A coggins and brand inspection are not a health certificate and therefore require nothing more than visual inspection (and a blood test). If the filly did have a health certificate done, usually all that consists of is matching a horse with the coggins, quick visual, heart rate and temperature. This may have been done quickly enough that the strangles was missed, or what might be more likely is that strangles is not a reportable disease meaning that the vet might not even have to note it on the paperwork.
I checked oregon's import requirements, and they only list negative coggins, health and permit #, so the horse could easily enter with strangles.
Strangles isn't the end of the world, BUT it is an absolute responsibility that you/the barn needs to practice good management and sanitation to prevent spread and end the incubation.
She did have all of that, but it took her a while to get the health certificate because the vet wasn't sure about her so they waited till she was better to let me take her. She came from a ranch in Washington and none of their other horses had strangles so I'm not sure how she could have gotten it?
I just emailed her breeder to check and see what their vet had said about Scarlett and I mentioned Strangles so I will let you all know once I get a reply back from that as well.