INTRODUCTION — In the genus Yersinia, three species are important human pathogens: Yersinia pestis, Yersinia enterocolitica, and Yersinia pseudotuberculosis [1,2]. The yersinioses are zoonotic infections of domestic and wild animals; humans are considered incidental hosts that do not contribute to the natural disease cycle.

Y. enterocolitica and Y. pseudotuberculosis cause yersiniosis, a diarrheal illness. Illness due to Y. enterocolitica is more common than illness due to Y. pseudotuberculosis. Overall, Y. enterocolitica infection occurs more frequently in Europe than in North America [3].

The epidemiology of yersiniosis (infection with Y. enterocolitica or Y. pseudotuberculosis) will be reviewed here. The microbiology, pathogenesis, clinical manifestations, diagnosis, and treatment of yersiniosis are discussed separately. (See "Microbiology and pathogenesis of Yersinia infections" and "Clinical manifestations and diagnosis of Yersinia infections" and "Treatment and prevention of Yersinia enterocolitica and Yersinia pseudotuberculosis infection".)

Y. pestis causes plague and is discussed separately. (See "Epidemiology, microbiology and pathogenesis of plague (Yersinia pestis infection)" and "Clinical manifestations, diagnosis, and treatment of plague (Yersinia pestis infection)".)

RESERVOIRS OF INFECTION — Y. enterocolitica strains have been isolated from a variety of vertebrate hosts, including domesticated animals as well as wildlife [4]. Healthy pigs are frequently colonized with strains that cause human illness, such as serotype O:3 and serotype O:9. In pigs, the organism colonizes the tonsils and other oropharyngeal lymphoid tissues; from these sites it can be shed into the gastrointestinal tract. Y. enterocolitica in swine herds may spread as pigs are transferred from one pig farm to another, and then can persist on a farm for many years [5]. The organisms can contaminate retail pork products, including neck trimmings, tonsillar tissue, tongue, and tripe, and can be transferred to other cuts of meat during slaughter [6,7]. Y. pseudotuberculosis has been isolated from a variety of mammals and birds. It causes epizootic disease in European brown hares in Northern Europe, sheep in Australia, and farmed deer in New Zealand [8-10].

TRANSMISSION — Transmission of yersiniosis is largely foodborne and occasionally waterborne (table 1). There are also reports of infection related to exposure to household pets [11] and transfusion of blood products [12,13].

Updated information on outbreaks may be found on websites maintained by the United States Centers for Disease Control and Prevention and the US Food and Drug Administration.

Pork consumption is a major mode of transmission for Y. enterocolitica. Case-control studies of sporadic illnesses in Belgium and Norway have noted a strong association between consumption of undercooked or raw pork products and yersiniosis [14,15]. In a Swedish study, strains of Y. enterocolitica isolated from retail pork had the same PFGE profiles as patient isolates [16]. In Norway and New Zealand, sporadic yersiniosis has been associated with drinking untreated surface water [15,17].

Yersiniosis has been associated with preparation of chitterlings, an ethnic winter holiday dish comprised of pig intestines [18-21]. This was first described during an investigation of bacteremia among bottle-fed African American infants in Atlanta, Georgia in 1989 [19]. Illness was thought to occur as a result of cross contamination of infant bottles or formula during chitterling preparation.

Other outbreaks have been related to contaminated food sources other than pork [22-26], including tofu packaged in untreated spring water [22], post-pasteurization contamination of milk, and, in Norway, mixed salad with radicchio rosso [23-27]. In 2019, an outbreak in Denmark was linked to imported spinach that was also shipped to Sweden, where a simultaneous outbreak occurred; the same strain was identified in both outbreaks through whole-genome sequencing [28]. In one outbreak, an infected food handler was a suspected source of contamination [29], although in general, direct fecal-oral person-to-person transmission has rarely been described.

In rare cases, Y. enterocolitica septicemia has been observed following transfusion with packed red cells [12]. Yersinia organisms are ferrophilic and capable of multiplying slowly in stored units of red cells at cold temperatures before they are transfused. In one prospective study conducted in 1998 to 2000, the incidence of transfusion associated Yersinia sepsis was 1 in 23.7 million red cell transfusions [30]. The number of such infections could potentially be reduced by testing blood products for bacterial contamination and by reducing storage time [31]. (See "Transfusion-transmitted bacterial infection", section on 'Microbiology'.)

Data on the transmission of Y. pseudotuberculosis are limited. Outbreaks have been associated with consumption of contaminated lettuce, carrots, and milk [32-36].

Risk factors — Risk factors associated with yersiniosis include consumption of undercooked or raw pork products, exposure to untreated water, blood transfusion, derangements of iron metabolism (such as cirrhosis, hemochromatosis, aplastic anemia, thalassemia, and iron overload), and other comorbid conditions (such as malignancy, diabetes, malnutrition, and gastrointestinal illness).

Individuals with thalassemia are at increased risk for yersiniosis, as patients with thalassemia can develop iron overload in the setting of red cell transfusion and the organism is ferrophilic. However, in one series of patients with thalassemia, severe yersiniosis was observed even when the iron burden was not greatly elevated; the reasons for this observation are not fully understood [37].

INCIDENCE OF DISEASE — Sporadic yersiniosis has been observed worldwide. The incidence of disease around the world increased substantially in the 1970s [38]. It was reported frequently in northern Europe, particularly in Belgium, Norway, and the Netherlands; it is rarely observed in tropical countries [39]. The reasons for these geographic patterns are not clear but may reflect the underlying frequency of infection in food animal reservoirs.

