Microscopic entities with distinct topology and Euler characteristics, such as instantons and wormholes, at the spacetime foam level, within the framework of Euclidean quantum gravity, initiate changes in spacetime topology. These alterations could theoretically influence the field equations derived from the semiclassical variation of gravitational actions. While in the case of the Einstein-Hilbert action the presence of microscopic wormholes yields no significant outcomes, the inclusion of the Gauss-Bonnet term into the gravitational action brings about an effective topological variation procedure. This process generates an effective cosmological term that is influenced by both the Gauss-Bonnet coupling and the density of wormholes. Given that wormhole density can vary with time in a dynamic spacetime, this mechanism introduces a topologically derived effective dark energy component.