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A closer look at Salmonellosis
Two recent wide-scale Salmonella food-poisonings and an unusual event in farm animals, reported from distant parts of the globe, will be briefly described in this column. Before doing so, a summarized review of Salmonellosis may be in place.
According to the latest nomenclature, the genus Salmonella consists of two major species: Salmonella enterica and Salmonella bongori.
S. enterica is divided into six subspecies, which are distinguishable by certain biochemical characteristics. Strains of Salmonella are classified into serovars on the basis of extensive diversity of lipopolysaccharide antigens (O) and flagellar protein antigens (H) in accordance with the Kauffmann–White scheme. Currently, approximately 2,500 serovars are recognized and this number is constantly being increased. The most common serovars that cause infections in humans and food animals belong to subspecies enterica. The serovars of the other subspecies are more likely to be found in poikilothermic (cold-blooded) animals and in the environment, but are occasionally associated with human disease.
Salmonellosis is an infectious disease of humans and animals caused by organisms of the two mentioned species, S. enterica, and S. bongori. Although primarily intestinal bacteria, salmonellae are widespread in the environment and commonly found in farm effluents, human sewage and in any material subject to fecal contamination. Salmonella organisms are etiological agents of diarrheal and systemic infections in humans, most commonly as secondary contaminants of food originating from animals and the environment, usually as a consequence of subclinical infection in food animals leading to contamination of meat, eggs and milk or secondary contamination of fruits and vegetables that have been fertilized or irrigated by fecal wastes, as the case may have been in one of the examples included in this column.
Human salmonellosis is one of the most common and economically important zoonotic diseases. Salmonella organisms may also be found in feedstuffs, causing gastro-intestinal asymptomatic carriage or infectious disease in animals, particularly poultry and pigs. Salmonellosis has been recognized in all countries but appears to be most prevalent in areas of intensive animal husbandry, especially in pigs and calves and some types of poultry reared in confinement. Reptiles are commonly asymptomatic carriers of Salmonella. Several serovars are host specific (eg, S. Abortusovis in sheep, S. Typhi in humans, S. pullorum in poultry) or host adapted (e.g. S. Choleraesuis in swine, S. Dublin in bovines).
The disease can affect all species of domestic animals; young animals and pregnant and lactating animals are the most susceptible. Enteric disease is the most common clinical manifestation, but a wide range of clinical signs, which include acute septicemia, abortion, arthritis and respiratory disease, may be seen. Many animals, especially pigs and poultry, may also be subclinically infected. Such animals may be important in relation to the spread of infection between flocks and herds and as sources of food contamination and human infection.
Outbreaks
Between April and July 2008, more than 1,300 persons infected with S. Saintpaul with the same genetic fingerprint have been identified in the United States (in 43 states and the District of Columbia) and Canada. Epidemiological studies comparing ill persons to well persons in the affected areas found that ill persons were more likely to have recently consumed raw tomatoes, fresh jalapeño peppers and fresh cilantro. Since the items were commonly, though not always, consumed together, the study could not determine which item(s) caused the illnesses. An FDA laboratory detected S. Saintpaul with the outbreak strain fingerprint pattern in a sample of jalapeño pepper obtained from a distribution center in McAllen, Texas. The peppers were grown in Mexico; investigators are working to determine where they were contaminated. Contaminated jalapeño pepper was discovered also in Colorado, but final conclusions were not available at the time of writing (late August 2008), when the outbreak seemed to be ongoing though with fewer new illnesses reported.
Another serovar, S. agona, has been incriminated as the causative agent of an outbreak of gastroenteritis affecting residents in the United Kingdom, Ireland, Finland, Sweden and France. Investigations led to the suspicion that this outbreak, which probably started in February 2008, is possibly linked to a meat plant in Ireland. The suspicion of an unusual event was raised when the Irish National Salmonella Reference Laboratory reported six isolates of S. Agona received over the previous three weeks, regarded as an unusual finding since there were a total of three isolates in 2007, five in 2006, and 10 in 2005. The temporal association of six isolates of an uncommon serotype suggested a possible link between cases. An outbreak was declared on July 16, 2008. Colleagues in the United Kingdom Surveillance Centers were notified on that day. Colleagues in England, Scotland and Wales reported an increase in reports of S. Agona during the end of June and early July 2008. Alerts were posted through the Food- and Waterborne Diseases network and the European Union Early Warning and Response System. Subsequently, single cases were reported in Northern Ireland, England, Finland, Sweden and France. The epidemiological descriptive study has demonstrated that at least 10 cases had eaten sandwiches containing one of the products from the company in question.
The third Salmonella-related event is not of the same magnitude and is limited to animal involvement. It is rather exceptional in view of the severe control measures applied. The case relates to five farms in Skane, Sweden, where animals were found infected by S. Reading, not an overly common serovar. The authorities were worried that while all infected farms are in a small area, they don’t have any contact links other than geography. A common link between some of the herds is a stream, where samples have intermittent occasional contamination of both water and sediment. It was decided to destroy 700 calves, lambs, and bulls on the farms and to compensate the owners.
Control policy
The “stamping-out” of 700 domestic ruminants because of contamination with Salmonella bacteria may seem an excessive zoosanitary measure but deserves to be seen in the context of Sweden’s unique Salmonella control policy. According to this policy, all serotypes of Salmonella are regarded as unacceptable, and the legislation on salmonella control includes all serotypes because they are considered potentially pathogenic for humans. Any suspicion of Salmonella in animals is notifiable, and restrictions must be put on the infected holding, such as a ban on all animal movements. The overall strategy of the Swedish salmonella control program is to prevent Salmonella in any part of the production chain, from feed to food of animal origin, to monitor the whole chain, and to eliminate infection/contamination with salmonella whenever found.
In case of positive samples, trace back and trace forward investigations are made. A stamping-out policy is practiced whenever Salmonella is detected in poultry. This is followed by thorough cleaning and disinfection, and environmental sampling before repopulation is permitted. In other animal species, the on-farm eradication strategy depends on the situation and type of production.
The World Health Organization provides information on the development of appropriate measures for the prevention and control of food-borne diseases, including Salmonella infections, of humans. The most common vehicles of infection are eggs and egg products, poultry meat and meat from other food animals, and meat products. Contaminated salad crops and spices have also been involved in numerous outbreaks. S. Enteritidis and S. Typhimurium are the most widespread serovars in many European countries while S. Typhimurium is the dominant serovar in North America.
A Salmonella control policy for public health purposes should cover all stages from ‘the stable to the table’. It should include the mandatory reporting of all outbreaks of the disease and animal, food and feed testing. Feed monitoring includes sampling of compound feed and other feed materials that are fed unprocessed, and ingredients, as well as sampling during feed processing. Health and hygiene controls at slaughter are essential, and special precautions should also be applied when slaughtering potentially infected herds. Decontamination measures should be implemented during processing.
Vaccines are increasingly used to reduce Salmonella in poultry, and it is essential that these can be distinguished from field infections for monitoring purposes and to ensure that the vaccines do not spread beyond the vaccinated group of animals.
Another essential element in the prophylaxis of human salmonellosis is retailer and consumer education; in particular, awareness of safe handling and storage of food, kitchen hygiene and proper cooking to limit the risk of infection.
For more information:
- Arnon Shimshony, DVM, is Associate Professor at the Koret School of Veterinary Medicine Hebrew University of Jerusalem, Rehovot, and is the ProMED-mail Animal Diseases and Zoonoses Moderator. Dr. Shimshony was Chief Veterinary Officer, State of Israel, from 1974 to 1999.