Identifying antimicrobial resistance genes of human clinical relevance within Salmonella isolated from food animals in Great Britain.

OBJECTIVES: To investigate the occurrence of antimicrobial resistance genes of human clinical relevance in Salmonella isolated from livestock in Great Britain. METHODS: Two hundred and twenty-five Salmonella enterica isolates were characterized using an antimicrobial resistance gene chip and disc diffusion assays. Plasmid profiling, conjugation experiments and identification of Salmonella genomic island 1 (SGI1) were performed for selected isolates. RESULTS: Approximately 43% of Salmonella harboured single or multiple antimicrobial resistance genes with pig isolates showing the highest numbers where 96% of Salmonella Typhimurium harboured one or more resistance genes. Isolates harbouring multiple resistances divided into three groups. Group 1 isolates harboured ampicillin/streptomycin/sulphonamide/tetracycline resistance and similar phenotypes. This group contained isolates from pigs, cattle and poultry that were from several serovars including Typhimurium, 4,[5],12:i:-, Derby, Ohio and Indiana. All Group 2 isolates were from pigs and were Salmonella Typhimurium. They contained a non-sul-type class 1 integron and up to 13 transferrable resistances. All Group 3 isolates harboured a class 1 integron and were isolated from all animal species included in the study. Most isolates were Salmonella Typhimurium and harboured SGI1. CONCLUSIONS: Salmonella isolated from livestock was shown to harbour antimicrobial resistance genes although no or little resistance to third-generation cephalosporins or ciprofloxacin, respectively, was detected. The preponderance in pigs of multidrug-resistant Salmonella Typhimurium makes it important to introduce control measures such as improved biosecurity to ensure that they do not pass through the food chain and limit human therapeutic options.

Ecological meta-analysis of density-dependent processes in the transmission of lymphatic filariasis: survival of infected vectors.

The survival rate of infected vectors represents one of the fundamental components that influence the transmission dynamics of mosquito-borne diseases. Despite the occurrence of a number of studies investigating mosquito survival after infection with filarial worms, there remains conflicting evidence from both laboratory and field experiments as to the existence and mechanism for parasite-induced mortality among filarial mosquitoes. Here, we used a mixed effects meta-analytical framework to combine the data from all available vector-human host blood feeding experiments to evaluate the evidence for the impact of parasite load on the mortality rates of the three major lymphatic filariasis transmitting mosquito genera, Culex, Aedes, and Anopheles mosquitoes, over the extrinsic incubation period of parasitic infection. The results show that, despite the application of this approach, or in the case of Anopheles using a convention fixed effects logistic regression analysis supplemented with additional survival analysis of longitudinal data, no strong association between mortality rate and microfilariae (mf) uptake for either of the three mosquito genera is apparent in the combined data. Instead, a key finding is that study effects played a more crucial role in determining the levels of mortality observed in these experimental studies. This was most revealing in the case of Culex, given that the largest single study in terms of both the number of data points and range of mf intensities, in contrast to smaller studies, showed a significant positive association between mf intensity and mortality, indicating that in this genus at least, the detrimental effect of infection may be manifested only at the highest mf intakes. Although no density dependence in vector mortality was also observed for Aedes, possibly because of the use of restricted human mf intensity range in previous studies, an intriguing finding was that a significantly higher overall mortality was observed for this genus over mfintake ranges that produced much less corresponding mortality in Culex and Anopheles. The results also indicate that currently very little can be said about the survival rate of Anopheles mosquitoes infected with filarial worms because of the striking paucity of data for this genus. Further studies, using standardized methods and covering an appropriate range of mf uptake intensities and using study frameworks that allow the design and comparison of data from both experimental and field experiments, are clearly indicated if we are to reliably quantify the likely effect of filarial infection on vector survival.

Farm practices to control E. coli O157 in young cattle--a randomised controlled trial.

A randomised controlled trial was used to investigate the effect of three complex management intervention packages to reduce the burden of E. coli O157 in groups of young-stock on cattle farms in England and Wales. All intervention farms were assigned measures to avoid buying in new animals and having direct contact or sharing water sources with other cattle. Furthermore, package A (7 farms) aimed to keep a clean environment and closed groups of young-stock; package B (14 farms) aimed for improved water and feed hygiene, whilst package C was assigned both A and B. The control farms (26 farms) were asked not to alter their practices. Farms, which were assigned intervention package A, exhibited a 48% reduction in E. coli O157 burden over the 4.5 months (average) of observation, compared to 18% on the control farms. The effect of package A compared to the control farms in a crude intention-to-treat model was RR = 0.26 (p=0.122). When the risk ratio was adjusted for actual application of the different measures, the effect of intervention package A became stronger and statistically significant (RR = 0.14 p=0.032). Statistical evidence (p< 0.05) showed that dry bedding and maintaining animals in the same groups were the most important measures within the package and weak evidence (p< 0.1) showed that a closed herd policy and no contact with other cattle may also be of importance. Compliance with the other measures in package A had no influence on the effect of the package. No evidence of effect of the other two intervention packages was found.

Identification of management risk factors for VTEC O157 in young-stock in England and Wales.

We conducted a cross-sectional study on 255 cattle farms in England and Wales to identify risk factors for verocytotoxin-producing E. coli O157 (VTEC). Exposure variables were collected at the levels of the farm and of the group of young-stock within the farms. On each farm a group of young-stock (6-18 months of age) was sampled to establish VTEC status. In our multiple logistic regression, farm VTEC status was associated with access to springs (OR: 0.31, CI95%: 0.12, 0.78) and assessing the wetness of the bedding material less frequently than daily (OR: 3.89 CI95%: 1.5, 10.2). At group-level we found no associated risk factors for animals housed outdoors in fields. Significant for groups housed in pens were wet bedding (wet OR: 3.43, CI95%: 1.3, 9.4; very wet OR: 4.24, CI95%: 1.2, 14.6), number of animals in the group (10-15 OR: 2.72, CI95%: 0.75, 9.9, 16-24, OR: 3.78, CI95%: 1.2, 12.3; >25 OR: 3.78, CI95%: 1.1, 12.7) and feeding straw (OR: 2.29, CI95%: 1.2, 5.5).

Mathematical models and lymphatic filariasis control: endpoints and optimal interventions.

The current global initiative to eliminate lymphatic filariasis is a major renewed commitment to reduce or eliminate the burden of one of the major helminth infections from resource-poor communities of the world. Mathematical models of filariasis transmission can serve as an effective tool for guiding the scientific development and management of successful community-level intervention programmes by acting as analytical frameworks for integrating knowledge regarding parasite transmission dynamics with programmatic factors. However, the power of these tools for supporting control interventions will be realized fully only if researchers address the current uncertainties and gaps in data and knowledge of filarial population dynamics and the effectiveness of currently proposed filariasis intervention options.