Increased Prevalence of Antimicrobial Resistance in Vibrio cholerae in the Capital and Provincial Areas of Zambia, January 2023–February 2024

Cholera is an acute diarrheal disease caused by Vibrio cholerae infection. Increased prevalence of multidrug-resistant strains has been a public health concern in Africa as antibiotics are recommended treatment options for severely ill patients in addition to rehydration therapy.^1,2 Zambia has experienced over 30 cholera outbreaks and over 10,000 cases cumulatively between 1977 and 2019.³ The outbreak that occurred in the capital, Lusaka, between October 2017 and May 2018 caused more than 5,000 cases and resulted in 90 fatal cases.⁴ This called for a large-scale public health response, which was coordinated by the Ministry of Health through the Zambia National Public Health Institute (ZNPHI).⁴

In 2023–2024, cholera cases were reported from known cholera hot spot districts (e.g., Nchelenge and Lusaka districts) and those where cholera outbreaks had not been previously reported (e.g., Vubwi district).^5,6 In Lusaka district, the first cholera case was reported in the Kanyama subdistrict, a high-density periurban area of the city, on October 15, 2023.⁷ The number of cases increased rapidly across other districts in Lusaka province, and the epidemic reached its peak on January 8, 2024. As of March 1, 2024, a total of 20,577 cases and 699 deaths were reported nationwide.⁸ During the outbreak, the Zambia National Public Health Reference Laboratory (ZNPHRL), operated by ZNPHI, conducted laboratory testing for suspected cholera cases as part of its mandate as a center of excellence in providing public health laboratory services. This study aimed to determine the antimicrobial susceptibility profile of V. cholerae isolated during the 2023–2024 outbreaks in Lusaka and other areas of Zambia by analyzing laboratory test results at ZNPHRL.

We tested a total of 2,384 stool samples that were collected from patients suspected for cholera at health care facilities in Eastern, Lusaka, and Luapula provinces in Zambia between January 1, 2023 and February 29, 2024 (Table 1). Although all provinces were eligible, samples were submitted only from Eastern, Lusaka, and Luapula provinces. Suspected cholera cases were defined as individuals of any age group who presented with three or more loose stools within 24 hours. Stool samples were submitted to ZNPHRL through the Provincial and District Health Offices for identification of V. cholerae using standard bacterial culture and serotyping. Culture-positive samples were subjected to antimicrobial susceptibility testing (AST) against panels of antibiotics, including ampicillin (ABPC), azithromycin (AZM), chloramphenicol (CAM), ciprofloxacin (CPFX), cotrimoxazole (SXT), and tetracycline (TC), through the Kirby–Bauer disc diffusion method using Muller Hinton agar. Selection of antibiotic discs and laboratory procedures in AST was based on the Clinical & Laboratory Standards Institute (CLSI) Guideline M45.⁹ Sensitivity to AZM was assessed only for samples collected from pediatric cases under 8 years old because the first-line antibiotic treatment option for this age group was AZM because of the contraindication of TC. The inhibition diameters were interpreted and categorized as being susceptible, intermediate, or resistant based on the CLSI Guideline M45.⁹

The secondary use of the laboratory test results, which were obtained as part of the public health services of ZNPHRL, for analysis and publication was approved by the National Health Research Authority (reference number NHRA-1453/09/08/2024).

A total of 549 samples (23.5%, n = 549 of 2,341) were identified to be culture positive for V. cholerae O1, including 117 (21.3%) of serotype Inaba and 401 (73.0%) of Ogawa. The median age (interquartile range) of the patients was 25 years old (15–36), and male gender was overrepresented (55.1%, n = 267 of 485). AST results were available for 431 (78.5%) of the 549 culture-positive samples. The proportion of V. cholerae being sensitive for TC, the first-line antibiotic treatment of adults, was 84.5% (n = 316 of 374) in Lusaka, whereas it was 100% among the tested samples in Eastern (n = 34 of 34) and Luapula (n = 20 of 20) provinces (Figure 1). The first TC-resistant V. cholerae was identified in epidemiological week 48 (EW48) of 2023, and it continued to be identified until EW8 of 2024 (Figure 2). One sample was resistant for AZM, the first-line antibiotic treatment of children, in Lusaka province (1.6%, n = 1 of 61), and no samples were shown to be resistant for AZM in other provinces (Figure 1). Sensitivity for CPFX, the second-line antibiotic treatment, was 81.8% (n = 260 of 318) in Lusaka province, whereas it was 100% among the tested samples in Eastern (n = 34 of 34) and Luapula (n = 20 of 20) provinces. CAM sensitivity was 100% in Luapula province (n = 20 of 20), 46.2% (n = 139 of 301) in Lusaka province, and 0 in Eastern province (n = 0 of 34). ABPC and SXT sensitivity was similar in all provinces, ranging from 0 to 2.8% (n = 9 of 318, SXT, Lusaka) (Figure 1).

