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abstract & commentary
Synopsis: Nontyphoidal Salmonella account for an estimated 2-3 million cases a year of foodborne infection in the United States. A global food market, increasing numbers of immunocompromised patients, and emerging resistance to both fluoroquinolones and third-generation cephalosporins will guarantee a place for these bacteria at the top of the public health agenda and in the headlines for the foreseeable future.
Source: Hohmann EL. Nontyphoidal salmonellosis. Clin Infect Dis. 2001;32:263-269.
The 2500 plus serotypes (or serovars) among the nontyphoidal Salmonella, strictly speaking, belong to a single species, Salmonella cholerasuis. Nonetheless, nomenclature can be confusing, even bewildering, and so Salmonella enterica has been proposed as an inclusive name followed by a serotype (eg, serotype Typhimurium). However we name these bacteria, they infect an estimated 1-3 million people annually in the United States. Infection is mainly by ingestion of infected or contaminated foods, particularly undercooked chicken and uncooked eggs, but it seems almost any food item can harbour Salmonella—even dried cereals. Fresh vegetables are probably now an unsafe bet—particularly those that are likely to be eaten raw like lettuce. The increasingly global food market ensures produce is distributed all over the world, an opportunity which no self-respecting Salmonella could refuse. Livestock and pets are also world travellers and perfect vehicles for distributing Salmonella (if nothing else, the current foot-and-mouth crisis in Britain shows how quickly and how far livestock travel and how little control there is over it). Consequently, the potential sources of Salmonella are more likely to expand than contract.
The pool of potential victims of gastrointestinal (GI) salmonellosis is also greater as more people take H2 blockers and proton pump inhibitors breaking down the barrier created by stomach acid. In addition, antibiotic use continues to rise leaving a distinctly perturbed resident microbial flora in its wake.
GI salmonellosis is generally self-limiting, and treatment with antibiotics is not warranted for otherwise healthy individuals. When prescribed, antibiotic therapy might shorten clinical illness by a couple of days and temporarily suppress viable bacteria in stools, but carriage of Salmonella will not, in fact, be ended and might even be prolonged.
In the United States, some 5% of patients with GI salmonellosis develop bacteraemia, and a proportion of these will go on to develop more serious complications such as arteritis and endocarditis. The immunity (in the broadest sense of the word) of these patients is almost invariably compromised (corticosteroids use, malignancy, diabetes, HIV). In fact, bacteraemia due to non-typhoidal Salmonella should always prompt inquiries to uncover compromised immunity. Assuming the risk of bacteremia is 1 in 20 and strikes 2-3 million victims of GI salmonellosis per year in the United States, some 500,000 patients with compromised immunity fall prey to this tenacious bacterium each year. This number is set to rise inexorably as more patients receive treatment for malignancies and haematopoietic stem cell transplants providing a ready supply of yet more potential victims of systemic disease. These patients will need treatment but with what? This used to be relatively straightforward: ampicillin or sulfamethoxazole-trimethoprim. Such certainties are firmly a thing of the past, and treatment should be guided by the susceptibility of the strain isolated from each individual patient.
Comment by J. Peter Donnelly, PhD
The rise to prominence of Salmonella DT104 which carries the genes for resistance to ACSSuT (ampicillin, chloramphencol, streptomycin, sulfamethoxazole, tetracycline), has further dented confidence and demonstrated again just how adaptable and robust Salmonella are. The fluoroquinolones provided some solace, but this too was short-lived when, predictably and inevitably, DT104 strains resistant to the fluoroquinolones entered the arena as more cases of GI salmonellosis due to this strain began to appear. Although this was linked to use of enrofloxacin to treat livestock, it could just as easily have been brought about by prescribing fluoroquinolones for patients, a practice that provides more psychological than physical relief. More worrying, further selective pressure is already abroad since Salmonella appears receptive to a plasmid shuttling among its enteric cousins that confers resistance to third-generation cephalosporins and aminoglycosides as well as ACSSuT. The spectre of losing cephalosporins for therapy because of resistance looms large.
How to halt this trend is anybody’s guess. In Europe, at least, there is little confidence in regulatory authorities being able to do anything except wring their hands when confronted with yet more "food issues." The catalogue of crises involving Salmonella, E coli, dioxins, bovine spongiform encephalopathy, and genetically modified foods has seen to that. Consumers now feel that they are effectively being warned to consume food "at their own risk." Many are already choosing to go without suspect foods rather than take any more risks. Public enthusiasm for intensive farming has waned perhaps irreversibly and bio- or eco-foods are now in political and economic demand. This should at least help decrease a number of reservoirs of Salmonella and perhaps even reduce the opportunities for dissemination. The little remaining confidence in government officials who pledged to safeguard the public’s health will not be so easy to reverse. Perhaps governments will also now have the guts to start dismantling some of the artificial boundaries separating human and animal welfare. Encouraging food producers to adopt a more environmentally friendly and humane way of rearing livestock for us to eat would seem a good place to start.