We brought our cats with us from the US and it wasn't a problem at all... a few things to consider:
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www.exportapet.com. We used them to bring our cats in and wouldn't do it any other way. They guide you through the process and paperwork; pick up and ease your animals through customs and deliver them to your home. Start early with them as they are a bit slow and langugage can be a barrier but everything turned out fine.
-- No quarantine from the US AS LONG AS YOU HAVE ALL REQUIREMENTS... i.e. certain shots with paperwork during the right timeframe (Exportapet will guide you if you ask them) and they look healthy when they arrive.
-- Your pets will need to go manifest cargo - with luggage... temperature controlled but some airlines like United won't let your pets fly if the temperatures could be dangerous. For instance, we cut it close when we flew our cats out in the summer because it was getting hot. The concern isn't when the plane is in the air but if it gets delayed/stuck on the tarmac and the animals could freeze/overheat.
-- crate.. make sure you have an approved crate for each animal. They must be able to stand up inside and turn around. Also needs to have air holes on all sides and constructed so that if the crate were pushed up against boxes, it won't be flush and cover the airholes.
-- must include some mechanism for water... a snap on dish with ice or a water bottle like those for mice. Tape a baggie of dry food to the crate so that if there's a delay or a problem, someone can feed the animals. (they usually are too nervous to eat or drink much anyway).
-- Try to get a direct fly from the US to HK. We actually drove from Portland to SF so the cats would only need to board a plane once
-- don't drug your animals as you dont 'know how they will react to the meds in the air and if it's a long flight, the drugs could wear off in the middle of it, making them even more disoriented and scared. After the take off, they tend to be ok
-- I was super nervous/scared for the cats; cried when i dropped them off at cargo so let me know if you have any other questions i can help answer... jsut PM me.