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United Airlines is scaling back its animal transportation program after several pet deaths and mix-ups. Under the new policy, which takes effect June 18, United will only transport dogs and cats in the cargo hold. It is also banning several short-nosed dog breeds. The airline has transported more animals in its cargo hold than any other U.S. carrier, and has flown with ocelots, rabbits and geckos.

The tighter restrictions come after a passenger's French bulldog puppy died after the bag it was in was stored in an overhead bin aboard a United flight in March. United flew two other dogs to the wrong destinations shortly afterward. Last year, a traveler's spaniel died while in United's care. A giant rabbit that was flying United from London to Chicago also died. Under the new rules, United will no longer transport more than 20 dog breeds and four breeds of cats in its cargo holds "out of concern for higher adverse health risks." Some of the banned dogs include Boston terriers, boxers, pugs, and Persian cats. The restrictions present a challenge to travelers moving long distances with their pets but also to animal breeders who use airlines to deliver dogs, cats and other animals to customers.

Breeds banned from cargo hold:

That puts United's policy more in line with that of its competitors Delta Air Lines and American Airlines. The airline is working with American Humane, an animal welfare group to improve its pet transportation program. United reported to the Department of Transportation the highest number of animal deaths of any U.S. carrier: 18. American and Delta each reported that two animals died on their planes last year. Those figures refer to animals that were transported in the cargo hold, not the cabin. United is also limiting when and where pets can fly because of concerns about high temperatures. United will not accept cats and dogs in the cargo hold between May 1 to Sept. 30 to or from Las Vegas, Palm Springs, California, Phoenix or Tuscon, Arizona. United had suspended its pet transportation program following the death of the puppy and the two mix-ups in March. It has since started issuing brightly-colored bag tags for animals traveling in the cabin with passengers. The new rules announced Tuesday do not affect in-cabin animal transport, but CNBC reported earlier that it is reviewing its in-cabin pet policy.

WATCH: United halts shipments of pets