This!
I live in CA and had a really bad year with PF. Several horses had external and one nearly died from the internal version (saved only with weeks of meds).
I did not quarantine the affected horses because it’s really pointless – the bacteria is in the soil and is carried around by flies. So I had a mixed turnout group, some who wound up with PF and some who didn’t.
Without exception, the horses who tended to have chronic belly sores from flies were the ones who got pigeon fever. I believe it’s transmitted by flies and when the horse lays down on infected soil.
I am pretty hard-core these days about putting Swat, vaseline, anything greasy on those midlines during summer. To date, not a single case of PF, and my pasture definitely has the bacteria sitting in it.