative agent of diarrhoea in developingcountries are scarce. We aimed to determine the prevalence ofSalmonella enteritidis as a causative agent of diarrhoea in anurban area in Zimbabwe.

Materials and methods

The study was conducted over a period of 8 months duringthe hot and wet season (October – May), when diarrhoeacaused by bacteria peaks in Zimbabwe (Cruickshank &Zilberg 1976). We investigated stool specimens from allpeople with diarrhoea presenting as outpatients at primarylevel health centres in the city of Harare. Specimens were col-lected in sterile containers and processed within 5 h.

The specimens were cultured for Salmonella, Shigella andCampylobacter species, enteropathogenic Escherichia coliand Yersinia enterocolitica. We also looked for Giardialamblia, Cryptosporidium and Entamoeba histolytica para-sites. Xylose-Lysine-Desoxycholate agar medium was usedfor culturing Salmonella species. Inoculated agar plates wereincubated at 37 �C for 24 h. Colonies which appeared pink-red with a black centre were presumptively identified asSalmonella species and biochemically tested (Cheesbrough1985). Isolates which gave positive reactions for Salmonellawere serotyped using polyvalent and monovalent antisera(Wellcome Diagnostics, Dartford, UK). The first 46 S. enteri-tidis isolates were sent to the Instituto Superiore della Sanitàin Italy for phage-typing. Other bacterial enteric pathogenswere identified by standard procedures (Cheesbrough 1985).The formol-ether concentration technique (Allen & Ridley

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Tropical Medicine and International Health volume 5 no 7 pp 503–506 july 2000

C. Simango & C. Mbewe Salmonella enteritidis diarrhoea in Zimbabwe

1970) was used for parasites and staining with modified ZiehlNeelsen stain (Cheesbrough 1987) for Cryptosporidiumoocysts.

The Kirby–Bauer disc diffusion method was used to testfor susceptibility to ampicillin (10 �g) chloramphenicol(30 �g), cotrimoxazole (25 �g), tetracycline (30 �g), gentam-icin (10 �g), nalidixic acid (30 �g), ciprofloxacin (5 �g), andceftriaxone (30 �g) using Mueller Hinton agar medium.

Results

We processed 4155 stool specimens for the detection ofenteric pathogens. Their distribution is shown in Table 1.Shigella species were the most common bacterial entericpathogens, followed by Salmonella and then enteropatho-genic Escherichia coli. S. enteritidis comprised 30.7% of the

Salmonella isolates. Giardia lamblia was more common thanCryptosporidium. Multiple infections were observed in 23specimens but only one specimen had S. enteritidis isolatedtogether with another pathogen.

The distribution of S. enteritidis by age group is shownin Table 2. The highest incidence was observed in people� 50 years (3.8%), the lowest in children � 10 years (1.2%).The most common S. enteritidis phage type was 4 (78.3%)followed by types 7, 9 and 23 (Table 3). All S. enteritidis iso-lates were sensitive to gentamicin, ciprofloxacin and ceftri-axone as shown in Table 4. Less than 10% of the isolateswere resistant to chloramphenicol, nalidixic acid, cotrimoxa-zole and tetracycline. The highest resistance was observedagainst ampicillin with 13.5% of the isolates being resistantto this drug. About 19% of the S. enteritidis isolates wereresistant to one or more drugs (Table 5). One isolate showedmultidrug resistance to ampicillin, chloramphenicol, cotri-moxazole and nalidixic acid.

Discussion

Salmonella enteritidis infections are on the increase in manydeveloped countries and have been associated with consump-tion of contaminated poultry, eggs and egg products. We foundthat S. enteritidis is the most common Salmonella serotype in

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Table 1 Incidence of bacterial and parasitic enteric pathogens inisolates from patients with diarrhoea

Pathogen Number of isolates (n � 4155) %

Shigella flexneri 0250 06.0Shigella sonnei 0192 04.6Shigella dysenteriae 0088 0 2.1Shigella boydii 0033 00.8Salmonella enteritidis 0074 01.8Salmonella Group B 0061 0 1.5Salmonella Group C 0060 01.4Other Salmonella Group D 0029 00.7Salmonella Group E 0003 00.1Other Salmonella species 0014 00.3EPEC 0086 02.1Campylobacter species 0053 0 1.3Yersinia enterocolitica 0 17 00.4Giardia lamblia 0057 0 1.4Cryptosporidium 0021 0 0.5Specimens with pathogens 1015 (24.4)

EPEC � Enteropathogenic Escherichia coli.

