Last Wednesday, Nghe An police had rescued 17 tigers held captive by two families in Do Thanh Commune, Yen Thanh District.

The tigers were kept in basements by Ho Thi Thanh, 31, and Nguyen Thi Dinh, 50.

The two women told the police that the tigers were kept in separate metal cages for months after being transported from Laos to Vietnam as cubs.

The rescue team had given each tiger with an anaesthetic shot before transferring them to the Muong Thanh ecological area in Dien Lam Commune of Dien Chau District to be taken care of while the case was being investigated.

Eight of the tigers were confirmed dead on arrival at the area.

Hoang Thi Thu Thuy, a veterinarian with Save Vietnam's Wildlife (SVW), a wildlife conservation non-profit organization founded to advocate more effective solutions to secure a future for Vietnam’s wildlife, said the fact that tigers were kept in a closed basement system for a long time means they were not in good health and prone to cardiovascular, liver and kidney diseases.

In addition, rescue procedures have great effects on the physical and psychological health of animals held captive, she said.

The process of rescuing large wild animals is "very complicated" and normally, they must be examined and monitored comprehensively before being anesthetized, and the dose of anaesthetic depends on their actual health and psychological conditions, she added.

For tigers, the anaesthetic dose is normally 3-4 mg per kilo of body weight, but for stressed tigers the dose can be higher, at 10 mg per kilo.

However, if the anaesthetic dose is administered based on the average weight of a tiger, it could be risky and dangerous.

In nature, an individual tiger normally weighs 80-100 kg but the rescued tigers are much heavier, weighing 200-265 kilograms each because they have been held captive in a small space without sunlight, and the people keeping them have feed them on purpose to make them fat.

"Given such a health background, if the anaesthetic is overused, the risk is very high," said Thuy.

Thuy said that the only anaesthetic that veterinary authorities are allowed to use at present is Zolecyl.

This drug, if overused, can cause airway obstruction and heart failure.

SVW director Nguyen Van Thai explained further that large animals can only be moved two to hour hours after anaesthesia.

They must be let to lie in a quiet, enclosed cage to create a sense of security before being taken to the safe area. The transportation of tigers must also be gentle to avoid causing panic.

"The dosage of anaesthetic for a resting tiger is very different from that of a stressed, restrained tiger. When animals are stressed, the ability to inhibit anesthetic drugs is very high," he said.

Nghe An police said at a Monday meeting that relocating the captive tigers was "a humane solution" and the death of eight of 17 tigers was "unintentional."

Explaining why the tigers were removed from the scene, Nguyen Duc Hai, deputy director of Nghe An Province’s Police Department, said the tigers are evidence in the criminal case and if they were not moved, the culprits could tamper with the evidence and compromise the investigation.

He said the process of transferring the tigers was "carefully planned, and in consultation with wildlife experts."

There has been no official report released so far on the cause of death.