oxfordlearnersdictionaries.com:
scabies - [uncountable]
britannica.com:
scabies - [plural]
Do I understand correctly that it doesn't matter whether a verb after "scabies" has the singular form or the plural form?
That is, "scabies" is a collective noun, right?
Britannica is wrong, it may have been tricked by a Latin noun ending that looks like an English plural, or this may be American usage. In British English it is singular and always takes a singular verb.(example from Cambridge corpus):
Scabies was obviously a problem of national importance...
The word "scabies" is borrowed directly from a singular Latin word meaning "itch, roughness". While it is possible for a singular word to be become plural in English, this doesn't match actual use. We say "Scabies is a disease". The word is singular and strictly uncountable, it doesn't have a plural form in British English.
To qualify that, one occasionally sees "scabies" used to mean "scabies mites" and in this sense it might be plural "We examined some scabies under the microscope". Moreover, the singular-plural confusion is such that one can find examples of "scabies" used as a plural noun. "Scabies were diagnosed in 35% of the patients". This may be considered a variant, or non-standard use.
