The 100%-positive market vs 0/1,107 farm contrast, and especially the same owner’s civets being negative on the farm yet positive at the market, force the inference that infection occurred at the market rather than on the farms or being intrinsic to the animals.
Reasoning
The within-owner comparison controls for the animals’ genetic stock, husbandry, and farm of origin: the 169 farm civets and the 17 market civets came from the same owner/source, so a difference in infection status cannot be attributed to which animals they were. The variable that changed is location (farm vs market). Since the farm animals were uniformly negative (0/169, and 0/1,107 across 25 farms in 12 provinces) while the market animals were uniformly positive (17/17, and 91/91 civets plus 15/15 raccoon dogs, with 22/24 environmental swabs positive), the market environment - dense mixing of many individuals and species over shared cages and surfaces - is where transmission and amplification occurred. This rules out the farms as the site of civet infection and supports the market as the amplification and spillover locus. The design does not by itself identify the ultimate reservoir that seeded the market.
Verdict — approved (checked)
Reconstruction. Premises: (i) farm and market civets from the same owner shared genetic stock/husbandry/origin; (ii) they were uniformly negative on the farm and uniformly positive at the market; (iii) the broader farm sample was 0/1,107 and the market sample uniformly positive incl. environment. Load-bearing step: a controlled contrast holding the animals constant and varying only location assigns the difference in infection status to location — i.e. a differential-comparison (matched-control) inference.
Evaluation. Assuming the premises, the step goes through for the two claims that carry weight: infection is not intrinsic to the animals (the same stock was negative earlier) and did not occur on the farms. The one surviving undercutting defeater is that infection could have happened during transport/trade between farm and market rather than physically inside the market building; this does not deny the premises. But it does not rescue the farm or the animal-intrinsic alternative, and it collapses the “market vs farm” dichotomy the argument actually tests into “market/trade chain vs farm” — immaterial to what the argument feeds (H-29’s market-amplification claim). The argument itself disclaims identifying the ultimate reservoir. The step is real; approved. Traced directly from the tabulated counts, author-blind.