It’s a nightmare for any dog or cat owner: suddenly developing an allergy to their pet. Belly rubs and cuddles bring red eyes and a runny nose. Some people can no longer even be in the same room as a pet without their respiratory system sounding the alarm.
Medical professionals’ advice in such cases is unanimous: it’s best to find the animal a new home because the odds of an allergy disappearing are extremely low. In fact, living with the animal increases the risk that allergic rhinitis, commonly known as “hay fever,” will turn into more serious allergic asthma.
But saying goodbye to an animal friend is an unbearable thought for many allergy sufferers. And for some, it’s not just an emotional problem but also an existential one. This is the case for people who need a guide dog, for example, or have a job in which they can’t avoid contact with animals.
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The good news: there is a therapy that can significantly alleviate or even cure allergies. The bad news: the treatment is laborious and not entirely risk-free.
An allergy sufferer’s immune system acts like an overzealous security guard that keeps mistaking harmless visitors for dangerous intruders. If an innocent visitor fits a certain pattern, the guard automatically detains them and sends messengers to alert the entire security apparatus of an apparent threat.
In this analogy, the classic allergy drugs—antihistamines—take over various security posts, where they prevent the messenger substances—histamines—from raising an alarm. But this only temporarily relieves symptoms. The security guard still thinks harmless visitors are dangerous.
Retraining the Immune System
For more than 100 years, however, there has been a procedure that can address an allergy’s root cause: specific immunotherapy, also called hyposensitization. As early as 1911 pathologist Leonard Noon used it to successfully relieve hay fever. To this day it remains the only known treatment that can permanently reeducate the immune system.
Hyposensitization works like this: An allergy sufferer’s body is exposed to very small—and then gradually larger—amounts of the harmless visitors (allergens), and the immune system learns over time that they are not really dangerous. But if too much is introduced at once, the guards panic. And the immune system learns slowly: retraining it takes at least three years.
In this process, there are two ways to introduce allergens into the body in a controlled manner: injections under the skin (subcutaneous immunotherapy) or drops or tablets dissolved under the tongue (sublingual immunotherapy). The injections, or allergy shots, require visiting a doctor, say, weekly at the beginning of treatment and later, say, about every four weeks. The tablets or drops are taken under medical supervision the first time, then daily at home. (Allergy shots for animal dander are available in the U.S. Allergy tablets are only approved by the Food and Drug Administration for ragweed, grass pollen and dust mites. Drops are not approved by the FDA. Europe has approved more subcutaneous and sublingual treatments, including injections, drops and tablets.)*
Specific immunotherapy has proved effective for tree and grass pollen, and experts assume in principle that it also works for allergies to animals. “The effect of both methods is about the same, for cats as well as dogs. In 70 to 80 percent of people who finish the therapy correctly, complete success occurs in the sense that they hardly notice any symptoms,” says Karl-Christian Bergmann, an allergist at the Institute of Allergology at Charité University Medicine Berlin.
The first improvements are usually seen after three or four months, Bergmann says, “and then they can take a cat on their lap, and it’s very likely that they won’t have a cold or watery eyes anymore.” Once a three-year treatment is completed, the effect usually lasts for several years. “But that doesn't mean the allergy is cured forever. It’s possible that the symptoms will come back after a few years,” Bergmann says. No one can say how likely that is.
Preventing the Spread of Severe Allergy Symptoms
The best argument for hyposensitization is that it reduces the risk of the potentially dangerous spread of allergy symptoms from the upper to the lower respiratory tract—from allergic rhinitis to allergic bronchial asthma, for example. The first signs that such a change has occurred are often chest discomfort and an irritating cough. In severe asthma, the bronchi become chronically inflamed and constricted. There is a risk of shortness of breath or even life-threatening respiratory distress.
In principle, this spread of allergies can be prevented by specific immunotherapy. But the study record for animal allergies, unlike for plant pollen allergies, is modest. Reports of success come only from very small studies, and the covered time periods are short. For dog allergy sufferers, there are no large studies at all. The 2022 German guidelines on specific immunotherapy mention “limited evidence” for efficacy in allergies to cats and “insufficient” evidence in dogs. They therefore call for critically weighing the benefits and risks.
This is because there is evidence that serious side effects occur more frequently with treatment involving animal allergens than with other types, warns Helmholtz Munich’s allergy information service. One example is severe anaphylactic shock, a life-threating bodily shutdown. The first signs can be harmless, similar to mild side effects that usually pass quickly: after a shot, the skin around the injection site reddens; after taking drops, the oral mucosa itches. But if the larynx swells up or the circulatory system collapses, the situation quickly becomes dangerous: there is a risk of respiratory distress, unconsciousness and organ failure—an emergency that must be treated immediately.
