Risk factors detection methods and control procedures

14.7.1 Risk factors

Age is an important factor in determining risk of Y. enterocolitica infection. Persons at extremes of age are most susceptible, infants aged less than 1 year being at greatest risk. Susceptibility to Yersinia infection remains high, however, until about 14 years of age. Although there is a general susceptibility to Y. ente-rocolitica among the immunocompromised, the most important predisposing conditions involve cirrhosis, or other liver disorder, and iron overload. Liver disorders and iron overload also predispose to more serious systemic infections. Hospitalised patients, in general, are at enhanced risk of Yersinia infection. Bio-serotypes not usually considered pathogenic have been involved on some occasions. Hospitalised patients, especially children, may also be at risk from Y. frederickensii and other species not usually considered pathogenic.

Most cases of yersiniosis are sporadic. The organism has been isolated from a wide range of foods, but has been associated only with outbreaks involving milk, pork, tofu and water. The carriage rate of Y. enterocolitica is high in pigs, the tonsil and surrounding tissue usually being involved, although the incidence can be particularly high when diarrhoeal symptoms are present in the animals. Carriage of Y. enterocolitica in pigs usually involves bio-serotype O:3/4, most commonly involved in human infections, although typing by pulsed-field gel electrophoresis suggests strain differences (Asplund et al., 1998). Contamination of meat inevitably occurs during butchery, although a direct causal link between pork and yersiniosis has been established in only a small number of cases. A strong epidemiological link has been established between consumption of raw pork mince and yersiniosis in Belgium, one of the few countries where raw pork is widely eaten (Tauxe et al., 1988).

The highly virulent serotypes O8 and O21 are not associated with pigs and it is unlikely that pork is a vehicle of infection. It has been suggested that wild rodents form a reservoir for these serotypes and that transmission to humans largely involves fleas, although contamination of water is also a possibility. Fleaborne transmission implies poor living conditions as a risk factor for Y. ente-rocolitica infections, although there is likely to be occupational risk for workers in agriculture, sewer maintenance and pest control.

14.7.2 Approaches to detection and enumeration

It is not usual to examine for Y. enterocolitica on a routine basis. Where examination is made, cultural methods are most commonly used. Most media will not differentiate between species of Yersinia and identification is required as part of the detection and enumeration procedure. The situation is further complicated by the fact that only certain bio-serotypes are recognised pathogens and that further testing may be required. The level to which putative isolates of Y. enterocolitica are identified depends on circumstances and the purpose of the work. In surveys, for example, identification to species level is usually sufficient and, in cooked foods, the presence of other Yersinia species may well be considered cause for concern.

Detection procedures, based on cultural methodology, have been developed specifically for Y. enterocolitica. Enrichment is necessary and may involve either incubation at low temperatures (4-15 °C) in non-selective broth (cold enrichment), or selective enrichment at 25 °C. Selective enrichment is now preferred, bile-oxalate-sorbitol (BOS) usually being the medium of choice. Alkali treatments may be applied in conjunction with enrichment where the number of competing microorganisms is high, but its value in many circumstances is doubtful (Varnam and Evans, 1996). Cephaloridin-irgasan-novobiocin medium (CIN) is widely used and is generally effective, although serotype O3 may be inhibited. If necessary, a second medium should be used to recover this serotype, a modification of Salmonella-Shigella agar being recommended.

Confirmation of identity of Y. enterocolitica may be made using commercial identification kits (incubated at 30°C). Relatively simple cultural methods also exist for determination of virulence, although interpretation can be difficult (Varnam and Evans, 1996). Rapid genetic methods for detection of Y. enteroco-litica have been developed. A number of workers have developed polymerase chain reaction (PCR) methods, the most effective selecting primers directed both at chromosomal genes and the plasmid-borne virulence, virF, gene. Advantages have been claimed in terms of selectivity and specificity (Thisted-Lambertz et al., 1996), but use in the food industry is limited and commercial kits are not currently available.

14.7.3 Control procedures

Control procedures for foodborne Y. enterocolitica are similar to those for other zoonotic pathogens; reduction of levels of contamination in the pre-process food chain, destruction by thermal processing and adequate domestic cooking and prevention of recontamination. Y. enterocolitica, however, is generally considered to be much less important a cause of human morbidity than Campylobacter or Salmonella and relatively little effort has been put into specific precautions against the organism.

Reduction of contamination in the pre-process food chain A high incidence of Y. enterocolitica carriage in pigs has been associated with particular agricultural practices, especially 'buying in' of pigs to the farm. This system is dictated by economic circumstances and is likely to continue. Some control of carriage rates can be exerted by good hygiene and it is common practice to quarantine incoming pigs before entry into a flock. Carriage of Y. entero-colitica, however, is long term and, usually, asymptomatic and the scope for control of infection at this stage is limited. Control by vaccination is a possibility, but introduction seems unlikely at present. Incidence of carriage can be modulated by a number of factors, including feeding of growth-promoting antimicrobials (Asplund et al., 1998).

Contamination of meat with zoonotic pathogens at a high level is often associated with poor slaughterhouse practice and improvements in technology may reduce carriage (Andersen, 1988). Concern over E. coli O157:H7 has enhanced efforts to reduce carcass contamination by decontamination procedures. Many decontamination treatments have been developed, 'steam-vacuuming' being con sidered particularly effective (Corry et al., 1995). Although some decontamination procedures are effective in reducing the incidence of pathogens, the possibility of enhanced growth if recontamination occurs is a general concern (Jay, 1996) and has been discussed above with respect to St. aureus. Work with Y. ente-rocolitica on pork suggests little difference in growth rate on decontaminated meat (Nissen et al., 2001), although longer shelf-life, resulting from reduction in the spoilage microflora could, potentially, permit a significant increase in numbers.

Although porcine carriage of Y. enterocolitica may involve colonisation of the gastrointestinal tract, the tonsils are more commonly involved. Control by improving standards of butchery has been proposed, with particular emphasis being placed on preventing contamination of pork mince. Although obviously desirable, the extent to which this can be achieved in industrial-scale butchery is questionable.

Elimination by processing

Yersinia enterocolitica is not unusually heat resistant and will be eliminated by adequate thermal processing, including high-temperature short-time pasteurisation of milk and generally applied processes for cooking of meat. Although there has been relatively little work on alternative processing methods in relation to the bacterium, it is unlikely that response will be significantly different from that of other members of the Enterobacteriaceae. Equally generic measures taken to prevent recontamination after processing are likely to be effective against Y. ente-rocolitica. Contamination with Y. enterocolitica, however, is potentially serious owing to its ability to grow at 4°C. Vacuum-packing offers no protection and sensitivity to preservatives is similar to that of other Enterobacteriaceae, although sodium lactate, in combination with mild heat processing, has been found to be effective in control of Y. enterocolitica in sous vide foods (McMahon et al., 1999). Carbon dioxide is also effective in limiting growth of Y. enterocolitica in modified atmosphere packed fish (Davies & Slade, 1995).

With respect to risk of contamination from food handlers, human carriage is known, although there is disagreement over the extent and significance. Figures suggesting a higher carriage rate than non-typhoid salmonellas have been quoted, but it is not clear if this is true, or convalescent, carriage. Precautions against contamination by human carriers are the same as those devised for Salmonella and include exclusion from work during acute illness, but return to be permitted after recovery, providing stools are well formed and personal hygiene good. Convalescent persons should not, however, be allowed to handle foods to be consumed without cooking until three consecutive stool samples have tested negative for Y. enterocolitica.

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