Human salmonellosis : impact of travel and trade from a Swedish perspective

Abstract

Background: Salmonellosis is one of the most important gastrointestinal infections in humans. The vast majority of Swedish Salmonella cases have acquired the disease abroad, reflecting both a large number of Swedish travellers and a very favourable domestic Salmonella situation compared to other countries, mainly due to various control programs implemented in the animal food production and import regulations (e.g. on reptiles). Some of these programs and regulations were changed when in 1995 Sweden joined the European Union (EU). The aim of this thesis is to describe and analyse the impact of travel and trade on the epidemiological situation of human salmonellosis in Sweden.

Methods: The basis for the five studies in the thesis is the national database on notified Salmonella infections, from which we extracted case-based information on age, sex, area of residence, country of infection, Salmonella serotype, and reptile contacts. For comparison in Papers II and III, we used a comprehensive database on the patterns of over-night travels abroad among Swedish residents as travel denominator to calculate risks per 100,000 travellers, but also to estimate the incidence of this infection in the various EU countries, using Norway as reference. In Paper IV, Salmonella isolates isolated from humans and sewage sludge from the same residential areas were compared using genetic typing method and antibiotic susceptibility testing. Paper V was a case-control study, comparing knowledge and behaviours and risk of salmonellosis in reported travel-associated cases and randomly selected travellers from destinations outside EU.

Results: In Paper I, we could show rapidly increasing incidences of reptile-associated salmonellosis, after EU harmonisations of import rules, but also how this trend could be broken by active information. In Paper II, we showed the risk of travel-associated salmonellosis to be highest in East Africa and the Indian subcontinent. Children aged 0-6 years were at higher risk than travellers of other ages. There are also marked geographical differences in serotype distribution between various regions of the world, in Europe S. Enteritidis was especially dominating. In Paper III we estimated the "true" European salmonella incidence to be highest in Bulgaria, Turkey and Malta, and that a severe underreporting in the official figures from some countries make official data useless for comparisons. As the egg-related S. Enteritidis was the dominating serotype, limiting Salmonella in European poultry could have a major public health impact. In Paper IV, we could demonstrate that Salmonella isolates from sewage treatment plants probably originated from infected humans and survived the treatment at the plants. It also highlighted the risk of spreading antibiotic resistant Salmonella from sewage sludge to the environment. In Paper V, we could show that knowledge and advice are not enough to decrease the risk of travel-associated salmonellosis, while actual behaviour of strictly avoiding high risk food-items reduces the risk by approximately 70%. Still the weekly risk for salmonellosis in these travellers would be more than 100 times higher compared to staying at home in Sweden.