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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Enterobacteriaceae

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Escherichia Coli Infections : Clinical Aspects

The most common symptoms of E. coli UTI are dysuria and urinary frequency and do not differ significantly in character from those produced by the other less common Gram-negative urinary pathogens discussed.

E .coli INFECTIONS : CLINICAL   ASPECTS

MANIFESTATIONS

Opportunistic Infections

The most common symptoms of E. coli UTI are dysuria and urinary frequency and do not differ significantly in character from those produced by the other less common Gram-negative urinary pathogens discussed. If the infection ascends the ureters to produce pyelonephritis, fever and flank pain are common and bacteremia may develop. Although E. coli may have enhanced virulence in the production of pneumonia as well as soft tissue and other infections, no clinical features distinguish these cases from those caused by other members of the Enterobacteriaceae.

Intestinal Infections

Infections caused by all of the E. coli virulence types usually begin with a mild watery di-arrhea starting 2 to 4 days after ingestion of an infectious dose. In most instances, the du-ration of diarrhea is limited to a few days, with the exception of EAEC diarrhea, which can last for weeks. With ETEC and EPEC, the diarrhea remains watery, but with EIEC and EHEC, a dysenteric illness follows. Some EPEC cases may also become chronic. EHEC disease begins like the others but often also includes vomiting. In 90% of cases this is followed in 1 to 2 days by intense abdominal pain and bloody diarrhea, but fever is not prominent. Some EHEC cases develop into a dysentery that is less severe than that seen in shigellosis. Colonoscopy reveals edema, hemorrhage, and pseudomembrane for-mation. Resolution usually takes place over a 3- to 10-day period, with few residual ef-fects on the bowel mucosa.

HUS develops as a complication in about 10% of cases of EHEC hemorrhagic colitis, primarily in children under 10 years of age. The disease begins with oliguria, edema, and pallor, progressing to the triad of microangiopathic hemolytic anemia, thrombocytopenia, and renal failure. The systemic effects are often life-threatening, requiring transfusion and hemodialysis for survival. The mortality rate is 5%, and as many as 30% of those individ-uals who survive suffer sequelae such as renal impairment or hypertension.

DIAGNOSIS

Like the rest of the Enterobacteriaceae, E. coli is readily isolated in culture. For the diag-nosis of intestinal disease, separating the virulent types discussed above from the numer-ous other E. coli strains commonly found in stool presents a special problem. A myriad of immunoassay and nucleic acid methods have been described that are able to detect the toxins and genes associated with virulence. These methods work but are still too expen-sive to be practical, especially in the developing countries where ETEC, EIEC, EPEC, and EAEC are prevalent. A screening test for EHEC takes advantage of the observation that the O157:H7 serotype typically fails to ferment sorbitol. Incorporating sorbitol in place of lactose in MacConkey agar provides an indicator medium from which suspect (colorless) colonies can be selected and then confirmed with O157 antisera. This proce-dure has become routine in areas where EHEC is endemic.

TREATMENT

Because most E. coli diarrheas are mild and self-limiting, treatment is usually not an is-sue. When it is, rehydration and supportive measures are the mainstays of therapy, regard-less of the causative agent. In the case of EHEC with hemorrhagic colitis and HUS, heroic supportive measures such as hemodialysis or hemapheresis may be required. Treatment with trimethoprim/sulfamethoxazole (TMP-SMX) or quinolones reduces the duration of diarrhea in ETEC, EIEC, and EPEC infection, but neither the course of hem-orrhagic colitis nor the risk of HUS are altered by antimicrobial therapy. Because the risk of HUS may be increased by antimicrobial treatments, many physicians feel that treat-ment is not indicated. Antimotility agents are not helpful and are contraindicated when EIEC or EHEC might be the etiologic agent.

PREVENTION

Traveler’s diarrhea is usually little more than an inconvenience. Because the infecting dose is high, the incidence of the disease can be greatly reduced by eating only cooked foods and peeled fruits, and drinking hot or carbonated beverages. Avoiding uncertain water, ice, salads, and raw vegetables is a wise precaution when traveling in developing countries. High-priced hotel accommodations have no protective effect. Chemoprophy-laxis against traveler’s diarrhea is not routinely recommended. TMP-SMX or ciprofloxacin have been recommended for a short-term ( 2 weeks) for those at high risk for disease resulting from such chronic conditions as achlorhydria, gastric resection, pro-longed use of H2 blockers or antacids, and underlying immunosuppressive diseases.

These public health measures apply equally to EHEC, but here prevention is more dif-ficult because the infecting dose is so low. Cooking hamburgers all the way through is sensible, but no one is recommending abstinence from salads when at home. Recent US recommendations for the irradiation of meats and the extension of pasteurization require-ments to fruit juices are largely designed to stem the spread of EHEC.

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