In July 2015, in Pymble, on Sydney’s upper North Shore, Nicole Lenoir-Jourdan was bitten by a tick. Her head “swelled up like the Elephant Man”, she tells me. Her naturopath, a “miracle worker” she had been seeing for 20 years, was unavailable so Lenoir-Jourdan, a writer and journalist who runs a PR company, went to a variety of GPs to get a diagnosis. None of them appeared concerned, she says.

Within two weeks, one night after eating some yoghurt, Lenoir-Jourdan was struggling to breathe. She woke up the next morning feeling fine but consulted the internet about her symptoms. Here she learnt about “alpha-gal”, otherwise known as galactose-alpha-1,3-galactose, a carbohydrate that is found in all mammalian blood, aside from that of certain primates, including baboons, gorillas and chimpanzees – and humans. Visiting a sixth doctor, she was told that she wouldn’t be experiencing such a severe allergic reaction to alpha-gal because it’s very rare. “But I asked to be tested [for antibodies directed against alpha-gal] and I had it.” The doctor’s response was some surprising dietary advice.

“She just said, ‘Don’t eat meat,’” Lenoir-Jourdan says, taking a sip of tea laced with soy milk.

When those who are sensitised to alpha-gal digest mammalian meat and related products, such as dairy, they can experience allergic reactions ranging from abdominal pain to anaphylaxis. Remarkably, humans seem to acquire the allergy after being bitten by a tick.

This phenomenon has been reported in 17 countries where humans and ticks coexist (on all continents except Antarctica). A significant cluster of mammalian-­meat-allergy sufferers is found around Sydney’s northern beaches, with a prevalence so high that at one point anyone who presented to Mona Vale Hospital with a middle-of-the-night case of anaphylaxis was diagnosed with this allergy until proven otherwise.

The link between the allergy and tick bite was uncovered by Sheryl van Nunen, an expert in allergies who headed the department of allergy at Royal North Shore Hospital from 1985 to 2012. Van Nunen was perplexed at a marked increase in patients presenting with anaphylactic reactions in the wee hours. Those coming to the hospital seemed to have reacted to eating red meat, an occurrence that was previously rare in van Nunen’s decades of practice. Unlike in the case of a standard food allergy reaction, which occurs one or two hours after digesting the substance, van Nunen’s patients were arriving eight to 10 hours after eating. And what united an otherwise disparate spate of new cases was that all had been bitten by a tick. The link was bizarre but noteworthy. It was a connection made serendipitously by van Nunen, as her hospital was in an area where tick bites had become increasingly prevalent.

Contrary to popular belief, people aren’t born with allergies. Instead allergies are a condition that our immune system somehow acquires, misrecognising mundane molecules as hostile invaders and triggering an arsenal of responses, usually reserved for defence against parasites. In many cases – peanut allergy, for example – we don’t know why people develop the reaction. But in the case of mammalian meat allergy, we know – for the first time – exactly how it has occurred.

“It’s an allergy in a box,” says van Nunen, the founder of TiARA (Tick-induced Allergies Research and Awareness) and now a clinical associate professor at the University of Sydney’s Northern Clinical School.

“It’s a paradigm [for an allergy] because when the tick injects you it bypasses the usual tolerising mechanisms of the immune system.”

Ticks produce alpha-gal in their own gut, possibly due to an evolutionary response to infectious agents they may have been exposed to. When a tick bites a small mammal, such as a bandicoot, attaching itself to their body and feeding on their blood until it is engorged, the alpha-gal from either the tick or its victim binds to a tick protein, forming a fusion that the tick later injects into a human victim.

Allergies in humans are usually triggered by a protein, whereas alpha-gal is a carbohydrate. But because it is introduced into our bodies with the tick-protein companion, and is injected directly into the bloodstream, our immune systems are often aroused accordingly.

“It’s presented into our bodies unvetted and the immune system’s default position is to declare it an unwanted intruder,” van Nunen says.

The reaction doesn’t occur straight away. Sometimes it takes months, but somewhere between the tick bite and the first instance of an allergic reaction the immune system has been modified and trained to react to alpha-gal.

Of course, not everyone has a reaction to the protein – sometimes there is no allergy at all. A reaction can also be localised, occurring only at the area where a person was bitten. But more significantly, the result can be anaphylaxis.

After her tick bite, Lenoir-Jourdan, who has been vegetarian for 30 years, experienced severe reactions to dairy and products with concealed meat ingredients. After eating cashews one night Lenoir-Jourdan’s stomach bloated. “I looked like I was pregnant,” she says, caressing an imaginary bump. “Then I was in childbirth-like agony.”

The nuts she consumed were “sprayed with God knows what”.

Then there’s wine, which sometimes uses dairy in the fining process. “I couldn’t drink French champagne – I had to find vegan champagne … Every love I had was gone.” She vomited after drinking orange juice and became fearful of food in general, losing 14 kilograms in six months.

Mammalian meat and its byproducts turn up in unexpected places. Toothpaste contains glycerol, which can be derived from animal fats, and sweets such as marshmallow contain gelatine.

Medicine, too, can be hazardous. Gel-capped tablets contain gelatine, and vaccines have bovine derivatives. Aspects of the alpha-gal mystery were uncovered in the United States in 2002 when some cancer patients had severe allergic responses after being enrolled in a trial for cetuximab, an anti-cancer drug manufactured using mouse cells. The patients who reacted, from southern US states, were found to be allergic to alpha-gal and were living in an area with high numbers of lone star ticks. Researchers are also investigating whether alpha-gal may trigger an allergic reaction to snake antivenoms.

The one reassuring thing about mammalian meat allergy is that in many cases it eventually does go away. Lenoir-Jourdan, who experienced the allergy for three and a half years, credits a naturopath who prescribed probiotics for her eventual desensitisation to alpha-gal, but van Nunen believes the allergy can disappear without any treatment. “There are many people out there who have lost it. Some after three or four years, some after quite a long time,” she says. “The key is not to be bitten by a tick again.”

This is more challenging than it sounds. The Australian environment, with its increasingly warm and humid climate, is favourable to ticks. Some tick bites cause severe reactions without the complication of alpha-gal. In Australia four people died between 1997 and 2003 from tick anaphylaxis after being bitten by a paralysis tick, a relatively common variety on the country’s east coast.

Allergies are on the rise, too. Today, around one in 20 children have food allergies and one in 50 adults. There are many theories about why: cleaner environments, too much processed food, too little vitamin D. “The rate of presentation to the emergency department has quadrupled for our children [in the past decade],” van Nunen says. “We need to know why.”

This makes van Nunen’s discovery of the link between ticks, alpha-gal and allergies important beyond its specifics: it provides an approach that might help us learn the cause of other allergies. “When you have something that gives you a paradigm for allergies and it’s environmental, we can look at environmental things for other allergies. Maybe we will find something.”