Hundreds of thousands of puppies that are raised in puppy mill conditions are advertised online each day, according to a report released by the nonprofit the International Fund for Animal Welfare Tuesday.

Nine investigators from the animal rights group and 16 volunteers spent a full day in July tracking ads on six high-volume puppy sale websites -- Animaroo, DogsNow, NextDayPets, PuppyFind, PuppyTrader, and TerrificPets -- and three sites that sell puppies in addition to other goods -- Craigslist, eBay Classifieds, and Oodle. Investigators then spent two months analyzing the collected ads.

In one day, more than 733,000 puppies were advertised for sale, with prices ranging from $1 to thousands of dollars each, according to the report. The investigators estimated that 62 percent of the nearly 10,000 ads from the six puppy sale websites were likely from puppy mills, commercial breeders that prioritize profit above the animals' welfare.

“The Internet is the Wild West of puppy mills. It’s a 24-7 unmonitored, anonymous marketplace, the world’s largest marketplace. It’s very difficult to monitor and therefore easy for sellers to circumvent laws," Jeff Flocken, director of the IFAW office in Washington, told The Huffington Post.

Investigators used a variety of factors to determine if a facility was likely a puppy mill, including if the seller screens potential owners, if puppies under eight weeks old are offered, and if the same puppy is advertised as a different puppy elsewhere. Other criteria the investigators searched for included if sites have a no refund or return policy, if 20 or more puppies are advertised at a time, if registration papers for the puppy are offered, if sale slogans such as “Christmas Pets” or “Easter Pets” are used, and if puppies appear photoshopped into artificial settings.

A telltale sign, Flocken said, is if the seller will only ship the puppy or meet some place other than the kennel. “Never buy an animal unless you can see the facility in which it was raised,” he said.

The online advertiser with the largest percentage of puppy mill animals, according to study, was Animaroo, where 85 percent of the ads appeared to from puppy mills. Next was PuppyTrader, with 64 percent “likely puppy mill” ads, DogsNow with 62 percent, NextDayPets with 61 percent, PuppyFind with 55 percent and TerrificPets with 44 percent.

The site with the highest quantity of puppies being advertised was Oodle.com, which was advertising more than 440,000 puppies, with over 212,000 ads, on July 18. Oodle was followed by eBayclassifieds.com, which was advertising more than 79,000 puppies, with more than 38,000 ads, that same day.

Dean Hamill, president of PuppyTrader.com, told HuffPost that if he gets a complaint with merit from a buyer, he removes the breeder from the site and forbids any advertising from the breeder. "Breeders of bad reputation are usually found out quickly with this type of feedback," he said.

Hamill said that he thinks the report is "deceptive and highly biased." "The writers even admit to the report's likely inaccuracy by using their criteria to come to a conclusion that the advertisers were to be classified as only 'likely a puppy mill,'" he said.

He also said that pet stores and brokers -- not individual buyers -- keep problematic breeders in business.

The other websites in the report did not respond to HuffPost's request for comment.

According to the nonprofit, this is the first investigation of this magnitude. Flocken said investigators were shocked to find out how many ads there were in one day.

And the problem is getting worse as the demand for dogs increases, according to IFAW. The number of dogs living in U.S. homes is at an all-time high at more than 78 million, according to the American Pet Products Association’s 2011-2012 National Pet Owners Survey.

The most effective solution, the report says, is consumer awareness. IFAW held a press conference with actor Ben Stein in Los Angeles Tuesday to announce the report.

“Consumers opting to purchase puppies over the Internet are duped into believing they are buying from reputable breeders,” Stein said in a press release. “The cute puppy images shown on many seller websites hide the heartbreaking reality of the overcrowded and unsanitary conditions in which the dogs are housed.”

Puppies from puppy mills often have serious health conditions that either result in costly veterinarian expenses or premature death, according to the group. "Sometimes a puppy dies within days of joining a family," Tracy Coppola, campaigns officer of IFAW, told HuffPost. "Even if they do live, they're shell-shocked, permanently emotionally disturbed."

In addition to using the report to inform consumers, the nonprofit hopes to urge websites to refrain from posting puppy mill ads and to persuade the U.S. Department of Agriculture to put into action the online puppy mill sales regulation it proposed in May.

"The USDA recently recognized that online sales were a loophole in the outdated 1966 Animal Welfare Act. We hope they use this report to put an end to this abuse," Coppola said.

The nonprofit plans to take the report to Capitol Hill in the spring to lobby Congress to provide increased funding to hire more inspectors for the USDA Animal Care Program.

Since 2004, IFAW has performed numerous investigations on Internet wildlife trade, including a report that convinced eBay to institute a global ban on the sale of ivory products on its site.

In October, Los Angeles became the largest city in the U.S. to ban the sale of commercially-bred pets, requiring pet stores to sell rescues only. LA Councilman Paul Koretz introduced the ban after his own Bichon Frise died due to an illness he said he believes was caused by conditions at the puppy mill that sold him the dog.

"Dogs have become members of our family in the U.S.," Coppola said to HuffPost. "We're really talking about profit-making entities that are treating family members as crops, as agricultural commodities. This hurts dogs, and it hurts families."

UPDATE: Animaroo sent the following statement to HuffPost:

Animaroo is, and has been for over five years, the only online site that takes the time and performs the personal contact needed with each and every breeder that advertises on its website. Be looking to Animaroo.com in the coming weeks for more stringent programs, a guaranteed puppy mill-free environment, and a respectable place for breeders and dog lovers to interact.

PuppyFind sent the following statement:

PuppyFind was launched in 2003 to help the public connect "directly" with dog breeders, instead of relying on pet stores -- which have been known to sell puppies from puppy mills.

We screen all new accounts and do not allow pet stores or known puppy mills to advertise. We limit the number of listings posted to each account, developed a breeder rating and review system, and added a "Buyer Tips" page to arm buyers with the information they need to find great breeders and the perfect puppy.

Earlier on HuffPost:

Check out images from No-Kill Los Angeles (NKLA), a campaign to dramatically reduce animal euthanasia by 2017: Courtesy of TBWA\Chiat\Day.
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