Recent flooding brings return of water-borne diseases
Following is part of a series of health articles supplied and supported by Pfizer Korea, a leading pharmaceutical company, and written by physicians practicing in Korea. In today's column, Dr. Kim June-myung, professor of division of infectious diseases of department of internal medicine at Severance Hospital of Yonsei University College of Medicine, discusses causes and treatments of shigellosis. - Ed.
Dr. Kim June-myungShigellosis is an acute inflammatory disease due to the Shigella species, which are a common cause of bacterial diarrhea worldwide. Most infected persons have only mild watery diarrhea. But in a few cases, classic dysentery shows with bloody diarrhea, high fever and abdominal cramps (convulsion). In Korea, until 1997, shigellosis had diminished to near extinction. After the heavy summer rain of 1998, it suddenly increased, 905 cases were infected during last year. Health officials are alarmed the recent prevalence of shigellosis. Because the spread and transmission of shigellosis are so fast, it is classified as a class 1 national communicable disease in Korea.
Microbiology: Shigella can be grouped into four species according to their antigenic and biochemical properties. S. dysenteriae causes serious bacterial dysentery. On the other hand, S. flexneri and S. boydii show moderate symptoms, and S. sonnei commonly causes mild disease. In Korea, S. sonnei is the prominent species, accounting for more than 75 percent of infections. S. flexneri accounts for 20 percent. S. dysenteriae is rarely identified (0.3 percent). Shigella is highly host-adapted and is a natural pathogen only of humans and a few other primates like monkeys and chimpanzees; it can barely survive in natural environments. Therefore patients or carriers act as pathogenic sources. Shigella is often found in water contaminated by human feces. It grows in temperatures of 10-45?C, and it is susceptible to heat.
Epidemiology: Worldwide, 200 million cases of shigellosis occur with 650,000 associated deaths annually. Most cases of death involve preschool children and elderly individuals who tolerate dehydration poorly. Shigella usually occurs sporadically or epidemically mainly during summer and early fall. In Korea, it is found year-round and tends to become endemic; a patient was reported at the beginning of this year. Anyone can get shigellosis, but it is recognized more often in young children. Those who may be at greater risk include children in day care centers, elementary and middle school students who eat in a common place, foreign travelers to certain countries, people in institutions and active homosexuals.
Transmission: The usual mode of transmission is person-to-person by the fecal-oral route, generally via direct contact but sometimes through contaminated vectors such as food, water and flies. Shigella species, in contrast to other intestinal pathogens, require a small inoculum to cause disease. The infectious dose for S. dysenteriae is as few as 10-100 organisms. As a result, Shigella can be easily transmitted by fecal contamination of hands. Therefore, transmission in kindergarten or elementary school students during group activities can be much faster. Shigellosis is associated with a high rate of secondary household transmission. As many as 40 percent of children and 2 percent of adults who are household contacts of a case will develop Shigella infection. In developing countries, transmission via contaminated food and water is prominent. For industrialized countries, transmission often occurs in direct physical contact between people. In the past, unsanitary water supplies caused many mass outbreaks. These days, water borne outbreaks are rarely found because sanitary conditions have much improved. Now outbreaks occur only after a natural calamity such as flood or earthquake. Although all Shigella species bring food borne outbreaks, S. sonnei is the most common contaminator of food; other species are more involved with contaminated water. Among those foods are cold salads (potato, tuna, shrimp, macaroni, and chicken), raw vegetables, milk, and poultry. The most common causes for contamination are feces-tainted water and unsanitary cooking.
Pathogenesis: Shigella is relatively resistant to killing by stomach acid. Thus, ingested bacteria pass into the small intestine where they multiply, so that more bacteria passes into the colon (large intestine). Shigella in the human body expresses some virulent factors when heated by the body temperature; it attaches then penetrates into the epithelial cell of the mucus membrane of the large intestine and spreads into adjacent cells. Eventually, characteristic mucus membrane ulceration occurs and rectal bleeding and severe dehydration can be induced.
Clinical features: The symptoms may appear 1 to 7 days after exposure but, usually within 2 to 3 days. The disease typically begins with constitutional symptoms such as fever, abdominal pain, anorexia and malaise; diarrhea initially is watery, but subsequently contains blood and mucus. Tenesmus, the cramping pain in rectum after defecation, is a common complaint. Most people show symptoms, but infants and seniors suffer more severely. The frequency of diarrhea is up to 10 times per day or more, but the volume of stool is small, thus significant fluid loss does not typically occur. These are characteristic symptoms with diarrhea due to a colon infection since the colon functions as a storage organ.
Normal healthy persons mostly recover without treatment and will not suffer for more than 7 days. In rare cases, infants or young children show complications such as intestinal obstruction or perforation. Various general complaints by patients are possible, and Shigella thrives in patients having poor nutrition.
Management: Most shigellosis patients recover without treatment. Generally, patients need fluid to protect against dehydration. Patients with severe symptoms require hospitalization and are given antibiotic treatment. Also, patients, who are possible transmitters, such as food handlers, health care workers, and children in day care centers should consider antibiotic treatment. Most patients fully excrete Shigella in their feces for 1 to 2 weeks. Antibiotics can shorten this carrying period. However, it is uncertain whether antibiotic treatment can reduce the patient's suffering. Antibiotics should be used carefully since their active usage also leads to increased antibiotic resistance. The use of antimotility (antidiarrheal) agents is not a good idea in the early phase of disease. These drugs are suspected of enhancing the severity of the disease by delaying excretion of organisms and thus facilitating further invasion of the mucosa.
Prevention: Transmission can be prevented by proper improvements in environments and personal hygiene. One of the most important prevention methods is hand washing after using the toilet. Patients with active diarrhea or those who are unable to control their bowel habits should be isolated from physical contact with others to avoid transmission. Most infected people may return to work or school when their diarrhea ceases, provided that they carefully wash their hands after toilet visits. Food handlers, children in day care and health care workers should be checked for absence of Shigella by culture test before returning to their normal routines. No effective vaccine is available.
For further information, contact Dr. Kim June-myung at: Tel: 02-361-5431 / Fax: 02-393-6884.