SENECA , Mo. — When authorities raided J.B.’s Precious Puppies, they discovered more than 200 dogs standing in their own excrement, crammed three and four to a cage. Some were so sickly they were missing clumps of hair. The skeletal remains of puppies and adult dogs were found inside pet-food bags.

Missouri is home to more than 4,000 shoddy and inhumane dog-breeding businesses, by one estimate. But the ...