Yersinia enterocolitica is a Gram-negative, toxigenic, pleomorphic, non-lactose fermenting, aerobic and facultative anaerobic, food-borne and water-borne rod-like bacterial pathogen or coccobacillus in the genus Yersinia. The motility of Y. enterocolitica varies with temperature. At 37oC, Y. enterocolitica is non-motile but at 25oC the organism is motile. Y. enterocolitica produces enterotoxin but this is usually attributed to some strains or serotypes of the organism. Y. enterocolitica causes gastroenteritis, an intestinal infection in humans especially in children and infants. The intestinal infection caused by Y. enterocolitica is similar to the gastroenteritis caused by Shigella species and Salmonella serotypes in humans. Y. enterocolitica is mainly reserved in the environment and it is frequently found in domestic animals from where human infection can occur. Most human infection with Y. enterocolitica occurs through the contamination of food and water with the excreta of pets, rodents and domestic animals including pig, cattle, cats and dogs.
Y. enterocolitica is a zoonotic bacterium, and it can also be transmitted to humans through fomites contaminated with animal excreta. Yersinia species including Y. pestis (the causative agent of plague) and Y. enterocolitica are common biological agents used to perpetrate bioterrorism in human population. Y. enterocolitica is also associated with some autoimmune diseases in humans (e.g., Graves’s disease and Reiter’s syndrome). Y. enterocolitica can survive at a very low temperature (e.g., 4oC), and they can thrive in refrigerated food products to cause gastroenteritis in humans. The organism is a common temperate bacterium but Y. enterocolitica is now found in non-temperate and tropical regions of the world such as in the African continent where some disease episodes have been previously recorded.
PATHOGENESIS OF YERSINIA ENTEROCOLITICA INFECTION
Upon ingestion of Y. enterocolitica through contaminated food or water, the pathogen migrate to the gastrointestinal tract (GIT) where it penetrates the mucous layer and/or epithelial cells of the small intestine or ileum from where they are transported to the mesenteric lymph nodes by macrophages. The incubation period of the disease is usually 4-8 days – during which Y. enterocolitica multiplies rapidly in the mucosal cells of the ileum where they are attached. Inflammation and ulceration follows Y. enterocolitica multiplication in the mucosa of the small intestine. An inflammatory diarrhea known as dysentery (usually accompanied with blood and mucous in faeces) ensues in intestinal invasion by Y. enterocolitica. Y. enterocolitica expresses several virulence factors including enterotoxins and invasions that provoke their virulence in vivo. The toxin produced by Y. enterocolitica is heat-stable and resembles the shiga-toxin (ST-1) produced by E. coli (i.e., EHEC strains). Extra-intestinal infections by Y. enterocolitica rarely occur, and the infection is usually self-limiting and may relapse after about three days of initial infection.
Y. enterocolitica is an invader of the intestinal mucosa (especially those of the ileum), and it causes enteric fever-like symptoms associated with dysentery. The pathogen is usually transmitted via the feacal-oral route through contaminated food and water. Diarrhea, severe abdominal cramp, vomiting, watery stool (usually accompanied with mucous and blood in children) are signs and symptoms of gastroenteritis or mesenteric adenitis caused by Y. enterocolitica. Mesenteric adenitis is a clinical condition that bears a resemblance to acute appendicitis (especially in adults), and in which Y. enterocolitica is implicated. Contaminated food products such as pork, unpasteurized milk, meat and water are sources via which Y. enterocolitica infection in humans can occur. Gastroenteritis caused by Y. enterocolitica is a zoonotic infection that requires an animal host in the transmission of the pathogen. Human-to-human transmission of the organism or disease rarely occurs.
LABORATORY DIAGNOSIS OF YERSINIA INFECTION
The laboratory diagnosis of gastroenteritis caused by Y. enterocolitica is usually based on the isolation and identification of the organism on culture media. Fresh stool specimens and rectal swabs from infected patients are usually required for culture in the microbiology laboratory. Since Y. enterocolitica is a temperate organism and can thrive under cold conditions, stool samples and rectal swabs can be placed in physiological saline (pH 7.6) and kept at refrigeration temperature (e.g., 4oC) for about 2 weeks prior to subculture onto solid growth/culture media for the isolation of the organism. Most faecal bacteria do not survive at 4oC but Y. enterocolitica thrives efficiently at low or cold temperatures. Y. enterocolitica grow on MacConkey agar (producing non-lactose fermenting colonies). It also grows on Salmonella-Shigella agar (SSA), as well as on Cefsulodin-Irgasan Novobiocin (CIN) agar – where it produces red-like colonies as shown in Figure 1. Cefsulodin-Irgasan Novobiocin (CIN) agar is a selective culture media that is used for the selective isolation of Y. enterocolitica from clinical and/or environmental samples.
Biochemically, Y. enterocolitica is Voges-Proskauer (VP) – negative, citrate-negative, oxidase-negative and urease-positive. Y. enterocolitica also produces a red slope and yellow butt with no H2S or gas production in triple sugar iron agar (TSIA) medium. Y. enterocolitica is a non-lactose fermenter but it hydrolyzes other sugars such as glucose, sucrose and mannitol.
TREATMENT OF YERSINIA ENTEROCOLITICA INFECTION
Gastroenteritis caused by Y. enterocolitica is a self-limiting water-borne or food-borne infection that usually resolves on its own even without prior antibiotic therapy. However, severe infections (especially in extra-intestinal spread of the organism) can be treated with antibiotics such as sulphamethoxazole-trimethoprim (co-trimoxazole), fluoroquinolones and cephalosporins. Supportive therapy using oral rehydration is also important in patients experiencing severe fluid loss from the body. Oral rehydration will help to replace the fluids and electrolytes lost from the body as a result of diarrhea or other related conditions.
PREVENTION AND CONTROL OF YERSINIA INFECTION
Y. enterocolitica is a zoonotic organism. Human Y. enterocolitica infection (i.e., zoonoses) only occurs via the ingestion of food and water contaminated by the excreta of animal reservoirs of the bacterium. Animal reservoirs of Y. enterocolitica include pig, rodents, cats and dogs. People should safeguard their food from the excreta of animal reservoirs of Y. enterocolitica. Good personal hygiene and environmental sanitation is critical to the prevention and control of gastroenteritis caused by Y. enterocolitica. No vaccine currently exists for Y. enterocolitica infection.
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