What’s behind the sharp rise in tick-induced meat allergies?
Unlike most food allergies, tick-induced meat allergies are entirely preventable, and correctly removing ticks can save patients from potentially life-long allergies.
“The most common allergy for carrying an EpiPen as an adult where we live is mammalian meat allergy or tick anaphylaxis,” Associate Professor Sheryl van Nunen, an allergist at the Tick-induced Allergies Research and Awareness Centre, tells The Medical Republic.
While peanut allergy is the main cause of anaphylaxis worldwide, meat allergies are the leading cause in metropolitan Sydney north of the harbour bridge.
Professor Van Nunen discovered the tick-induced mammalian meat allergy back in 2007.
She says there are around 1000 people in Australia with this condition. “But in the US – because they have a greater population – they probably have about 5000 people with it.”
Mammalian meat allergy generally develops a few months after a person is bitten by the paralysis tick nymph, Ixodes holocyclus. The tick injects saliva into the host over four to eight days, stimulating an immune response in some individuals.
However, appropriate tick bite management can prevent the allergy from developing.
“From the general practitioner’s point of view, kill the tick,” Professor Van Nunen says.
For a larval tick, which is about one-millimetre in length, apply the permethrin-containing cream Lyclear. A nymph tick, which looks like a black splinter in the skin, should be hit five times with a freezing agent such as Wart-Off.
The tick should be left to drop off by itself, which takes one to three hours.
“If you have to take it out then use very fine tipped surgical forceps and you need a steady hand,” says Professor Van Nunen.
Disturbing or squashing a tick can cause it to inject allergens into the bloodstream. This can cause subsequent meat allergies, but also has the added risk of triggering anaphylaxis in people with allergies to the tick saliva itself.
Identifying the allergy
Tick-induced meat allergies are diagnosed primarily on their history, but a diagnosis can also be confirmed through a blood or skin test.
“The time course is quite peculiar for this [allergy],” says Professor Van Nunen. “With allergy tests, whether it’s a blood test or a skin test, it is extremely important that the history lines up with it.”
Patients with this condition will report allergies two to 10 hours after eating mammalian meat. Allergy onset will occur several months after a tick bite.
Some people will have gut symptoms after eating meat, while others will have local or full-body reactions. But some people experience life-threatening anaphylaxis, particularly when red meat consumption is combined with amplifying factors, such as alcohol or exercise.
According to the Australasian Society of Clinical Immunology and Allergy, the following three blood tests will be positive for people with mammalian meat allergy: Beef, Lamb, Pork ImmunoCap; Alpha-galactose ImmunoCap; and Elevation of tryptase.
Professor Van Nunen says skin tests can confirm a diagnosis, but only if done with fresh, raw, organic meat. “The bottle stuff is hopeless,” she says.
While there is no available treatment to “switch off” the red meat allergy through desensitisation, the allergy does disappear on its own in some individuals. “If you don’t have another tick bite then some people lose the allergy within three to four years,” says Professor Van Nunen.
The allergy can be mild in some people and severe in others.
“If you have someone who has very severe anaphylaxis you want to be looking for mastocytosis, whether it be a reaction to a wasp sting or a bee sting or meat or ticks,” Professor Van Nunen says.
A subgroup of patients with tick-induced mammalian meat allergy will also be allergic to mammalian milks and animal-derived gelatin. Gelatin is used as a binding agent in some medications. It is also present in intravenous blood substitutes.
There have been seven deaths worldwide due to very severe anaphylaxis in patients with meat allergies who have received the chemotherapy agent cetuximab (Erbitux).
Professor Van Nunen says GPs with meat allergic people in their care should alert oncologists to this issue upon referral. “That makes it doubly safe for the patient.”
GPs also need to know where the ticks are. The east coast of Australia is a hotspot for tick bites, with the hinterland around Noosa in Queensland and the south coast of NSW being particularly infested. Ticks can also be found up to 100km inland and are also present in the ACT and around Launceston in Tasmania.
How do ticks cause meat allergies?
Through their lifecycle ticks have several hosts. If they feed on an animal and then bite a human they can transfer a sugar molecule called alpha-gal between hosts in their saliva.
Humans can normally tolerate this sugar molecule when eating mammalian meat, but some people develop an immune response if it is injected into the bloodstream directly.
For these unlucky individuals, red meats may forever cause an allergic reaction.
Any meat containing alpha-gal can cause a problem, which includes all mammals except great apes and old world monkeys (which, like humans, evolved to not produce this sugar).
This usually means beef, pork and lamb are off the menu for life, but seafood and chicken are still tolerated.
Tick-induced meat allergies have only occurred recently, says Professor Van Nunen. “It’s an emergent allergy. I’ve seen the odd case from 1987 or even earlier, but it’s really taken off since 2003.”
“[But] just because it’s new doesn’t mean it’s not present. A lot of people think if they haven’t seen it before, it’s not happening.”
A perfect storm of factors has created the problem, she says.
More people are living near tick-friendly environments, microclimate changes have boosted tick populations and people are more allergy-prone because of lack of exposure to dogs, dirt and vitamin D.