In Europe, overall annual incidence was 1.8 cases per 100,000 in 2016, and was highest in Finland (7.4 per 100,000) and Lithuania (5.9 per 100,000) [40], with little change year to year. Incidence in Belgium peaked at 14.7 per 100,000 in 1986, subsequently declined by 2003 to 3.3 per 100,000, and was 3.1 per 100,000 in 2016 [39,41]. The majority of Belgian isolates have been serogroup O:3; 5 percent are serogroup O:9 [41]. In Norway, the incidence has also declined for reasons that may include a combination of public education efforts to reduce consumption of raw or undercooked pork products, changing dietary habits, and efforts by the meat industry to reduce contamination of carcasses during processing [42].

In the United States, the incidence of yersiniosis is lower than in Europe and has decreased since the 1990s, though it has been increasing as more diagnoses have been made using multiplex polymerase chain reaction (PCR) syndromic panels. Although the incidence of diagnosed yersiniosis reported to the Foodborne Disease Active Surveillance program (FoodNet) of the US Centers for Disease Control and Prevention (CDC) had declined by approximately 59 percent from 1996 to 0.3 per 100,000 in 2015 [43], by 2019, the incidence had increased to 1.4 per 100,000 [44]. This increase likely reflects the greater frequency of testing with multi-pathogen syndromic panels [45]. Almost all yersiniosis in the United States is due to Y. enterocolitica; between 1996 and 2007, only 1 percent of reported yersiniosis was due to Y. pseudotuberculosis [46]. There is some geographic variation within the United States; in 2015, the highest incidence was in Oregon (0.5 cases per 100,000); the lowest was in Tennessee (0.1 cases per 100,000) [43]. Before 1990, the most common serotypes were O:5,27 and O:8; since 1990, O:3 and O:9 have predominated, as in Europe [18]. Accounting for underdiagnosis and under-reporting, the actual number of Y. enterocolitica infections occurring annually in the United States is estimated to be 117,000 [47]. Reported incidence did not change significantly in 2020 during the COVID-19 pandemic [48].

High and increasing rates of Y. enterocolitica infection have been described in New Zealand since the 1990s [49,50]. Incidence there was 24.1 per 100,000 in 2019 [51].

The incidence of Yersinia infections is highest in young children, although the age distribution may be changing in the era of multi-pathogen syndromic diagnostic panels. In Europe, in 2016, the incidence in children <5 years of age was 7.5 per 100,000 [40]. In the United States, between 1996 and 2009, children <5 years of age comprised 47 percent of Yersinia infections reported to the CDC's FoodNet program [52]. Over this time, the overall incidence among these children dropped from 9.2 to 1.9 per 100,000, and the most substantial decrease was among African-American children <5 years old, in whom the incidence dropped from 41.5 per 100,000 in 1996 to 3.5 per 100,000 cases in 2009 (figure 1) [52]. As of 2015, there was little difference in overall incidence across racial or ethnic groups [43]. Yersiniosis is rare among observant Muslim individuals who do not consume pork [14].

PREVENTION — Basic tenets of safe food production and preparation are important for prevention of yersiniosis (www.foodsafety.gov) [42]. Education about hazards associated with consuming raw pork and changes in the slaughter process may have helped to diminish the incidence in Belgium and Norway [41,42]. Education about the importance of separating chitterling preparation and childcare tasks in the United States may have helped to lower rates among children [53].

Reducing the carriage of Y. enterocolitica in pigs has been proposed through the use of attenuated live Yersinia strains that induce cross-protective immunity [54]; this area requires further investigation. Reducing contamination of produce before and after harvest is an unsolved challenge.

SUMMARY

●Yersinia enterocolitica and Yersinia pseudotuberculosis cause yersiniosis, a diarrheal illness. Illness due to Y. enterocolitica is more common than illness due to Y. pseudotuberculosis. Overall, Y. enterocolitica infection occurs more frequently in Europe than in North America; it is rarely observed in tropical countries. (See 'Introduction' above and 'Incidence of disease' above.)

●Y. enterocolitica strains have been isolated from a variety of vertebrate hosts, including domesticated animals as well as wildlife. Healthy pigs are frequently colonized with strains that cause human illness. In pigs, the organism colonizes the tonsils and other oropharyngeal lymphoid tissues; from these sites, it can be shed into the gastrointestinal tract. The organisms can contaminate retail pork products including neck trimmings, tonsillar tissue, tongue, and tripe, and can be transferred to other cuts of meat during slaughter. (See 'Reservoirs of infection' above.)

●Transmission of yersiniosis is largely foodborne. Pork consumption is a major mode of transmission; other outbreaks related to food contamination have also been reported. In rare cases, Y. enterocolitica septicemia has been observed following transfusion with packed red cells. Yersinia organisms are ferrophilic and capable of multiplying at cold temperatures prior to transfusion. (See 'Transmission' above.)

●Risk factors associated with yersiniosis include consumption of undercooked or raw pork products, exposure to untreated water, blood transfusion, derangements of iron metabolism (such as cirrhosis, hemochromatosis, aplastic anemia, thalassemia, and iron overload), and other comorbid conditions (such as malignancy, diabetes, malnutrition, and gastrointestinal illness). (See 'Risk factors' above.)

●Basic tenets of safe food production and preparation are important for prevention of yersiniosis (www.foodsafety.gov). In particular, it is important to separate the task of preparing raw pork from that of caring for an infant. (See 'Prevention' above.)