Monkeys being flown to the US for laboratory research died in crates on board the plane, sources say.
Animal-protection activists, who described the deaths as “shocking and heartbreaking” called on the Spanish airline Wamos Air to stop carrying animals destined for experiments.
This year the airline has flown several shipments of monkeys out of Asia for lab tests.
The tragedy happened on Sunday when the holiday charter airline was flying 720 long-tailed macaques as cargo on board a flight from Cambodia to Houston, according to information passed to the Action for Primates (AfP) organisation.
It is not known how many died, but it is thought others also suffered badly.
The monkeys were flown for a total of 24 hours, with an extra six-hour stop in Tbilisi, Georgia, which included a three-hour delay, according to the informant in Spain. In addition, the primates were driven for hours to and from the airports.
Long-tailed macaques are the most heavily traded non-human primate species in testing, and the US is one of the world’s largest importers and users for research. Last year, imports of them from Cambodia by the US shot up by 82.8 per cent – from 8,571 in 2019 to 15,664.
Monkeys are carried on flights in small transit crates, usually too small to allow them to stand up in, which are treated as cargo.
Activists say the animals endure poor ventilation, unfamiliar and loud noises and extremes of temperature and humidity. The animals may become ill or die in transit, made worse if there are delays en route.
Sarah Kite, co-founder of Action for Primates, said: “This tragedy exposes the shocking reality of the suffering inherent in the transportation of these intelligent and sentient beings.
“It is simply not possible to confine non-human primates to small crates, away from familiar surroundings, and transport them on long journeys across the world without causing considerable distress, physical and psychological suffering.
“We know that deaths occur on airlines flying monkeys for research, but details are rarely publicised.
“This shocking and heartbreaking incident is a stark reminder of the very real suffering involved in the global trade and transportation in non-human primates for research.
“It’s time for Wamos Air to join the long list of airlines that now refuse to be a part of the cruel global trade in monkeys for research.”
Ms Kite said that anxiety and stress in primates on flights can lead to infections and the onset of disease that may remain latent until the animals reach their destination.
Photographs obtained by The Independent of a primate-breeding centre in Cambodia show the animals tattooed on their chests and kept in rows of outdoor cages.
According to Action for Primates, the conditions breach the guidelines of the International Primatological Society, which say animals’ welfare, physical, behavioural and psychological needs should be met as far as possible and they should be free to express normal behaviour patterns, including foraging.
Ms Kite said the unnatural environment was “a sharp contrast to the lush foliage of their forest homes”.
Wamos declined to comment on the deaths, referring The Independent to the laboratory involved.
The Madrid-based airline, a subsidiary of the Miami-based Royal Caribbean Group, has come under repeated pressure from animal-welfare organisations this year for carrying macaques for lab tests.
In 2016, the group, then known as Royal Caribbean Cruises, unveiled a plan with the World Wildlife Fund to reduce its environmental footprint, which included “eliminating procurement of highly vulnerable species”.
The IUCN Red List of Endangered Species this year moved macaques’ conservation status from “least concern” to “vulnerable” following a drastic decline in numbers. Threats listed include international trade for laboratory research.
Envigo, understood to be the laboratory importing the latest monkeys, carries out toxicology testing, but says it uses animals only when there is no alternative available or where research is mandated. “Animal welfare is a top priority,” its website says. “We adopt a humane and compassionate approach, actively fostering a culture of care toward our animals... Animals that are well cared for allow us to produce better science.”
The Independent has asked Envigo to comment but had not received a response by the time of publication.
The US Department of Agriculture Animal and Plant Health Inspection Service said in a statement that it was aware of the monkey deaths and that an animal welfare inspector was present when the flight arrived in Houston.
US group Rise for Animals says figures from the US Centers for Disease Control and Prevention show that last year four monkeys were found dead on arrival and 75 others died during quarantine in the US.
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