Seven infant monkeys were accidentally poisoned and died at one of the largest primate research centers in the US, internal documents shared with the Guardian have revealed.

The young macaques died after their mothers were marked with dye at the University of California, Davis, primate research laboratory.

Once reunited with their infants, the female macaques inadvertently transferred the dye, which proved toxic to them. Seven of the infants, none more than a few weeks old – with one just a day old – subsequently died.

UC Davis reported the incidents to federal authorities in April 2018, with further details provided in a letter sent a year later. In the latter document, UC Davis told the federal watchdog Office of Laboratory Animal Welfare that two of the infants were found to have “generalized weakness and respiratory distress”, with dye found on their lips and tongues.

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Both of these infants had “severe edema and swelling of the larynx and tongue” and, despite emergency treatment, died. The other deceased infants were “either found dead or euthanized upon arrival at the hospital”. All seven of them had some amount of dye on their fur, skin or around the mouth, which probably triggered a deadly allergic reaction.

In response, the Office of Laboratory and Animal Welfare told UC Davis that infants aged under six months should not be marked with dye and that mothers and babies should be kept apart longer to minimize the transfer of the markings. The correspondence has been released under Freedom of Information laws.

The tragedy is the latest blow to the primate research operation at UC Davis, one of the largest in the country. The university said it had about 4,200 primates, mostly rhesus macaques, which it keeps for research into HIV/Aids, Zika virus and other infectious diseases, respiratory disease, reproductive health, Alzheimer’s disease and ageing.

In 2016, it emerged that UC Davis was among nine federally funded research centers being investigated over mistreatment of primates. In March of that year, a primate fractured both legs after escaping through an unsecured door at the facility, with another primate subsequently injured following a similar incident.

UC Davis has been targeted by animal rights activists over these incidents, as well as a 2005 episode where seven monkeys died from apparent heat exposure. The US Department of Agriculture subsequently handed the university a $4,815 fine.

The latest deaths show that “seven infant rhesus monkeys ranging in age from one to 19 days of age were forcibly removed from their mothers, had a physical exam, were tattooed, had blood drawn, were marked with a dye with irritant capacity, and then placed back on their anesthetized and unresponsive mothers”, said John Gluck, an academic and former primate researcher.

“How could UC Davis fail to consider that this intensely stress-provoking experience would not place these infants at risk? Negligence is the word I would use.”

The New England Anti-Vivisection Society, which wants UC Davis and other facilities to reduce the numbers of primates used for research, said it has written to the USDA requesting that the university be fined $70,000 over the infant monkey deaths.

“It’s 2019, and we can no longer deny the science that shows non-human primates suffer like we would and feel pain like we do,” said Mike Ryan, head of government affairs at the group.

“Invasive and deadly experiments on captive primates who are abused in cages for their entire lives must end.”

The US government has curbed some primate research in recent years, with the National Institutes of Health announcing in 2015 it would no longer fund biomedical research on chimpanzees. Facing hefty public opposition to primate testing, some facilities, such as at Harvard, have wound down their programs and sent their apes and monkeys to sanctuaries.

Testing on primates in the US is far from over, however, with figures released last year showing nearly 76,000 non-human primates in captivity for research purposes. Supporters of primate testing argue it is essential in order to discover new cures for diseases that afflict humans.

“We strive to take the best possible care of animals in our charge,” said a UC Davis spokesman, who added that many of the captive primates are kept in extended family groups, get regular medical exams and can live for up to 38 years, double the expected lifespan in the wild.

UC Davis has typically used dyes to identify individual primates but following the infant deaths “center staff changed procedures so they no longer dye-mark monkeys under six months of age and take other steps to minimize dye transfer”, the spokesman said.