It may be that zoo animals are so well cared for they develop allergies as a side effect.

In 2018, staff at Lincoln Park Zoo in Chicago began to notice something unusual about a male polar bear named Siku. He was losing hair, revealing patches of black skin underneath. Evidently itchy, he was scratching and rubbing himself against his enclosure. Kathryn Gamble, the zoo's director of veterinary medicine, had a hunch what was wrong: allergies.

But what would a polar bear be allergic to? Gamble and her colleagues anaesthetized Siku and conducted a skin test, injecting small amounts of various allergy-provoking substances into his skin. Siku's skin reacted against house mites, as well as elm, mulberry and red cedar pollen. Oddly enough, "one of the things that he initially showed a very strong response to on his skin was actually human hair dander", Gamble recalls.

We often think that allergies only afflict people, but a broad range of other animals can also develop them. That includes pet dogs and cats – in which experts say allergies are on the rise – but also everything from rhinoceroses, bats, dolphins to chimpanzees and rhesus monkeys. Lincoln Park Zoo staff say they have seen allergy-afflicted black leopards, fennec foxes, saki monkeys, Japanese macaques and black bears.

Fortunately for itchy animals living in captivity like Siku, animal allergies are treatable, just like in humans. But these cases may also tell us something deeper about how and why allergies develop.

Scientists speculate that animals get allergies for similar reasons as people in many Western countries – especially those living in cities – who are currently experiencing a rise in allergies.

Some experts blame the fact that pets, as well as farm and zoo animals, are increasingly being raised in manmade environments where they're less exposed to the parasites, microbes and other pathogens that they evolved with, causing their immune system to go awry.

"In dogs and cats, the numbers definitely have gone up, as the numbers in people have gone up," says Annette Petersen, a veterinary dermatologist at Michigan State University. "I wouldn't be surprised if more zoo animals [are getting] allergies too," she adds – although no research has yet looked into whether or not this is the case.

Allergy symptoms can vary across animals. Humans and other primates that inhale allergy-causing substances through the nose often develop hay fever-type symptoms, with discharge, sneezing and weepy eyes. But most animals absorb allergy-provoking substances through the skin and develop skin conditions, says Andrew Rosenberg, a veterinary dermatologist and director of medical operations at the Animal Dermatology Group, a US-based veterinary company.

The cats Rosenberg sees will often scratch their face and neck, tear out hair, or develop pink plaques or crusted pimples on their bodies. Allergic dogs will lick itchy paws and scratch at armpits and groin areas – and tend to develop ear infections. Horses can break out in hives. Occasionally, Rosenberg also sees allergic zoo animals – like the itchy female

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