Nine monkeys – including three from a critically endangered species – who died in Hong Kong’s oldest zoo this week had been infected with an endemic disease, officials said on Friday.

The animals in the Hong Kong Zoological and Botanical Gardens contracted melioidosis, possibly after some digging work near their cages, that later caused them to develop sepsis, Secretary for Culture, Sports and Tourism Kevin Yeung said in a press briefing.

Yeung stressed that such infections typically occur through contact with contaminated soil and water and that there is generally no danger to humans from contact with infected animals or people.

“We’re saddened by the passing of the nine monkeys,” he said.

Eight monkeys were found dead on Sunday and another died Monday after displaying unusual behavior, officials said earlier this week. The deceased animals were a De Brazza’s Monkey, one Common Squirrel Monkey, three Cotton-top Tamarins and four White-faced Sakis.

Cotton-top Tamarins — a type of small, tree-dwelling monkey native to the tropical forests of South America — are considered one of the most endangered species of primates in the world.

Fewer than 6,000 of the monkeys are left in the wild, according to the Smithsonian’s National Zoo and Conservation Biology Institute.

What is melioidosis?

According to Hong Kong’s Center for Health Protection, melioidosis is caused by the bacterium Burkholderia pseudomallei, which is widespread in soils and muddy water.

Yeung said the park conducted digging work to repair some irrigation pipes under the flower bed near the monkey cages in early October and that the deaths might be related to that.

He said the monkeys might have come into contact with the bacteria after the park’s staff walked into their cages with possibly contaminated shoes. Another possibility is that some infected monkeys had close contact with other monkeys, he said.

“The incubation period for melioidosis in primates is about a week and this matched with the period after the soil digging work,” he said.

According to the Centers for Disease Control and Prevention, other animals known to get melioidosis include sheep, goats, pigs, hogs, boars, horses, cats, dogs and cows. The disease is mostly found in tropical climates, especially in Southern Asia and northern Australia.

Edwin Tsui, the controller of the center, said the incident only happened in a single zone and its impact on Hong Kong residents would be very low.

Yeung on Monday held an urgent interdepartmental meeting about the deaths with the Leisure and Cultural Services Department, the Agriculture, Fisheries and Conservation Department and the Department of Health.

Another De Brazza’s monkey also displayed unusual behavior and appetite but officials said its condition remained stable on Friday.

The Hong Kong Zoological and Botanical Gardens — the oldest park in the former British colony — fully opened to the public in 1871. It is a rare urban oasis in the downtown Central district of the financial hub, which returned to Chinese rule in 1997.

The deceased monkeys were among 93 mammals kept at the zoo, which also houses reptiles and birds.