This risk is the main reason immunotherapy is often not recommended for pet allergies, says Margitta Worm, an allergist also at Charité University Medicine Berlin. “Compared with immunotherapy against pollen, intolerance reactions occur disproportionately often,” she says, adding that deaths are extremely rare and that the danger of a reaction such as anaphylactic shock is likely highest for those who already suffer from asthma or other risk factors. Still, Worm says, “you simply can’t rule it out” in anyone. And treatment is especially difficult if patients continue to live with their pets because their exposure to allergens is not controllable.
Ear, nose and throat specialist Adam Chaker of the University Hospital rechts der Isar at the Technical University of Munich agrees. But he draws a different conclusion. The risk of a life-threatening allergic shock from immunotherapy for a pet allergy is one in a million, Chaker says. It is conceivable but highly improbable that a person could die in such a case if one can exclude risk factors such as cardiovascular diseases. He says specific immunotherapy is “extremely safe” if contact with the animal and its allergens can be minimized—the allergy must be under control.
Chaker is confident that hyposensitization works when it is used correctly. He and his team are researching the molecular mechanisms behind allergies, and they are currently focusing on various biomarkers that indicate—at the outset of specific immunotherapy—whether it will be successful after three years. There is no evidence that treatment is less effective for one respiratory allergen than another, he says.
The therapy can be more difficult, however, if it requires several different allergens from one species. That’s because the extracts used come from the animals themselves—and their compositions can vary. For cat allergy sufferers the issue is usually negligible as more than 90 percent of them react to the main cat allergen. For people with dog allergies, however, the problem tends to be spread across different allergens. Hyposensitization—as well as diagnosis—of dog allergies is therefore often more complicated.
There’s also less demand for immunotherapies against dog allergies, compared with those for cat allergies. “Allergies to dogs usually do not cause symptoms as severe as those to cats,” Worm explains. “Those affected are more likely to opt for symptomatic therapy with antihistamines and to banish the dog from the bedroom.” She advises against permanent treatment of the symptoms with cortisone because of side effects. Allergy sufferers who continue to live with their dog, however, do have a higher risk of developing asthma—albeit lower than they would with a cat. “The therapy of choice, therefore, is always to find the animal a new home,” Worm says.
Chaker agrees and says it would be even better not to get into this situation in the first place. He adds that people often tell him, “I want a dog, but I already have other allergies. Is that possible?” or “My child has neurodermatitis. Can we still get a cat?” In such cases, Chaker says, his answer is that this is “not a good idea. Anyone who is already allergic is in a high-risk group for further allergies and allergic asthma.”
The situation is different when contact with the animal is difficult to avoid, as with visually impaired people who need guide dogs. In such cases, Chaker recommends specific immunotherapy with the necessary precautions: At the beginning of the treatment, the symptoms must be reduced as much as possible, especially in the case of asthma. This is particularly crucial in the first weeks of therapy, when as few allergens as possible should be allowed in from the outside. In the case of a pollen allergy, the treatment should ideally start in the pollen-free season. This reduces the risks and increases the chances of success. “If you manage to keep the allergen exposure low and control the symptoms well with medication,” Chaker says, “you can also treat allergic asthma with immunotherapy.”
Additionally, Chaker sees a great need in a different population: people who do not have an animal in the house but suffer from the allergens that pet owners carry into the public. “These are the people we could do well to help with specific immunotherapy,” he says.
Say, one tablet a day for three years—that certainly seems manageable for those willing to take the risks. But there’s no guarantee of success. Symptoms are likely to lessen but not disappear completely, and they may return after a few years. Whether that prospect is enough to try it is something every pet owner must decide for themselves.
Tips for Allergies under One Roof
Though not a permanent solution, steps can be taken to reduce the amount of allergens in a dog or cat household. Experts recommend:
Keeping the animal away from as many rooms as possible, especially the bedroom.
Avoiding carpets and upholstered furniture.
Reducing blankets and pillows and vacuuming and cleaning regularly.
Changing and washing clothes frequently.
Damp mopping and steam cleaning the floor, seating and other surfaces regularly.
Bathing the animal once or twice a week.
Avoiding close contact with the animal.
Ventilating well on a regular basis or using air purifiers with high-efficiency particulate air (HEPA) filters.
This article originally appeared in Spektrum der Wissenschaft and was reproduced with permission.
*Editor’s Note (5/4/23): This paragraph was updated after posting to provide clarifying information on what allergy immunotherapies are currently approved and available in the U.S. and Europe.