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A pseudo-outbreak of salmonellosis
R E Joce1, F Murphy, M H Robertson
1Hertfordshire Health Agency, Welwyn Garden City, Herts.
Epidemiology and Infection
|August 1, 1995
Summary
A laboratory error caused a false Salmonella outbreak. Correcting the sample handling procedure stopped the pseudo-outbreak, highlighting the need to verify all suspected infectious disease outbreaks.
Area of Science:
- Clinical Microbiology
- Infectious Disease Epidemiology
- Laboratory Diagnostics
Background:
- An unusual increase in Salmonella hadar isolates was reported by a single laboratory in July 1991.
- An outbreak control team was assembled to investigate the potential infectious disease outbreak.
- Initial epidemiological interviews with 39 primary cases did not reveal common risk factors.
Purpose of the Study:
- To investigate the source of the increased Salmonella hadar isolates.
- To determine if an actual outbreak was occurring or if a laboratory artifact was responsible.
Main Methods:
- Case definition established for Salmonella hadar infection.
- Epidemiological interviews conducted with affected individuals.
- Review of laboratory procedures for sample processing and isolation.
Main Results:
- Interviews failed to identify common epidemiological links among cases.
- A procedural error in the laboratory was identified: the selenite enrichment medium was contaminated by the stool emulsification spoon.
- Salmonella hadar was isolated from the saline used for emulsification, indicating a pseudo-outbreak.
Conclusions:
- The observed increase in Salmonella hadar isolates was a result of laboratory contamination, not a true outbreak.
- The pseudo-outbreak underscores the critical importance of rigorous verification of laboratory procedures.
- Investigating inconsistencies between epidemiological data and laboratory findings is crucial for accurate outbreak detection.