Vol. 17, No. 12

THE JOHNS HOPKINS MICROBIOLOGY NEWSLETTER

Monday, March 23, 1998

A. Provided by Marguerite Hawkins, Epidemiology and Disease Control Program, Division of Outbreak Investigation, Maryland Department of Health and Mental Hygiene

From March 12, 1998, through March 18, 1998, 4 outbreaks have been reported to the Maryland Department of Health and Mental Hygiene as follows: 1 influenza-like illness, 1 food-related gastroenteritis, 1 presumed viral gastroenteritis and 1 presumed scabies.

Influenza Activity in Maryland - Update as of Friday, March 18, 1998:

To date, CDC has identified influenza A/Sydney/05/97-like (H3N2) virus in 2 isolates from Maryland. DHMH has received a total of 260 lab-confirmed cases of influenza A, which has occurred both sporadically and associated with outbreaks. For the last three weeks, influenza activity level has remained as regional.

B. The Johns Hopkins Hospital: Information provided by Dr. Frank Holmes, Dept. of Pathology

Case History

An 81 year-old female with a history of metastatic ovarian carcinoma and recent chemotherapy presented to the hospital with increasing confusion and lethargy. The patient was initially started on gentamicin and clindamycin for a presumed infection, which was changed to IV vancomycin after sputum cultures grew methicillin-resistant Staphylococcus aureus. In addition, urine cultures were positive for yeast organisms, and she was placed on fluconazole. The patient also had stool cultures sent on admission, which were negative for bacterial enteric pathogens, ova and parasites, and for Clostridium difficile toxin. The patient's condition improved; however, she developed diarrhea around day 10 of her hospitalization. At that time, an enzyme immunoassay was positive for C. difficile toxin. She was started on oral metronidazole with improvement in her diarrhea.

Stool Cultures

Although Salmonella, Shigella, and Campylobacter species are common causes of community-acquired diarrhea, they are rare causes of nosocomial diarrhea. Similarly, stool specimens sent for detection of ova and parasites are most often positive in outpatients and in recently admitted inpatients and almost never positive in patients who develop diarrhea during hospitalization. Yet, stool cultures for these pathogens are commonly requested for patients who develop diarrhea during hospitalization. Because these tests are expensive and because of the need for laboratories to establish cost-accountable test acceptance and rejection policies, it is important to identify those patient populations in which the yield of stool cultures or stool examination for ova and parasites would be so low that it would not justify performing the test. In fact, several studies have shown that after three days of hospitalization, the yield of these tests in both adult and pediatric patients with diarrhea is well under 1%. On the other hand, these patients are much more likely to have a positive C. difficile toxin assay. Therefore, the microbiology laboratory will soon adopt a policy of rejecting stool samples sent for culture or examination for ova and parasites in patients hospitalized over three days, unless otherwise approved by the microbiology resident or a microbiology attending.

Clostridium difficile

C. difficile is the major cause of antibiotic-associated diarrhea and pseudomembranous colitis. It is carried asymptomatically as part of the gastrointestinal flora in as many as half of all healthy neonates during the first year of life. This rate decreases to the adult carrier rate of about 2% in children older than two years. Among adults who have received antibiotic therapy, however, carriage rates as high as 46% have been reported, and hospitalized patients frequently become colonized with this organism. Exposure to antibiotics is thought to allow proliferation of toxigenic C. difficile by disrupting the normal intestinal flora. Antimicrobial agents of all classes have been implicated, but the most commonly reported agents are ampicillin, clindamycin, and the cephalosporins. Several anticancer chemotherapeutic agents have also been responsible. Other risk factors include bowel stasis and bowel surgery, and some patients develop

C. difficile colitis with no known risk factors. Two distinct toxins, toxin A and toxin B, are associated with C. difficile disease, along with a substance that inhibits bowel motility. Toxin A has been described as an enterotoxin causing an increase in intestinal permeability with subsequent enteric fluid accumulation and diarrhea. Toxin B is a potent cytotoxin causing rounding of cells in culture. Clinical symptoms range from mild diarrhea to toxic megacolon, with possible bowel perforation and death, although these events are unlikely to occur in appropriately treated patients. The association of C. difficile with disease in infants under the age of two is not well understood. As mentioned before, infants in this age group are frequently colonized with toxigenic C. difficile and can have toxin present without symptoms of the disease.

Diagnosis

The main method used by the Microbiology lab for detecting C. difficile disease is an enzyme immunoassay for the direct detection of toxins A and B in stool. Microwells are coated with toxin-specific monoclonal antibodies. If either toxin is present in the patient sample, an antibody-antigen complex is formed, which is detected subsequently by a color reaction in the well. The intensity of the color is proportional to the amount of toxin bound in the wells and is compared to positive and negative controls. Positive reactions must have a certain level of color intensity, and indeterminate values are followed by a cytotoxin assay for confirmation. Compared to the cytotoxin assay, which is considered the gold standard, the enzyme immunoassay has a sensitivity of 97.0%, specificity of 99.6%, positive predictive value of 98.5%, and a negative predictive value of 99.2%. The cytotoxin assay is used to detect the presence of toxin B in patients' stool by cell culture with neutralization of the toxin by specific antiserum. Although extremely sensitive, this assay requires cell culture capability and up to 72 hours of incubation.

Treatment

C. difficile-associated intestinal disease is usually treated with oral vancomycin or metronidazole. Continuing the responsible antibiotic therapy often results in relapse of the disease, so discontinuation of the offending agent or change to an agent less likely to cause diarrhea should also be considered.

References

Bartlett JG. 1990. Clostridium difficile: clinical considerations. Rev Infect Dis. 12: S243-S251.

Fan K, Morris AJ, Reller LB. 1993. Application of rejection criteria for stool cultures for bacterial enteric pathogens. J. Clin. Microbiol. 31: 2233-5.

Koontz F. 1994. Limitation on parasite-ova specimen examinations [Editorial]. Clinical Microbiology Newsletter. 16: 124-6.