Y. enterocolitica and Y. pseudotuberculosis are zoonotic, food borne pathogens that spread through the fecal–oral route. Y. enterocolitica is a well-known human pathogen but rarely causes disease in animals; on the contrary, Y. pseudotuberculosis is a well-known animal pathogen that very infrequently causes disease in humans. Both of the Yersinia species are nonlactose-fermenting, pleomorphic Gram-negative rods that are urease positive and oxidase negative. They grow best at 25°C and are motile at 25°C but nonmotile at 37°C. They are found in the intestinal tracts of a variety of animals, in which they may cause disease, and are transmissible to humans, in whom they can produce a variety of clinical syndromes.

Y. enterocolitica exists in more than 70 serotypes; most isolates from human disease belong to serotypes O:3, O:8, and O:9. There are striking geographic differences in the distribution of Y. enterocolitica serotypes. Y. enterocolitica can produce a heat-stable enterotoxin, but the role of this toxin in diarrhea associated with infection is not well defined.

Y. enterocolitica has been isolated from rodents and domestic animals (eg, sheep, cattle, swine, dogs, and cats) and waters contaminated by them. Transmission to humans probably occurs by contamination of food, drink, or fomites. Person-to person transmission with this organism is probably rare.

Y. pseudotuberculosis is commonly found in water and soil, as well as in various domesticated and wild animals. The organism is less frequently isolated in the United States, but is more commonly found in northern Europe and Asia. Rodents, rabbits, and wild birds are the main natural reservoir for Y. pseudotuberculosis, and humans usually acquire the infection via contaminated food and water.

Pathogenesis and Clinical Findings

 An inoculum of 10⁸–10⁹ yersiniae must enter the alimentary tract to produce infection. During the incubation period of 4–7 days, yersiniae multiply in the gut mucosa, particularly the ileum. This leads to inflammation and ulceration, and leukocytes appear in feces. The process may extend to mesenteric lymph nodes and, rarely, to bacteremia.

Y. enterocolitica typically causes enterocolitis and early symptoms include fever, abdominal pain, and diarrhea. Diar rhea ranges from watery to bloody and may be caused by an enterotoxin or due to invasion of the mucosa. Some patients may present with a mesenteric adenitis or terminal ileitis, and at times, the abdominal pain can be severe and located in the right lower quadrant, clinically suggesting appendicitis. One to 2 weeks after onset of diarrhea, some patients with histocompatibility antigen HLA-B 27 may develop arthralgia, arthritis, and erythema nodosum, suggesting an immunologic reaction to the infection. Other, rarer immunologic complications include ankylosing spondylitis and acute reactive arthritis (formerly known as Reiter’s syndrome, with arthritis, urethritis, and conjunctivitis). Very rarely, Y. enterocolitica infection produces pneumonia, meningitis, or sepsis; in most cases, the majority of gastrointestinal infections are self-limited.

Y. enterocolitica has also been associated with transfusion-related infections caused by contaminated red blood cells. This is a consequence of the ability of the organism, transmitted by an asymptomatic donor, to multiply at refrigeration temperatures.

Y. pseudotuberculosis usually affects children and young adults and presents clinically as mesenteric adenitis, an acute appendicitis-like syndrome. When an exploratory laparotomy or appendectomy are performed, the appendix has usually a normal appearance, but enlarged mesenteric lymph nodes and an inflammation of the terminal ileum can often be appreciated. The infection is usually self-limited, and antibiotic therapy is probably not necessary for patients with mesenteric adenitis.

Diagnostic Laboratory Tests

A. Specimens

Specimens may be stool, blood, or material obtained at surgical exploration. Stained smears are not contributory.

 B. Culture

The number of yersiniae in stool may be small and can be increased by “cold enrichment”: a small amount of feces or a rectal swab is placed in buffered saline with a pH of 7.6 and kept at 4°C for 2–4 weeks; many fecal organisms do not survive, but Y. enterocolitica multiplies. Subcultures made at intervals on MacConkey agar may yield yersiniae. Alternatively, most clinical laboratories use a Yersinia-selective agar such as cefsulodin-irgasan-novobiocin (CIN) agar incubated at room temperature for several days. Y. enterocolitica colonies have a bull’s eye appearance with a red center on CIN agar. Y. pseudotuberculosis can be differentiated from other non-pestis Yersinia species by various biochemical test characteristics.

 C. Serology

 In paired serum specimens taken 2 or more weeks apart, a rise in agglutinating antibodies can be shown; however, cross-reactions between yersiniae and other organisms (vibrios, salmonellae, and brucellae) may confuse the results.

 Treatment

Infections due to Y. enterocolitica or Y. pseudotuberculosis cause a mild diarrhea that is usually self-limited, and the possible benefits of antimicrobial therapy are unknown. Y. enterocolitica is generally susceptible to aminoglycosides, chloramphenicol, tetracycline, trimethoprim–sulfamethoxazole, piperacillin, third-generation cephalosporins, and fluoroquinolones; it is typically resistant to ampicillin and to first-generation cephalosporins. Y. pseudotuberculosis is usually susceptible to ampicillin, tetracycline, chloramphenicol, cephalosporins, and aminoglycosides. Sepsis and meningitis due to Y. enterocolitica and also Y. pseudotuberculosis carry a high mortality rate, but deaths occur mainly in immunocompromised patients. Sepsis due to non-pestis Yersinia species can be successfully treated with third-generation cephalosporins (possibly in combination with an aminoglycoside) or a fluoroquinolone (possibly in combination with another anti-microbial); however, sepsis due to Y. pseudotuberculosis has a high mortality rate even when appropriate antibiotic therapy is given. In cases in which clinical manifestations strongly point to either appendicitis or mesenteric adenitis, surgical exploration has been the rule unless several simultaneous cases indicate that Yersinia infection is likely.

Prevention and Control

 Contact with farm and domestic animals, their feces, or materials contaminated by them probably accounts for most human infections. Contaminated meat (especially pork and pork products) and occasionally dairy products have been indicated as sources of infections, and group outbreaks have frequently been traced to contaminated food or drink. Conventional sanitary precautions are probably helpful, and consumption of raw or undercooked meats should be avoided. In addition, public health measures focusing on safe food handling and processing practices can be helpful to decrease the risk of contamination of food and dairy products. In blood banks and blood donor centers, volunteers are asked about recent fever, abdominal pain, or diarrhea prior to blood donation, in order to decrease the risk of collecting potentially contaminated blood products.