Which statement best defines a pathogen-free status in a mouse colony?

• A. Pathogen-free status is determined by random sampling.
• B. Pathogen-free status means only the presence of bacteria.
• C. Pathogen-free status means no animals can ever be sick.
• D. Pathogen-free status is absence of defined pathogens in the colony as determined by health surveillance.

Answer: (D)

Pathogen-free status is defined by the absence of a defined set of pathogens in the colony, verified through a structured health surveillance program. In practice, this means the facility tests for a specific panel of defined pathogens (viruses, bacteria, parasites) and uses validated methods (serology, PCR, culture, etc.) to ensure none are present. It’s not about random sampling alone, and it isn’t merely the absence of bacteria. It also isn’t a guarantee that animals can never get sick—they can still become ill from noninfectious causes or pathogens not included in the surveillance panel. So this status specifically reflects the absence of defined pathogens as determined by ongoing health surveillance.