A vector species that facilitates pathogen transfer between ecologically or taxonomically distinct host populations or between different ecological settings (for example wildlife and humans) by acquiring the pathogen from one host group and transmitting it to another under natural conditions.

Principle

A bridge vector contributes to spillover when it both contacts multiple host groups (or habitats) and is competent to transmit the pathogen between them; both exposure and vector competence are required.

Demonstration

Illustrative scenario — Situation: A mosquito species feeds on infected forest birds and later feeds on humans entering the forest. Recognition: Field data show feeding on both hosts and laboratory assays confirm competence for the virus. Action: Vector control and exposure reduction at the forest–human interface are prioritized. Consequence: Interrupting bridge‑vector contacts reduces spillover events to humans.

Misapplication

Assuming any vector specimen that tests positive for pathogen nucleic acid is a bridge vector. The error is equating pathogen carriage with demonstrated ecological bridging (i.e., feeding behavior and demonstrated transmission).

Consequence

Correctly identifying bridge vectors focuses interventions at interfaces where cross‑species transmission occurs; misidentification can divert resources to vectors irrelevant to actual spillover pathways.

Reversal

A species may regularly contact multiple host groups but be an incompetent transmitter for a specific pathogen, so it does not function as a bridge vector for that pathogen. Alternatively, environmental or direct transmission routes can bypass vectors entirely, rendering vector‑focused interventions ineffective for that pathogen.

Boundary

Clearly within: a vector species with documented feeding on two host groups and demonstrated transmission from one group to the other. Boundary case: a vector that occasionally contacts another host group but with no evidence of onward transmission. Clearly outside: vectors that are host‑specific and never contact the second host group under natural conditions.

Semantic Tension

Management priorities can conflict between targeting specific bridge vectors to prevent spillover and broader vector control that reduces overall nuisance and disease but carries greater ecological or logistical cost.

Synthesis

Bridge vector designation requires both ecological bridging (multi‑host contact across interfaces) and biological transmission competence; absence of either component negates bridge status.