  The past decade has experienced an explosive increase of optically-discovered
tidal disruption events (TDEs) with the advent of modern time-domain surveys.
However, we still lack a comprehensive observational view of their infrared
(IR) echoes in spite of individual detections. To this end, we have conducted a
statistical study of IR variability of the 23 optical TDEs discovered between
2009 and 2018 utilizing the full public dataset of Wide-field Infrared Survey
Explorer. The detection of variability is performed on the difference images,
yielding out 11 objects with significant (>$3\sigma$) variability in at least
one band while dust emission can be only fitted in 8 objects. Their peak dust
luminosity is around $10^{41}$-$10^{42}$ erg/s, corresponding to a dust
covering factor $f_c\sim0.01$ at scale of sub-parsec. The only exception is the
disputed source ASASSN-15lh, which shows an ultra-high dust luminosity
($\sim10^{43.5}$ erg/s) and make its nature even elusive. Other non-detected
objects show even lower $f_c$, which could be one more order of magnitude
lower. The derived $f_c$ is generally much smaller than those of dusty tori in
active galactic nuclei (AGNs), suggesting either a dearth of dust or a
geometrically thin and flat disk in the vicinity of SMBHs. Our results also
indicate that the optical TDE sample (post-starburst galaxies overrepresented)
is seriously biased to events with little dust at sub-pc scale while TDEs in
dusty star-forming systems could be more efficiently unveiled by IR echoes.
