Yersinia enterocolitica: Epidemiological Studies and Outbreaks

Yersinia enterocolitica is the most common bacteriological cause of gastrointestinal disease in many developed and developing countries. Although contaminated food is the main source of human infection due to Y. enterocolitica, animal reservoir and contaminated environment are also considered as other possible infection sources for human in epidemiological studies. Molecular based epidemiological studies are found to be more efficient in investigating the occurrence of human pathogenic Y. enterocolitica in natural samples, in addition to conventional culture based studies.


Introduction
Foodborne diseases are a widespread and growing public health problem in developed and developing countries [1]. Amongst those, yersiniosis due to infection with the bacterium Yersinia enterocolitica is the frequently reported zoonotic gastrointestinal disease after campylobacteriosis and salmonellosis in many developed countries, especially in temperate zones [2]. Within developed countries, incidences of yersiniosis and foodborne outbreaks are appeared to be lower in the United States than many European countries [3][4][5]. In European countries, numbers of reported cases of human in England and Wales are lower than those in other European countries where fewer than 0.1 cases of yersiniosis per 100,000 individuals were reported in the United Kingdom in 2005, in contrast to 12.2 in Finland and 6.8 in Germany [6]. On the other hand, the high prevalence of gastrointestinal illness including fatal cases due to yersiniosis is also observed in many developing countries like Bangladesh [7], Iraq [8], Iran [9], and Nigeria [10], which indicates major underlying food safety problems in low-and middle-income countries. Worldwide, infection with Y. enterocolitica occurs most often in infants and young children with common symptoms like fever, abdominal pain, and diarrhea, which is often bloody. Older children and young adults are not out of risk. The predominant symptoms within these age groups are right-sided abdominal pain and fever, sometimes confused with appendicitis. Occasionally, the Y. enterocolitica associated complications such as skin rash, joint pains, or spread of bacteria to the bloodstream can also occur.
Although Y. enterocolitica is a ubiquitous microorganism, the majority of isolates recovered from asymptomatic carriers, infected animals, contaminated food, untreated water, and contaminated environmental samples are nonpathogenic having no clinical importance [11]. At the same time, the epidemiology of Y. enterocolitica infections is complex and remains poorly understood because most sporadically occurred cases of yersiniosis are reported without an apparent source [3,[12][13][14] Other foods (oysters, mussels, shrimp, blue crab, fish, stewed mushrooms, cabbage, celery, carrots, etc.) for Yersinia virulence) [15] that encodes various virulence genes (tccC, yadA, virF, ysa) with traditional chromosomal virulence genes (inv, ail, yst) whereas other pathogenic strains, having no pYV plasmid, produce a thermostable enterotoxin (ystA) [16][17][18]. These virulence genes located in chromosome or plasmid of pathogenic Y. enterocolitica has been widely used to identify pathogenic strains in epidemiological studies for example, chromosomal ail gene [19,20].

Epidemiological Studies and Outbreaks
Many factors related to the epidemiology of Y. enterocolitica, such as human and nonhuman sources, and contamination routes in foods remain obscure in developing countries and tropical regions of developed countries. Additionally, epidemiological data on the prevalence of pathogenic Y. enterocolitica in animals in developed countries are missing as the reporting of this pathogen in animals is not mandatory in most European countries [26].

Animal Reservoirs Involved in Zoonosis.
Animals have long been suspected of being significant reservoirs for Y. enterocolitica and, therefore, sources of human infections [3]. Numerous studies have been carried out to isolate Y. enterocolitica strains from a variety of animals ( Figure 1) [56]. Interestingly, most of the strains isolated from the animal kingdom carry unique serotypes of Y. enterocolitica compared to the strains isolated from humans with yersiniosis.
Pigs have been shown to be a major reservoir of pathogenic Y. enterocolitica involved in human infections, particularly for strains of bioserotype 4/O:3 which has been almost exclusively isolated in European countries like Denmark, Italy, Belgium, Spain, and Sweden [24,64]. The rate of isolation of Y. enterocolitica including bioserotype 4/O:3 from tonsils and tongues of pigs is generally greater than the rate of isolation from cecal or fecal materials [20].
Occasionally, pathogenic Y. enterocolitica strains, mostly of bioserotype 4/O:3, have also been isolated from dogs and cats [82]. Although pigs are the primary source of human infection with Y. enterocolitica throughout world, these pets may also be a potential source of human infection with pathogenic Y. enterocolitica because of their intimate contact with people, especially young children [28].
In addition with mostly isolated bioserotype 4/O:3, Y. enterocolitica strains of biotypes 2 and 3 and serotypes O:5,27, O:8, and O:9 have also been isolated from slaughter pigs, cows, sheep, and goats; however, the reservoir of these bioserotypes is not clearly established [81,[83][84][85]. In above cases, contamination of pluck sets (tongue, tonsils, and trachea hanging together with thoracic organs such as lungs, liver, and heart) and carcasses with enteropathogenic Yersinia from tonsils and feces may occur during the slaughtering stage [5,82,[86][87][88]. On the other hand, strains of very rare bioserotypes, such as bioserotype 5/O:2,3, have been isolated from sheep, hares, and goats and bioserotype 3/O:1,2a,3 from chinchillas (small rodent). Thus, the patterns of the pathogenic strains isolated from humans with yersiniosis compared to those from the animals suggest that the human infection due to Y. enterocolitica originated from the animals.

