The death of a giant rabbit that had flown on a United Airlines flight has rattled animal lovers around the world.

It’s still unclear what killed Simon, a three-foot-long giant continental rabbit, who was traveling from London’s Heathrow airport to Chicago, where his breeder said the animal was going to connect to another flight to meet his new owner.

While it’s become easier than ever to travel with your animal friends, deaths, injuries and other mishaps do occur on flights.

According to the US Department of Transportation, just over 0.9 in 10,000 pets traveling aboard a US airline, died, was injured, or lost last year.

Nine pets died on United Airlines last year, more than on any other airline, according to the government data. United transported 109,149 pets last year, about a fifth of the national total.

United told told Quartz that Simon the rabbit arrived in good condition to Chicago’s O’Hare International Airport but a kennel employee there later noticed he was motionless and said he “had passed away.” The airline said the owner declined to have an examination to determine the cause of death.

While airlines including United say they offer climate-controlled conditions for pets traveling as checked luggage or cargo, animals can undergo enormous stress while traveling due to separation from their owners, varying temperatures, unfamiliar conditions, or turbulence. Some airlines prohibit certain breeds from traveling at all, fearing the could have difficulty breathing.

The Humane Society recommends finding other ways to travel than air, or if it is absolutely necessary, try to bring the animal in the cabin, a method that has been embraced by those with emotional support animals, and hunting falcons.

If a staycation or leaving your dog at home is out of the question, here’s a look at how US airlines stack up when it comes to incidents involving animals: