Jessie's comment ·
Yes there is a risk of catching STD but the risk is minimum.
"As with other types of sexual activity, oral sex carries the risk of STIs. It may be possible to get some STIs in the mouth or throat from giving oral sex to a partner with a genital or anal/rectal infection, particularly from giving fellatio"
http://www.ashasexualhealth.org/stdsstis/oral-sex-stis/