An old shepherds trick is to stick a ewe who has aborted in with the ewe lambs (not yet tupped) so that they get exposed to, and immune to the disease before they get put to the tup the following year.
My understanding is that all exposed females will abort once and thereafter be immune. So the wily old shepherd forfeits the shearling's lamb(s) as being the smallest and least of all the crops she will hopefully bear.
I am happy to be corrected - but would like to read it in a journal, on the Eblex site, or similar, for confirmation, if you have a link?