The starting text "est itaque ut" is uncertain, another possible rendering of the text is:
“Excetra ut serpens, hominis quae tacta salivis
disperit ac sese mandendo conficit ipsa.” (DRN 4.638)
excetra = viper (it could be from ἔχιδνᾰ, traditionally held as a poisonous snake, viper, but perhaps also a constrictor) (Wiktionary)
So instead of the usual "there is a certain snake" we could have "there is the Excetra Snake" or "there is the Echidna Viper"
This idea is from Bailey's commentary, page 1258, where he says "most likely the corruption conceals the name of some snake"