(CNN) -- Packed tight into wire baskets -- sometimes 20 or more to a cage -- animal rights activists say as many as 200,000 live dogs every year are smuggled from northeast Thailand across the Mekong River destined for restaurants in Vietnam.
Dehydrated, stressed, some even dying of suffocation on the trip, the dogs are often stacked 1,000 to a truck on a journey that lasts for days.
"Obviously when you've got dogs stacked on top of each other they start biting each other because they are so uncomfortable, any kind of movement then the dog next to the one that's being crushed is going to bite back," said Tuan Bendixsen, director of Animals Asia Foundation Vietnam, a Hanoi-based animal welfare group.
When they arrive in Vietnam, the suffering doesn't end there. A common belief is that stress and fear releases hormones that improve the taste of the meat, so the dogs are placed in stress cages that restrict their movement.
Eventually, the dogs are either bludgeoned to death or have their throats cut in front of other dogs who are awaiting the same fate. In some cases, they've been known to be skinned alive.
"Dogs are highly intelligent animals so if you kill a dog and you have a whole cage of dogs next to the one that's being killed, those dogs that are going to be killed next know what's going on," Bendixsen said.
According to animal rights groups, dog smugglers round up everything from family pets to Thailand's ubiquitous strays -- known as soi dogs -- to sell the animals in Vietnam, or even as far away as China where a pedigree dog can fetch a premium price.
John Dalley of the Phuket-based Soi Dog Foundation estimates 98% of the dogs are domesticated and that some are even still wearing collars and have been trained and respond to commands.
"You can see all types of pedigree animals in these captured Thai shipments -- golden retrievers, long-haired terriers, you name it," says Dalley. "Some are bought. Others are snatched from streets, temples, and even people's gardens."
In the past, batches of stray dogs were traded for plastic buckets, but these days with demand soaring -- especially in the winter months when dog meat is regarded as a "warming" food -- a dog in Thailand can fetch up to $10. This figure jumps to around $60 once they are served up in restaurants in Vietnam.
Dalley says pet dogs, in particular, are targeted because they are friendlier and easier to catch.
Animal rights activists estimate that more than one million dogs are eaten each year in Vietnam; for the dog smugglers of the Mekong, business is booming.
While the trade is illegal in Thailand, and authorities have made a number of raids involving thousands of dogs, dog traders claim the laws are unclear and have even mounted counter protests against a series of crackdowns.
Smugglers are normally prosecuted under laws that prohibit the illegal trade and transportation of animals and, with no direct animal cruelty laws in Thailand, prosecutors attempt to charge smugglers with cruelty under Criminal Code laws.
The Soi Dog Foundation and the Thai Society for the Prevention of Cruelty to Animals are fighting to change that and are currently working through the Department of Livestock Development to get an Animal Welfare Draft Law through the Thai Parliament.
The reality, however, is that smugglers often receive light sentences of just a few months in jail. Animal activists also say thousands of impounded dogs -- rescued from smugglers -- that end up in quarantine centers sometimes find their way back onto the streets and in the dog meat circle again.
"This is not about whether it is right or wrong to eat dog meat," Dalley says. "It is about an illegal trade worth millions of dollars per year organized by criminals. The way in which these dogs are transported and, if they survive, killed, is horrific.
"Some of the footage we receive is so horrific it's too strong even for the media to run. It's so inhumane ... it's quite literally hell on earth."