Table 2 Age distribution of patients with diarrhoea due toSalmonella enteritidis

Age group Number of specimens Number of specimensin years processed positive for S. enteritidis %

0–10 2261 28 1.211–20 0239 06 2.521–30 0593 13 2.231–40 0596 15 2.541–50 0230 03 1.351 + 0236 09 3.8

Table 3 Distribution of phage types of Salmonella enteritidis

Number of isolatesPhage type (n � 46) %

04 36 78.307 04 08.709 01 02.223 01 02.2Untypable 04 08.7

Table 4 Antimicrobial susceptibilities of Salmonella enteritidisisolates

Number sensitiveAntibiotic (n � 74) %

Gentamicin (10 �g) 74 100Ciprofloxacin (5 �g) 74 100Ceftriaxone (30 �g) 74 100Chloramphenicol (30 �g) 72 097.3Nalidixic acid (30 �g) 72 097.3Cotrimoxazole (25 �g) 71 095.9Tetracycline (30 �g) 70 094.6Ampicillin (10 �g) 64 086.5

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diarrhoeal stool specimens, accounting for 30.7% and detectedin 1.8% of all specimens cultured. Similar observations werenoted by Morris et al. (1992) in the United States where S.enteritidis accounted for 37% in Maryland, and by St Louis etal. (1988), who found that it comprised 28% of Salmonella iso-lates in the north-eastern United States.

We found S. enteritidis phage type 4 to be the most preva-lent, which agrees with findings in Europe (Ward et al. 1987;Cowden et al. 1989; Stanely et al. 1992; Schroeter et al. 1994)as well as Japan (Suzuki et al. 1995) and Thailand (Boonmaret al. 1998) since 1980. The predominance of this phage typein many countries suggests that S. enteritidis phage type 4strains may possess unique characteristics and pose a greatrisk to public health in some regions of the world. Phage type8, the most common in the United States (Hickman-Brenneret al. 1991) and Canada (Khakhria et al. 1991), was not foundin our study. Phage type 7 was the second most common typeamong Zimbabwean isolates but was comparatively rare.

Some studies have implicated poultry and poultry productsas the main source of human infection with S. enteritidis (StLouis et al. 1988; Humphrey et al. 1988, 1989; Cowden et al.1989; Hedberg et al. 1993). A study in Zimbabwe (unpub-lished data), which examined 113 S. enteritidis isolates inchickens and eggs from areas around Harare, identified phagetype 4 as the most common (77.9%) followed by phage type 7(14.2%), with a similar distribution pattern to that of humanisolates observed in this study. The similarity of phage typedistributions in humans and chickens suggests that chickenand egg consumption is an important cause of S. enteritidis,and that phage type 4 is the most important in humans andchickens in Zimbabwe. The other phage types were detectedin comparatively low rates and appear to play a minor role ininfections of both humans and chickens.

Although antimicrobial drugs are not recommended for thetreatment of uncomplicated salmonellosis in humans, theyare essential for the treatment of extraintestinal S. enteritidisinfections such as septicaemia, endocarditis, empyema andmeningitis. A few isolates of S. enteritidis in our study were

resistant to one or more of the eight antimicrobial drugstested, with one isolate displaying multidrug resistance toampicillin, chloramphenicol, cotrimoxazole and nalidixicacid. Resistance to antimicrobial drugs in S. enteritidis iso-lated from humans has been shown to be low, as confirmed inour study. All S. enteritidis isolates were sensitive to cipro-floxacin, gentamicin and ceftriaxone; hence these are poten-tially useful in cases showing resistance to the antimicrobials.However, S. enteritidis resistance to some antibiotics is grow-ing: a substantial increase in the incidence of resistance toampicillin was observed in England and Wales (Ward et al.1990), and Tassios et al. (1997) found a high proportion of S.enteritidis to be resistant to ampicillin and doxycycline inGreece. The increase in resistance of S. enteritidis to ampi-cillin is due to a 34-MDa conjugative plasmid (Vatopoulos etal. 1994) which is easily transferred to recipient cells (Tassioset al. 1997). Multiple resistance has remained rare amongsthuman isolates of S. enteritidis, as was also observed in thepresent study, where only one isolate displayed resistance tofour antimicrobial drugs.

Acknowledgements

We would like to thank the Istituto Superiore della Sanitàin Italy for phage-typing the Salmonella enteritidis isolatesand Dr Makaya of the Veterinary Laboratory Services inZimbabwe, who made the necessary arrangements.

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Table 5 Patterns of resistance in Salmonella enteritidis

Antibiotic Number resistant

Ampicillin 05Tetracycline 03Nalidixic acid 01Ampicillin � chloramphenicol 01Ampicillin � tetracycline 01Ampicillin � cotrimoxazole 02Ampicillin � chloramphenicol �

cotrimoxazole � nalidixic acid 01Total 14

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Salmonella enteritidis diarrhoea in Harare, Zimbabwe