To report a pseudoepidemic of Pseudomonas aeruginosa infections discovered during an investigation of postoperative joint infections.

A retrospective review of case patients’ hospital charts, operative reports, and laboratory data, as well as environmental culturing, polymerase chain reaction (PCR) ribotyping of outbreak isolates, and in vitro analysis of P aeruginosa growth characteristics.

A 510-bed, university-affiliated adult tertiary care hospital.

Between October 1 and December 1, 1992, seven postsurgical joint infections were diagnosed, including four caused by P aeruginosa. A bottle of “sterile” saline used to process tissue specimens was found to be contaminated with P aeruginosa. Further investigation revealed that P aeruginosa had grown from seven additional tissue cultures, all of which had been processed with the contaminated saline. PCR ribotypes of the contaminant matched those of the clinical isolates. In vitro, P aeruginosa strains were viable in commercial nonbacteriostatic saline, but never caused visible turbidity.

Six patients received antibiotics for their presumed infections; four patients had peripherally inserted central catheters placed, and one experienced severe anaphylactic reactions to several antibiotics.

Pseudoepidemics due to common organisms are often difficult to detect, and delayed recognition can result in substantial morbidity. This outbreak investigation illustrates the potential for contamination of diluents in the microbiology laboratory and emphasizes the need for meticulous quality control.

To evaluate measles seroprev-among cohorts of new employees and to evaluate vaccine responses of susceptible adult healthcare workers.

New employees were screened for measles susceptibility as part of employee evaluations. Anti-IgG measles antibody tests were completed on 2,473 workers. Demographic, measles history, and measles vaccination information was collected using a short questionnaire. Susceptible workers were vaccinated and screened for vaccine responses following vaccination.

Ninety-three workers (4%) were seronegative, and 56 (2%) were equivocal. Individuals in the youngest cohort (born after 1956) were significantly more likely to be susceptible than those in the middle cohort (born 1951 to 1956) and those in the oldest cohort (born before 1951) (P<0.01). The middle cohort included eight (5%) of the 149 seronegative or equivocal workers. Among the members of the youngest cohort, those from the United States were more likely to be susceptible (P<0.01) than those from outside the United States.

Of the 106 vaccinated susceptible workers whose follow-up serologies were determined, 90 (85%) developed positive IgG serologies, six had equivocal results, and 10 were seronegative. Eleven of the 16 non- or hyporesponders were revaccinated and re-evaluated; nine developed low positive IgG antimeasles levels, one exhibited an equivocal response, and one failed to respond.

A small but important proportion of healthcare workers are susceptible to measles. Whenever feasible, measles immunity programs for healthcare workers should include workers born before 1957. Of workers born after 1956, those from outside the United States are more likely to be immune than workers from inside the United States. Using the currently available vaccine, revaccination of initial non- or hyporesponders appears to be effective.

To develop a model to assess the impact of a program of testing surgeons for human immunodeficiency virus (HIV) on the risk of HIV acquisition by their patients.

A Monte Carlo simulation model of physician-to-patient transmission of human immunodeficiency virus (HIV) infection using three different rates of physician-to-patient transmission per percutaneous exposure event (0.15%, 0.3%, 0.6%). Data from the model were developed from a review of the medical literature and from subjective probability estimates when data were not available. We used this model to estimate on a national basis the annual number of cases of HIV transmission from surgeons to patients with and without surgeon testing and practice limitations.

The annual number of transmitted cases would range from 0.5 ( 0.3), assuming a surgeon HIV prevalence of 0.1% and a surgeon-to-patient transmission rate of 0.15%, to 36.9 (±11.6), assuming a surgeon HIV prevalence of 2% and a surgeon-to-patient transmission rate of 0.6%. After one screening cycle, a mandatory screening program would be expected to reduce the annual transmissions to 0.05 (± 0.03) and 3.1 ( 1. 1), respectively.

Patients are at low risk of acquiring HIV infection from an infected physician during an invasive procedure. The potential costs of such a program extended beyond the costs of testing and counseling. In communities with high HIV prevalence, screening surgeons and limiting their practices may decrease patient access to care. A disability insurance program also would be required to protect surgeons and trainees performing invasive procedures. Screening surgeons for HIV infection would be a costly undertaking that would reduce but not completely eliminate this risk.

Evaluate aerobiological monitoring for fungal spores during hospital construction and correlate results with an outbreak of invasive aspergillosis (IA).

Prospective air sampling for molds was done using the gravity air-settling plate (GASP) method.

A university medical center special care unit consisting of single-patient rooms with high-efficiency particulate air filtration under positive pressure.

Five neutropenic patients who subsequently developed IA.

Four of the five patients with IA were housed in rooms adjacent to a construction staging area. Aerobiological monitoring detected an increase in the number of airborne fungal spores including Aspergillus species in these rooms; however, increased counts preceded IA diagnosis by 1 to 7 days in only three of the five patients. Swab cultures of the exhaust vents within each room confirmed results from air-settling plates. Follow-up monitoring, using the GASP method, demonstrated that control procedures were effective in reducing air mold contamination.

The GASP method, although able to demonstrate that infection control measures reduced mold contamination of the air, was insensitive to detect levels of mold contaminates in time to prevent IA.

To evaluate the endemicity and epidemiology of toxigenic Clostridium difficile in a sustained outbreak of antibiotic-associated diarrhea.

University-affiliated, 465-bed tertiary care teaching hospital with adjacent cancer clinic in Hamilton, Ontario.

From August 8, 1991, through August 31, 1993, a total of 187 cases were investigated for epidemiologic analysis of toxigenic C difficile from stool cultures, to identify the endemic clone(s). To assess the nature of contamination, cultures of inanimate surfaces in the patient environment from the four most affected units (medical teaching, nonteaching medical, hematologic oncology, and the intensive care unit) were processed for C difficile. The 229 clinical strains and 24 environmental strains isolated were typed by numerical analysis of SDS-PAGE protein patterns.

A majority (81%) of cases in the epidemiologic analysis were associated with a toxigenic electrophoretic (EP) type 1 C difficile that was identical to the strain first isolated from an index case that occurred 18 months before the start of this study. Culture and typing of the C difficile strains from the inanimate surfaces in the four most affected units showed that the patient environment was contaminated with the toxigenic EP type 1 organism. Six other strains that occurred infrequently among cases also were found in the environment.

A single predominant toxigenic clone has been implicated in a sustained outbreak of antibiotic-associated diarrhea that affected elderly patients. The “endemic” clone transmitted for the 25-month study period was linked to an index case shedding a toxigenic EP type 1 strain that occurred 21 months prior to the initial outbreak on the medical teaching unit. The patient environment in the affected units was found to be contaminated with the same clone, possibly due to shedding of organisms by fecally incontinent symptomatic patients. The extrinsic factors contributing to the endemic transmission of this one clone still are not well understood