While it may be surprising to hear, many puppies and dogs purchased from pet stores come from puppy mills. Some say that 99% of all pet stores that sell puppies get them from puppy mills and not legitimate, professional breeders. Legitimate breeders typically do not sell their puppies and dogs to pet stores because these breeders usually prefer to interview prospective owners before adopting a dog out.
A puppy mill is capable of providing healthy animals to pet stores, but quite often the remaining puppies are kept in substandard and overcrowded cages. Veterinary care may be spotty or non-existent. If one puppy in a poorly-managed puppy mill contracts a contagious disease, then the chances are very high that other puppies will soon be infected. Canine diseases such as parvo often run rampant in a filthy, improperly constructed puppy mill.
Puppy mill operators may or may not be experienced breeders. Female dogs of breeding age can become exhausted if not given enough time to recover between litters, but inexperienced breeders rarely allow such rest periods. Puppies produced in a puppy mill are often the result of indiscriminate cross-breeding, which means the animal may inherit the worst traits of both breeds. Contact with caring humans may also be a rare event in a typical puppy mill, so a dog's socialization skills could also be compromised.
Operating a puppy mill is not technically illegal in many states, since the difference between a legitimate commercial breeding facility and an unscrupulous backyard breeder can be challenging to prove in court. A suspected puppy mill operator must be proven guilty of violating other laws, including cruelty to animals or operating a business without a license. Having a large number of puppies in small cages is not necessarily a crime in and of itself. It is not illegal to breed animals solely as a source of income, either. This is why animal protection societies have a difficult time shutting down a suspected puppy mill.
Consumers can take several steps to avoid dealing with a puppy mill. One important step is to ask pet store owners and private sellers specific questions about the animal's background. Commercial pet stores may post a strict "No Puppy Mills" policy, but this is no guarantee. Legitimate breeders should have documentation on the animal's veterinary care and place of birth. Puppy mill operators may not supply complete records, or the information may be inaccurate and misleading. Be aware of purebred puppies being offered at a significant discount — this could mean the animals are not registered purebreds or they may have undisclosed health problems.