There are a number of factors that contribute to determining if a pet is fit to fly. Each of these should be considered before certifying a pet fit to fly.
Breed: Brachycephalic breeds are prone to developing heat stroke. Many airlines will restrict brachycephalic breeds from flying in the cargo hold.
Age: Young pets may have an increased risk of hypoglycemia, especially small toy breeds. Older animals may experience issues when flying due to pre-existing medical conditions that come with age.
Weight: Colder weather can have more impact on underweight pets, whereas warm weather can more severely impact overweight pets.
Health: Pre-existing medical conditions should be taken into consideration before certifying that a pet is fit to fly. Considerations like;
Does the pet require daily medications that may be limited by flight schedules?
Does the pet have cardiovascular, respiratory, CNS or musculoskeletal disease?
Is the pet pregnant?
Does the pet have separation anxiety?