Bart Jansen

USA TODAY

The death of a giant rabbit on a United Airlines flight from London to Chicago focused the spotlight again on the carrier that has struggled with more than one-third of U.S. animal deaths aboard passenger flights during the last five years.

United had 53 animals die on its flights from January 2012 through February 2017, the most recent month available, according to the Transportation Department’s Air Travel Consumer Report. That compared with a total of 136 animals that died on all flights of airlines.

In a statement, United said it was saddened by news of the death of Simon, a 3-foot Continental Giant rabbit, on the flight to Chicago’s O’Hare International Airport.

“The safety and well-being of all the animals that travel with us is of the utmost importance to United Airlines and our PetSafe team,” United said in the statement. “We have been in contact with our customer and have offered assistance. We are reviewing this matter."

The rabbit's breeder, Annette Edwards, said the animal had an exam three hours before the flight and was fit as a fiddle.

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The rabbit incident came after United was under scrutiny for dragging a passenger off a flight April 9 at O’Hare to make way for a crew member. The airline was also criticized for preventing two girls from boarding a flight from Denver to Minneapolis while wearing leggings considered inappropriate for using guess passes given to employees and their relatives.

Onboard animal deaths don't necessarily mean an airline was negligent, as revealed in summaries of department investigations.

Among the four deaths on United flights in January, a Jan. 28 incident involving Hope, a 9-year-old cat, was suspected as heart failure, according to the department. Rocco, a dog, died on a flight Jan. 21 from a cardiac abnormality due to congenital heart disease, according to the medical exam. Two geckos were found dead upon arriving at Raleigh-Durham airport on Jan. 12, but no medical exam was performed.

The department requires passenger airlines to report any deaths, injuries or lost animals from flights with at least 60 seats.

Transporting pets has become contentious in recent years as more passengers seek to bring emotional-support animals in the cabin with them. While pleasing the owners, the larger number of animals that include birds, pigs and monkeys has sometimes upset fellow passengers.

The department considered limiting the species or sizes of animals but hasn't acted yet. Another concern for pet owners is what might happen when animals in portable boxes are transported with checked luggage.

United didn’t have the worst statistics when compared with how many animals it was transporting during the last couple of years.

During 2016, when United transported 109,149 animals, it had incidents of deaths or injuries in 2.11 out of every 10,000 animals, according the department. Hawaiian Airlines, which transported only 7,518 animals, had a higher rate of 3.99 deaths or injuries out of every 10,000 animals.

During 2015, when United transported 97,156 animals, it had 2.37 incidents per 10,000 animals, according to the department. Envoy Air, which transported only 1,673 animals, had 5.98 incidents per 10,000 animals.

United hasn’t always been near the top of these statistics. In 2010 and 2011, Delta Air Lines had the most deaths with 16 and 19, respectively, for nearly half the deaths in those years. But since then, Delta’s totals dropped significantly, to five deaths and five injuries last year, or 1.23 incidents out of every 10,000 animals.

Delta's latest animal policy updated in March 2016 allows for pets either in the cabin or cargo for flights less than 12 hours. If the animal's carrying case fits under a seat (other than international business or Delta One seating), it can count as one of two carry-on bags so long as the airline is notified 48 hours in advance. In cargo, a separate booking is required 14 days in advance. Members of the military and foreign service with orders to move can transport a pet as checked baggage.

"We know that pets are important members of the family, that's why we updated our pet travel options over a year ago to ultimately ensure that we have a high-quality, consistent service for pets when their owners choose to ship them with Delta Cargo," said Ashton Morrow, a Delta spokeswoman.

Seven airlines didn’t transport animals in cargo at all last year: Allegiant, Frontier, JetBlue, National, Southwest, Spirit and Virgin America.

Ingrid Newkirk, president of the advocacy group People for the Ethical Treatment of Animals, urged United to stop flying animals in checked baggage.

“This rabbit's death is not unique — more than 300 animals have died in cargo holds since 2005, and many more have been injured or lost,” Newkirk said. “PETA urges United to join JetBlue and Southwest in prohibiting companion animals from being flown as checked baggage in the confusion, noise, extreme temperatures, and improper pressurization of a cargo hold.”