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Antimicrobial susceptibility profile of Vibrio cholerae isolates identified in Eastern, Luapula, and Lusaka provinces from January 2023 to January 2024. Proportions (percentages) of isolates categorized as being sensitive, intermediate, and resistant (and not tested) to a panel of antibiotics, including tetracycline (TC), azithromycin (AZM), ciprofloxacin (CPFX), ampicillin (ABPC), chloramphenicol (CAM), and cotrimoxazole (SXT), are indicated with bar graphs. Data presented in a previous report from CDC² are included as a reference.

Citation: The American Journal of Tropical Medicine and Hygiene 112, 4; 10.4269/ajtmh.24-0558

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Temporal distribution of tetracycline-resistant isolates in Lusaka between October 2023 and February 2024. Numbers of samples identified with Vibrio cholerae categorized as being sensitive, intermediate, and resistant to tetracycline are indicated in the bar graph based on the epidemiological week of sample collection.

Citation: The American Journal of Tropical Medicine and Hygiene 112, 4; 10.4269/ajtmh.24-0558

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In this study, antimicrobial susceptibility profiles of V. cholerae for multiple antibiotics, including TC, AZM, CPFX, ABPC, CAM, and SXT, were determined for isolates identified in the outbreaks that occurred in Eastern, Luapula, and Lusaka provinces of Zambia between January 2023 and February 2024.

Notably, 10% of the isolates identified in Lusaka in 2023–2024 were found to be resistant to TC. Furthermore, the first AZM-resistant V. cholerae was identified in Lusaka in 2023–2024. During the previous major outbreak in Lusaka (2017–2018), it was reported that all isolates (n = 50) were 100% sensitive to TC, AZM, CAM, and SXT, and sensitivity to ABPC was 72%.⁴ These data suggest that resistant isolates to the first-line antibiotic treatment might have emerged or that the proportion of such resistant isolates increased significantly after the previous outbreak in Lusaka. Our findings were in line with the global trend of the increasing rate of TC-resistant strains.¹⁰ A previous report from the Democratic Republic of Congo (DRC) showed that loss of susceptibility to the antibiotics occurs over the years and that V. cholerae can acquire resistance to multiple antibiotics in those periods.¹¹ In DRC, TC-resistant isolates were first identified in 2007, and it became one of the predominant types in the successive outbreaks after 2008 after acquiring additional resistance to CAM.¹¹ This calls for the need to carefully monitor TC-resistant V. cholerae in future outbreaks in terms of its prevalence and acquiring additional resistance.

In addition, antimicrobial-resistant isolates for the first-line treatment options, TC and AZM, and the second-line treatment option, CPFX, were identified only in Lusaka province, whereas they were not identified in other provinces in 2023–2024. Multiple factors are assumed to be underlying in such geographical distribution patterns of resistant isolates, such as increased exposure to antibiotic treatment and transmission of resistant strains in Lusaka than in other provincial areas. In fact, previous studies documented high consumption of antibiotics, inappropriate use of antibiotics in health care facilities, and access to nonprescribed antibiotics in Lusaka district.^12,13 A report from Bangladesh describes the emergence of TC- and erythromycin-resistant isolates in the capital Dhaka and its spread to provincial cities within a few years.^14,15 Our results highlight the importance of improved prescribing practices in health care facilities and surveillance systems for such resistant strains in Lusaka and other parts of Zambia.

Limitations of this study include the potential effects of the larger number of samples during the outbreak in 2023–2024 compared with that in 2017–2018, which might have enhanced the surveillance capacity to capture a small proportion of resistant isolates. However, similar patterns of these isolates presenting reduced sensitivity to other antibiotics (e.g., ABPC, CAM, and SXT) support the possibility of increased resistance to TC and AZM. In addition, the study included samples collected from patients who visited health care facilities in 3 of 10 provinces in Zambia, which might have led to sampling bias.

Continuous efforts are required to prevent the emergence and spread of antimicrobial-resistant V. cholerae by careful and longitudinal monitoring of the regional antibiogram and reduction of antibiotic dependency (e.g., vaccine campaigns) in future cholera outbreaks in Zambia.