Two features of the genomic data jointly force a self-sustaining transmission chain. First, each farm’s sequences form a single tight cluster descending from one index genotype (the two farms diverge at 22 positions and do not intermix): if each mink had been infected independently from the surrounding human community, one would expect the farm samples to interleave with diverse community lineages, not to coalesce on one recent common ancestor per farm. Second, within-farm sequence variation has accumulated, which requires replication and onward passage inside the mink population. Combined with the epidemiological scale — a barn of 13,700 (NB1) or 7,500 (NB2) animals showing farm-wide respiratory disease and elevated mortality within roughly two weeks — the only parsimonious generator is mink-to-mink spread from a single (per-farm) introduction. Hence mink function as a competent host that sustains transmission chains, not a dead-end species.
Step 6 verdict — approved (checked)
Reconstruction. Premises: (i) each farm’s sequences form one tight monophyletic cluster from a single index genotype (the two farms 22 nt apart, non-intermixing); (ii) within-farm nucleotide variation has accumulated; (iii) thousands of animals per barn showed farm-wide disease within ~2 weeks. Load-bearing step: (i)+(ii)+(iii) are jointly generated only by self-sustaining mink-to-mink spread from a single per-farm introduction, not by many independent human-to-mink introductions.
Evaluation. The move is a best-explanation inference and it holds. Many independent community introductions would seed diverse lineages interleaving with community diversity, contradicting (i); the per-farm monophyly points to one seeding event, and independent human-to-each-of-thousands infection on a 2-week timescale is not credible, so onward animal transmission is required. Accumulated within-farm variation (ii) independently demands replication and passage in mink. I probed the one live defeater — a single common non-mink source (e.g. contaminated feed/environment) infecting all animals without mink-to-mink spread — but that reproduces neither the accumulated sequential/branching diversity (ii) nor the spreading respiratory-disease time-course (iii). So the conclusion of a competent, chain-sustaining host survives. Traced myself; author-blind.