New epidemic strain of Clostridium difficile has emerged

February 2005

A study has shown that the recent increase in rates of Clostridium difficile–associated disease in U.S. hospitals is due to the emergence of an epidemic strain with special virulence and antimicrobial resistance properties.

According to L. Clifford McDonald, MD, from the CDC, the incidence rates and the severity of C. difficile-associated disease have increased significantly during the past few years. McDonald recently spoke about C. difficile at the 42nd Annual Meeting of the Infectious Diseases Society of America, held in Boston.

Although there is no national surveillance of C. difficile cases, McDonald reported more than 168,000 cases of C. difficile listed as a diagnosis at the time of discharge from the hospital in 2002. This finding was reported at the Society for Healthcare Epidemiology of America meeting in Philadelphia in April 2004.

“We hypothesized that these increases may be due to the emergence of an epidemic strain. To test this hypothesis, we set forth to characterize C. difficile isolates from hospitals with outbreaks between 2001 and 2004,” he said.

Isolate characterization

Characterization included strain typing, detecting molecular markers that could suggest increased virulence and antimicrobial susceptibility testing. He studied acute care hospitals with C. difficile outbreaks between 2001 and 2004. These included hospitals located in Georgia, Illinois, Maine, New Jersey, Oregon and Pennsylvania. One hospital from each state was studied, with the exception of Maine, where two hospitals were studied.

In all of the hospitals, outbreaks were detected by increases in the number of positive routine clinical laboratory tests for C. difficile. Strain typing consisted of restriction enzyme analysis (REA). The patterns produced by REA were compared with a database of more than 6,000 historic isolates collected before 2001.

Additionally, pulsed field gel electrophoresis (PFGE) was used to characterize isolates. Finally, toxinotyping was performed. “Toxinotyping consists of analyzing restriction fragment length polymorphisms of the pathogenicity locus. The pathogenicity locus is the complex of toxin A and toxin B genes with surrounding regulatory genes. Molecular characterization of potential virulence factors included the detection of binary toxin,” he explained.

Other characterization involved detection of variations in tcdC. “It has been hypothesized that deletions in tcdC with loss of negative regulatory function may lead to increased toxin production. To look for such deletions, PCR [polymerase chain reaction] of the tcdC was performed followed by PFGE,” he added.

Antimicrobial resistance was determined using e-tests containing clindamycin, levofloxacin, gatifloxacin (Tequin, Bristol-Myers Squibb) and moxifloxacin (Avelox, Bayer).

Results

Using both REA and PFGE, a common epidemic strain was identified from each of the seven hospitals in six states. Five related isolates were found in the historic database. These isolates have been cataloged as BI strains by REA typing.

Of 62 epidemic strain isolates identified, including the five related historic REA type BI isolates, all were toxinotype 3 and binary toxin–positive and contained an 18 base-pair tcdC deletion. In contrast, of 36 nonepidemic strain isolates, none were toxinotype 3. Instead, 89% were toxinotype 0 or wild type. Only 6% of nonepidemic strain isolates were binary toxin–positive, and only one isolate contained an 18 base-pair tcdC deletion.

The epidemic strain accounted for 50% or more of the isolates in five of the seven outbreaks. “We found increased resistance in the epidemic strain isolates collected after 2000. Epidemic strain isolates were found to be somewhat more likely to be intermediate or resistant than nonepidemic strain isolates when tested against clindamycin and significantly more resistant when tested against fluoroquinolones,” he said.

Additionally, the researchers found increased resistance in contemporary epidemic strain isolates. “Contemporary epidemic strain isolates were significantly more likely intermediate or resistant than historic isolates to both clindamycin and the fluoroquinolones, with none of the historic BI strains resistant to fluoroquinolones,” he said.

This new epidemic strain of C. difficile possesses genes that encode binary toxin as a possible virulence factor. “A subset of epidemic strain isolates was tested, and all possessed genes for standard toxins A and B. Therefore, binary toxin as a possible virulence factor appears to be present in addition to the standard toxins A and B. This strain also possesses a deletion in the negative regulator gene tcdC. Because the function of this gene is to downregulate toxin A and B production, it is conceivable that this deletion could lead to increased toxin production,” he said.

While this strain has been in existence and has possessed these potential virulence factors for a number of years, it has only recently acquired resistance to clindamycin and the fluoroquinolones.

Recommendations

Based on the study findings, McDonald made the following recommendations:

• Hospitals should conduct surveillance. “This can consist of simply tracking positive lab results for C. difficile, such as the monthly number of positive toxin assays. In addition, hospitals should consider tracking outcomes, especially if they have noted a recent increase in rates,” he said.
• Early diagnosis and treatment is important for reducing severe outcomes. This should be emphasized to clinicians, McDonald said.
• Strict infection control should be applied in preventing and controlling outbreaks. “To facilitate this, we have updated fact sheets for health care providers on the CDC Web site (www.cdc.gov/ncidod/hip/gastro/ClostridiumDifficile.htm). These highlight contact precautions for C. difficile patients, an environmental cleaning and disinfection strategy and consideration for hand washing with soap and water as opposed to alcohol-based hand rub when caring for C. difficile patients in an outbreak situation,” he said.

For more information:

• McDonald LC. Emergence of an epidemic strain of Clostridium difficile in the United States, 2001-4: Potential role for virulence factors and antimicrobial resistance traits. LB-2. Presented at the 42nd Annual Meeting of the Infectious Diseases Society of America. Sept. 30-Oct. 3, 2004. Boston.
• McDonald LC, Banejee S, Jernigan DB. Increasing incidence of Clostridium difficile-associated disease in U.S. acute care hospitals, 1993-2001. Presented at the 14th Annual Scientific Meeting of the Society for Healthcare Epidemiology of America. April 18, 2004. Philadelphia.