  Cosmic string loops contain cusps which decay by emitting bursts of
particles. A significant fraction of the released energy is in the form of
photons. These photons are injected non-thermally and can hence cause spectral
distortions of the Cosmic Microwave Background (CMB). Under the assumption that
cusps are robust against gravitational back-reaction, we compute the fractional
energy density released as photons in the redshift interval where such
non-thermal photon injection causes CMB spectral distortions. Whereas current
constraints on such spectral distortions are not strong enough to constrain the
string tension, future missions such as the PIXIE experiment will be able to
provide limits which rule out a range of string tensions between $G \mu \sim
10^{-15}$ and $G \mu \sim 10^{-12}$, thus ruling out particle physics models
yielding these kind of intermediate-scale cosmic strings.
