  The propagation of very-high-energy gamma-rays (VHEGRs) in the extragalactic
space offers the opportunity to study astrophysical phenomena not reproducible
in laboratories. In particular, the deviation from predictions of the observed
photon flux from distant sources at the GeV energy scale still represents an
open problem. Commonly, this deviation is interpreted as the result of the
deflection out of the line of sight of the source of the cascade-produced
charged leptons by weak magnetic fields present in the intergalactic medium
(IGM). However, plasma instabilities could have an effect on the energy- and
momentum-distribution of the secondary electrons and positrons, modifying the
development of electromagnetic cascades. In this work, we study the influence
of plasma instabilities on the energy spectrum of electromagnetic cascades
through a parametric study, performed with the Monte Carlo code CRPropa. We
parameterize plasma instabilities with an energy loss length normalisation
$\lambda_0$ and a power law index $\alpha$ of its electron/positron energy
dependence. We simulate photon spectra at Earth for different blazar scenarios
and find that plasma instabilities can reproduce the suppression in the
GeV-photon flux of real astrophysical sources, such as 1ES 0229+200, when the
energy loss length of electrons/positrons at 1.0 TeV is $\lambda_0 \simeq
100\,\text{kpc}$ and $\alpha\simeq0$. The energy fraction lost by the secondary
electron pairs due to the instability is estimated to be about $1\%$ over the
typical interaction length of Inverse Compton scattering for these parameter
values.
