What’s not to love about a kitten that makes you giggle at its playful capers, or a cat that cuddles its warm body into yours and purrs blissfully while you stroke its soft fur? While many of us relish the company of our feline friends, for others it’s a miserable experience: sneezing, red eyes, hay fever, itchy rash, breathing difficulties and even asthma are some of the allergic reactions cats can trigger in humans.
But is this a defence mechanism that they unwittingly evolved to fend off enemies? That’s what a recent study involving an incredibly cute but atypically venomous primate – the dreamy-eyed slow loris – speculates could be the case. It was a chance discovery by Bryan Fry, associate professor at the University of Queensland, who embraced an opportunity to study the scientifically mysterious venom of the endangered Asian primate in an Indonesian wildlife rescue centre.
Slow lorises secrete their venom from a gland in the crook of their arms, and when threatened will hiss and raise their arms above their head, in a stance reminiscent of a cobra. From this position they can quickly suck the toxin from their armpit, ready to attack with a venomous bite.
Fry’s team sequenced the protein contained in the venom and used a computer program to compare it to all other known protein sequences. The cat allergen was the closest match.
The Fel d 1 protein – the array of amino acids that cat allergy sufferers can blame for their symptoms – is a known and potent allergen that lurks in cats’ saliva and skin glands and is further distributed when they lick themselves. This sticky substance gets around – somehow, it’s even been discovered in Antarctica – and is one of the most common allergens. People can develop a sensitivity or allergic response to felines without ever having owned one, or through repeated exposure.
Observing that a bite from an endearing little slow loris can sometimes trigger an anaphylactic shock, an extreme allergic response, Fry was intrigued and started to wonder if allergens are coincidences or something selected for in the wild, as an anti-predator defence.
Allergy experts are not convinced.
Wayne Thomas, emeritus professor at the University of Western Australia, says other studies have found proteins that resemble well-known allergens, “but many different types of proteins are important allergens showing very different biochemical properties and functions, so it is easy to make spurious associations.
“It is possible that the function of different proteins might influence their propensity to cause allergies and there has been much speculation about this.”
But he says the study doesn’t provide any insights about what part of the loris protein is responsible for its function as a toxin, nor reason to suggest that Fel d 1 might have a toxic function. The paper also contained no evidence that the protein in the slow loris toxin is what causes reactions to the venom or the anaphylactic response.
“I think the structural similarity between the loris protein and Fel d 1 could be sufficient for it to cause anaphylaxis in somebody allergic to cats,” he says, and animal handlers who become allergic to their charges could already have been sensitised by other animals, including the loris itself. “The same occurs with a pet hamster.”
Fry agrees that more research is needed.
“We are not saying the Fel d 1 has all the same bioactivities as the loris toxin; clearly there are differences as the loris toxin also causes pain. But the loris toxin also triggers allergy-like symptoms.”
Fry does have some direct evidence, albeit not published in a scientific, peer-reviewed paper, that the protein is directly responsible for anaphylaxis.
In an audacious experiment, a student (not one of his) popped the isolated loris protein in his mouth “to see what would happen”, and promptly went into allergic shock. “Darwin Award in the making,” says Fry. “But it was quite helpful in proving that it is the protein responsible for the effects. Thus, somewhere in the shared parts of the protein are the allergenic elements.
“Now we have worked out the sequence and structure of it, the next steps are to dissect it and see what makes it tick.”
He does think the line between allergens and toxins could be blurred. “If the immune system is being hijacked, then at what point does it become a toxin? For example, if it is something that is coincidental, like peanut allergies, then that is comfortably in the ‘sucks to be you’ kind of allergen. But if cats are deliberately secreting the allergens as a form of defence, does that make it a toxin?
“That’s the kind of fun philosophical question we are kicking around in the lab.”