Contaminated Food Involved in Infections.
Food has been proposed to be the main source of intestinal yersiniosis although pathogenic isolates have seldom been recovered from food samples [105]. The low recovery rates of pathogenic Y. enterocolitica in food samples may be due to limited sensitivity of culture methods [11]. However, Y. enterocolitica has been isolated from milk and milk products, egg products, raw meats (beef, pork, and lamb) and poultry, vegetables, and miscellaneous prepared food products. The occurrence of pathogenic Y. enterocolitica in natural sample including foods has been estimated by both culture-and molecular-based methods ( Table 1, Figures 2 and 3).

Contaminated Meat and Poultry Products Correlated with yersiniosis.
Indirect evidence considering food, particularly pork and pork products, indicates that there is an important link between consumption of raw, undercooked, or improperly handled pork product and human Y. enterocolitica infections [20]. This positive correlation between the consumption of raw or undercooked pork and the prevalence of yersiniosis has been demonstrated in case-control studies [32,64,[106][107][108][109]. Using molecular techniques, ail-positive Y. enterocolitica strains were detected in raw pork samples (loin, fillet, chop, ham, and minced meat) and in readyto-eat pork products [31]. However, the isolation rates of pathogenic bioserotypes of Y. enterocolitica have been low in raw pork, except for in edible pig offal, with the most common type isolated being bioserotype 4/O:3 (Table 2). In other studies, pathogenic yst-positive Y. enterocolitica strains have been isolated from ground beef [29] but not detected in chicken food samples [110].

Contaminated Milk and Milk Products Associated with
Human Disease. Y. enterocolitica has been isolated from raw milk in many countries, like Australia, Canada, Czechoslovakia, and USA. There were also a few reports on the isolation of this pathogenic strain associated with human disease from pasteurized milk [4,111]. It may be due to the malfunction in the pasteurization process leading to inadequate treatment or postprocess contamination, or it may be due to the contamination with heat-resistant strains of Y. enterocolitica. So, the presence of this pathogen in pasteurized milk should be a cause for concern. However, heat-resistant strains of Y. enterocolitica have not been still reported in milk samples.

Other Contaminated Foods Involved in Outbreaks.
Strains of Y. enterocolitica have been isolated from oysters, mussels, shrimp, blue crab, fish, salad, stewed mushrooms, cabbage, celery, and carrots [112]. In Korea, Lee et al. [113] isolated ail-positive Y. enterocolitica strain of bioserotype   [43]; selective agar media [11]; cold enrichment method [57]; biochemical & serological identification methods [58][59][60][61][62][63] 3/O:3 from ready-to-eat vegetables, which indicate that vegetables can be a source of human infection. Furthermore, Sakai et al. [34] reported an outbreak of food poisoning by Y. enterocolitica serotype O:8 in Japan where salad was proposed the cause of infection. Recently, Y. enterocolitica 2/O:9 has been isolated from chicken eggshell surfaces in Argentina [114]. Contamination of the egg surface might have occurred from contact with other Y. enterocoliticacontaminated animal products, such as pork product, during collection on farms or during transportation or handling in retail shops.

Contaminated Environment Reported as Source of Infection.
Most of the Y. enterocolitica isolates recovered from environmental samples, including the slaughterhouse, fodder, soil, and water, have been nonpathogenic [89,[115][116][117][118][119]. Occasionally, strains of bioserotype 4/O:3 have been isolated from the slaughterhouse [120,121] and sewage water [50]. Within the environmental sampling sites, drinking water has been relatively widely investigated and revealed to be a significant reservoir for nonpathogenic Y. enterocolitica. However, Sandery et al. [35] detected pathogenic Y. enterocolitica in environmental water by molecular studies. In a casecontrol study, untreated drinking water has been reported to be a risk factor for sporadic Y. enterocolitica infections in Norway [107]. Recently, Falcão et al. [122] tested 67 Y. enterocolitica strains isolated in Brazil from untreated water for the presence of virulence genes. They found that all 38 strains of serotype O:5,27 possessed inv, ail, and yst genes, suggesting that untreated water may be responsible for the human infection with Y. enterocolitica. In another study, Y. enterocolitica O:8 strains have been isolated from stream water in Japan, which indicate that stream water may be a possible infection source for human Y. enterocolitica O:8 infections [84,123].
the exploitation of apparent sources and contamination routes of human yersiniosis and in the development and implementation of effective control strategies to prevent future outbreaks. Efficient laboratory methods used for epidemiological study are also a vital requirement in Y. enterocolitica's monitoring and control purposes. Molecular methods should be needed with conventional culture methods to provide a better estimation of epidemiology of Y. enterocolitica particularly pathogenic strains in natural samples