The church of Santa Maria di Salute was built and dedicated to the Virgin as the Plague continued to ravage Venice. But other saints were also thought to be protectors against the plague and to intercede with God so I think "devotional" or "patron", "interceding" or even perhaps "protector" saints would work here. They were to be venerated or offered devotion to intercede with God and protect the city from the Plague.
"pietà" can tranlsate as patron or intercessor or compassion /pity so peoplevenerated these saints in the hope of gaining their compassion or intercession

http://catholicsaints.info/saints-in-art-05-patron-saints/

p115 here
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1900794/?page=6

https://en.wikipedia.org/wiki/Santa_Maria_della_Salute

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Note added at 18 hrs (2015-06-26 09:18:19 GMT)
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http://travel.yodelout.com/venice-the-four-great-plague-chur...
"the two great plague-saints, St. Sebastian, shot through with arrows, and San Rocco, lifting his garment to show his plague-spot. To the L., the two medical saints, Cosmo and Damian, with their surgical instruments and boxes of ointment : Damian seems to point to St. Roch’s symptoms, as if in consultation. The whole thus represents the preservation of Venice after a severe pestilence by the intercession of St. Mark, whose body she possesses, and of San Sebastian and San Rocco, to both of whom she has erected churches, while of one she holds the actual remains ; as well as by the skill and care of her medical profession, with the aid of the patron saints of the faculty. This is, perhaps, the most characteristic example you could find in Europe of a local plague-picture..."

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Note added at 6 days (2015-07-01 15:38:29 GMT) Post-grading
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glad to have helped. Personally, I think "Piety" wouldn't work at all here...