Yersinosis refers to infection by a genus of bacteria known as Yersinia. The two sub-types that are responsible for yersinosis are Yersinia enterocolitica and Yersinia pseudotuberculosis. The diseases produced by these organisms are called "zoonoses," because the bacteria is passed to humans from animal sources.
The name Yersinia comes from Dr. Alexandre Yersin, who was the first person to grow a much more deadly type of Yersinia known as Yersinia pestis, the bacteria responsible for what is now known as bubonic plague. This article, however, will deal with the more common forms of Yersinia, namely Y. enterocolitica and Y. pseudotuberculosis.
Yersinia are classified as gram-negative bacteria (bacteria that do not accept the color of a stain in a Gram stain test, which indicates the general chemical nature of the cell wall of the bacteria); they have a variety of appearances, and are therefore called pleomorphic. They belong to Enterobacteriacae, the large group of organisms that inhabit the intestinal tract. There are many different subtypes of Yersinia.
They are found worldwide and have been isolated from soil, fresh water, contaminated foods, and many wild and domestic animals. For reasons not entirely clear, disease caused by these organisms occurs more frequently in areas of northern Europe, especially Scandinavia. Infection, particularly in children ages one through four years, is quite common, though often these infections produce few symptoms. Studies have shown that infection with these bacteria is almost as common as that with Shigella or Campylobacter.
Causes and symptoms
Animals are the most important sources of bacterial infection for humans. Whether from pets or undercooked meat (especially pork), these bacteria almost always enter the human body through the mouth (oral transmission). An incubation period of one to eleven days passes before signs of disease develop. Rare cases have been transmitted by way of contaminated blood transfusions.
Yersinia produces several different types of disease. The most common form is a short-lived inflammation of the intestine known as enterocolitis. Most often the very end of the small intestine is involved, an area known as the terminal ileum. The result is gastroenteritis, with cramping abdominal pain, fever, and diarrhea. Diarrhea generally continues for two weeks or so, but can go on for many months. Up to 40% of patients also experience nausea and vomiting; and in one-third, inflammation of the intestine leads to bleeding.
In other patients, the same area of the intestine is involved, but instead of causing diarrhea, a syndrome resembling appendicitis occurs. In this syndrome, the lymph nodes surrounding the intestine are especially involved; this has lead to the term mesenteric adenitis. Although this syndrome resolves without serious consequences, it is often difficult to differentiate from appendicitis, and leads to surgery in some instances. Ultrasound exam may be able to demonstrate a normal appendix and avoid surgery. Why some patients develop symptoms of gastroenteritis, and others only inflammation, pain, and fever, is unknown.
In some patients, Yersinia produces infection of areas other than the intestinal tract. These include:
- Inflammation of the throat (pharyngitis) and tonsillitis; this can be quite severe and even lead to death, particularly in adults.
- Septicemia, or infection of the blood stream, with spreading of infection to other organs such as bone, meninges, kidneys, and others. Individuals with decreased immunity due to liver disease, diabetes, cancer, and other diseases are at increased risk for this complication.
Different parts of the body may be affected (such as joints, eyes, and urinary system) by changes in the immune system caused by Yersinia infection. Arthritis, which is especially frequent in Scandinavia, occurs in up to 10% of Yersinia infections. About one week after typical intestinal symptoms, swelling and pain in multiple joints occurs. The knees and ankles are most often involved, and become inflamed over a period of two weeks. In two-thirds of those affected, symptoms gradually resolve over one to three months without need for treatment. Rarely does chronic joint disease develop.
Inflammation of the heart muscle, called myocarditis, sometimes occurs together with the arthritis. In about 15-20% of patients, the skin develops a red, raised area, usually located on the shins, called erythema nodosum. This appears within a few weeks of the intestinal symptoms and disappears over a month or so.
Identifying Yersinia as the cause of all or any of these symptoms is not an easy task. It is possible to grow the organism from stool cultures, but this is difficult to do unless special methods are used.
A change in antibody levels can also be used to determine the presence of infection. To be accurate, levels must be initially examined early in the illness. Therefore, it is most important for the possible diagnosis and examination to be thought of early.
Since most of the symptoms caused by Yersinia are self limiting, specific antibiotic treatment is generally not needed. Patients with dehydration from gastroenteritis are given supportive therapy, including treatment aimed at replacing fluids.
Antibiotics are indicated, however, for those patients who develop more severe infections, such as invasion of the bloodstream (septicemia), or who develop infections at specific sites, such as bone. A variety of antibiotics have been used, but it is not clear which produces the best results.
No specific treatment is indicated for the joint, ocular, skin, or urinary symptoms that result from infection. As stated, these are not due to direct invasion by the bacteria, but are related to changes in immune reactions produced by the infection. However, treatment of those experiencing severe arthritic symptoms with NSAIDS (nonsteroidal anti-inflammatory drugs) or steroid injection at inflamed joints is used in selected cases.
As noted above, most of the time, Yersinia infection has an excellent outlook. However, when these bacteria invade the bloodstream or produce disease beyond the gastrointestinal tract, the outlook is less positive. This may be because more severe infections occur in those with decreased immunity. Death rate from septicemia has been reported to be as high as 50%.
Safe food handling procedures and food-preparation practices are by far the best means of avoiding infection. Undercooked food, especially pork or other animal products, should not be eaten.
"Bad Bug Book." Center for Food Safety and Applied Nutrition. 〈http://vm.cfsan.fda.gov/∼mow/intro.html〉.
Mesenteric adenitis— Inflammation of the lymph nodes which serve the small intestine. Has symptoms similar to appendicitis.
Septicemia— Systemic disease associated with the presence of microorganisms or their toxins in the blood; blood poisoning.