Molecular epidemiology of antimicrobial resistance (AMR) and Shiga toxin producing E. coli (STEC) in dairy herds of central Zambia
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Antimicrobial resistance (AMR) is a worldwide public health concern. While it is evident that the use of antibiotics creates selection pressure for the evolution of antibiotic resistance genes, there are still considerable knowledge gaps relating to the status quo of antibiotic use, emergence of resistant pathogens in different livestock production systems and spread within human and animal communities. This thesis includes a survey of antibiotic use in the dairy sector within a specific area of Zambia and analysis of AMR and virulence factors in E. coli isolated from dairy cattle and diarrhoea human patients with the following objectives. 1. To investigate the usage of antibiotics in the dairy sector and the drivers for use. 2. To determine the prevalence and patterns of antimicrobial resistance in E. coli isolated from faecal samples of dairy cattle. 3. To use whole genome sequencing (WGS) to investigate the molecular epidemiology of resistance determinants in E. coli strains isolated from both dairy cattle and humans. 4. To assess the zoonotic potential of isolated E. coli focusing on Shiga toxin-producing E. coli (STEC) and relationship to STEC associated with clinical disease in the UK. In view of these objectives, the first part of the work was carried out in Zambia and involved a questionnaire, a field survey, isolation of E. coli from dairy cattle faecal samples and phenotypic testing for AMR. In addition, E. coli isolates were obtained from another study that was focused on human patients presenting with diarrhoea at the University Teaching Hospital in Lusaka. The second part involved whole genome sequencing and molecular analyses of E. coli for resistance and virulence genotypes at the Roslin Institute (UK). For the field study, a stratified random sample of 104 farms was studied, representing approximately 20% of all dairy farms in the region. On each farm, faecal samples were collected from a random sample of animals and a standardised questionnaire on the usage of antibiotics was completed. An E. coli isolate was obtained from 98.67% (371/376) of the sampled animals and tested for resistance against the six types of antibiotics (tetracycline, ampicillin, sulfamethoxazole/trimethoprim, cefpodoxime, gentamicin and ciprofloxacin). These E. coli were then analysed together with those from humans for genotypes in the laboratory and from Illumina short read whole genome sequences using bioinformatics tools. Tetracylines and penicillin were the commonly used antibiotics in dairy herds. This finding was in line with the resistance phenotypes detected in E. coli isolated from the dairy cattle. The most prevalent AMR was to tetracycline (10.61; 95%CI: 7.40-13.82), followed by ampicillin (6.02; 95%CI: 3.31-8.73), sulfamethoxazole/ trimethoprim (4.49; 95%CI: 2.42-6.56), cefpodoxime (1.91; 95%CI: 0.46-3.36), gentamicin (0.89; 95%CI: 0.06-1.84) and ciprofloxacin (0%). The risk analysis indicated that AMR was associated with livestock diseases (lumpy skin disease and foot rot), exotic breeds (Jersey and Friesian), location, farm size and certain management practices. Analysis of whole genome sequences showed that isolates from humans had both higher levels and a greater diversity of resistance alleles than the cattle isolates. Common genotypes in both populations were: tetA (16%), tetB (10%), tetC (2%) for cattle isolates with tetA (32%), tetB (22%) and tetD (1%) in human isolates. Other common genotypes were blaTEM (56%), sul1 (29%), sul2 (66%), strA4 (57%) and strB1 (64%) in isolates of human origin while blaTEM (15%), sul1 (3%), sul2 (17%), strA4 (13%) and strB1 (19%) were in the cattle isolates. Whilst the E. coli isolates from cattle encoded resistance to common antibiotics of limited significance to human clinical medicine, isolates from humans had additional extended spectrum beta-lactamases (blaOXA, blaCMY, blaNDM, and blaDHA, blaOKP and blaCTX-M) that encode for resistance to essential antibiotics such as third generation cephalosporins and carbapenems. This was an evidence that AMR is an ongoing public health subject in Zambia but the exclusivity of certain resistances in the human population points to limited or no exchange of genotypes between E. coli of human origin and those from cattle. AMR in humans was probably independently selected by the use of antibiotics of clinical importance such as cephalosporin and fluoroquinolones. The virulence analysis focused on STEC, 11% (41/371) of E. coli isolates from cattle contained Shiga toxin genes (stx) while none (0/73) of the human isolates were positive. Phylogenetic analysis showed a random distribution of bovine STEC, with no indication of clonal spread. Although 89% (16/18) of the STEC tested had a cytotoxic effect on Vero cells, indicative of Shiga toxin production, only three (O45, O111, O157) belonged to one of the seven serogroups (O26, O157, O111, O103, O121, O145 and O45) associated with life-threatening enterohaemorrhagic E. coli (EHEC) infections in humans. In line with this, only the O157 serotype encoded a type 3 secretion system. This shows that, while Stx-encoding strains are common in these dairy herds of Zambia, they are not strain backgrounds known to pose an immediate threat to human health as they lack colonisation factors that are found in typical human EHEC. However, we must remain vigilant as emergence of EHEC strains in these animals remains an ever-present threat.