At the recent French-Speaking Allergology Congress (CFA 2025) in Paris, Sylvie Leroy, MD, PhD, a pulmonologist and allergist at the University Hospital of Nice and the RespirERA Institute, France, presented research showing that strategies to reduce cat allergies, from bathing and antibody-based diets to vaccines and genetic approaches, generally provide only partial or temporary relief.

With 16.6 million cats in French households, allergies are a common clinical concern. The symptoms range from rhinitis and conjunctivitis to eczema and asthma. 

Fel d 1, a secretoglobin primarily produced in the sebaceous, salivary, lacrimal, and perianal glands, is the main allergen driving reaction in humans. The protein spreads across the coat during grooming and can persist in the environment for several months through hair and dander.

However, beyond recommending pet avoidance in severe cases, what effective options can physicians offer?

Allergens

No evidence supports the use of a specific cat breed to reduce allergen exposure. A 2019 study in Missouri and Ontario, United States, and Canada, on short‑haired, mixed-breed cats, showed that Fel d 1 concentrations in saliva vary throughout the day, typically being higher in the afternoon than in the morning and also fluctuate seasonally within the same animal. 

Differences between individual cats are substantial, with some producing up to 80 times more Fel d 1 than others, regardless of coat color. 

Older cats showed modest reductions in Fel d 1 production, but this was insufficient to guide adoption. Male cats secrete more Fel d 1 than females, and this difference disappears after castration.

Standard advice includes restricting animals from bedrooms, removing carpets, ensuring proper ventilation, and regularly vacuuming surfaces. The evidence for high-efficiency particulate air (HEPA) filtration is stronger. 

In a 2020 double-blind, placebo-controlled trial in Strasbourg, France, patients with cat allergy and asthma were exposed to controlled allergen levels with or without HEPA H13 filters. 

Bronchial symptoms significantly decreased with HEPA H13 filtration. Early asthmatic responses occurred in 29.2% of the participants using the HEPA filter vs 87.5% without it, and late responses occurred in 16.7% vs 45.8%, respectively. 

The maximum decrease in forced expiratory volume in 1 second was 17.24% with the HEPA filter compared to 25.62% without the HEPA filter. Airborne allergen levels fell from 36.3 ng/m³ to 4.7 ng/m³ with filtration. No significant effects were observed on rhinoconjunctivitis symptoms (P  > .05). 

Leroy summarized, “These results confirm that HEPA filters reduce airborne cat allergen levels, although the clinical impact on symptoms remains variable.”

Bathing a cat lowers the Fel d 1 level, but only briefly. Weekly washing reduces the allergen load; however, allergen levels typically return within a week.

Daily bathing leads to the reaccumulation of Fel d 1 on the skin and coat within 1-2 days, with effects rarely lasting for more than 48 hours. 

“Bathing temporarily removes a substantial portion of the allergen and reduces its airborne spread, but its effect typically lasts less than 1 week,” Leroy noted.

Diet

Cat food (kibbles) supplemented with chicken egg-derived IgY antibodies targeting Fel d 1 is currently under study. In a 12‑week experimental study (without a control group) involving 106 cats, Fel d 1 levels in hair dropped by an average of 47%, with 86% of cats showing at least a 30% reduction and half reaching at least 50%, particularly among those with higher baseline levels. 

A proof-of-concept study of 49 allergic cat owners whose cats received these kibbles found symptom improvement, especially rhinitis, using the MASK‑air app(Mobile Airways Sentinel Network), a mobile health app that assesses the daily control of allergic rhinitis and asthma.

“These results provide encouraging proof-of-concept. However, their scope was limited by the absence of a control group, reliance on self-reported symptoms from cat owners, short study duration, and only partial response rates. A larger, controlled study in humans, with no competing interests (here, Nestlé Purina Research) is needed to confirm the effectiveness of this approach,” said Leroy.

Breeds

Breeds often described as hypoallergenic include the Siberian, which generally produces less Fel d 1 but shows wide individual variation; the Balinese, whose saliva contains fewer allergenic proteins; the Devon Rex and Cornish Rex, which have short hairs that reduce allergen deposits; the Sphynx, hairless but still producing Fel d 1 through saliva and skin; and some Bengal cats, which may be less allergenic in certain cases. However, evidence supporting these claims is limited.

One study on Siberian cats identified missense mutations in Ch1 and Ch2 that reduced Fel d 1 production. However, the proportion of cats carrying this variant and the Fel d 1 levels in other cats are unknown.

Overall, “no breed is allergen-free, and Fel d 1 production varies widely among individuals, even within the same breed.” Devices are available for measuring Fel d 1 in saliva; however, their clinical usefulness is limited.

Vaccine

A conjugate vaccine (HypoCat) combining recombinant Fel d 1 with virus-like particles was developed to induce neutralizing antibodies in cats. After administration, activated B lymphocytes produce specific anti-Fel d 1 IgG antibodies that bind to the Fel d 1 allergen , thus reducing the presence of the allergen in various secretions, including tears.

A 2019 meta-analysis of 54 cats reported that the vaccine was well tolerated, with no apparent adverse effects. All cats developed a robust and durable immune response, producing high-affinity antibodies with a significant neutralizing capacity. Tears from vaccinated cats showed reduced allergenicity, and the risk of toxicity was low.

Leroy noted that “the mechanism of action of vaccines is not fully understood. Responses were partial and varied between cats, although most showed some effect, with up to a 50% reduction in Fel d 1 production. The study duration did not allow conclusions to be drawn regarding long-term immunity. No data are available on the real effect on allergic symptoms in humans, whose allergic phenotype is not specified, nor on the environmental impact, particularly on Fel d 1 levels in ambient air.”

A small open-label study enrolled 10 allergic adults and 13 cats; the cats received three subcutaneous Fel‑CuMV injections (100 µg) at 3‑week intervals (weeks 4, 7, and 10), with a booster at week 56 (around 1 year).

The outcomes were a home-based provocation test (petting time to a visual analog scale of 5; Organ-Specific Symptom Score [OSSS]), weekly symptoms without provocation (General Weekly Symptom Score [GWSS]; monthly General Symptom Score [GSS], in the extension), and anti-Fel d 1 IgG titers in cat sera.

By week 24, the mean petting time increased from 16.9 to 27.7 minutes (P = .02), and GWSS decreased from 7.2 to 4.4 (P = .023).

Fel d 1-specific IgG rose approximately 300‑fold after two injections and correlated with owner-reported outcomes (petting time, OSSS, and GWSS).

After the week‑56 booster, provocation tests (weeks 58, 62, 78, and 98) and monthly GSS showed benefits that persisted but were smaller than those after the initial vaccination. However, broader ethical considerations remain unaddressed.

Transgenic Cats

Fel d 1 has existed for approximately 11 million years and is specific to felines, with genes expressed only in species of the Felidae family. 

Genetic evolution among the eight extant felid species showed significant interspecies variability, with sequence changes accumulating over millions of years. 

This genomic variability indicates that Fel d 1 is not highly conserved across Felidae and may not be essential in cats. However, their potential roles in immune modulation, epithelial protection, and chemical communication remain unclear.

The Fel d 1 protein is a tetrameric complex of 35-40 kDa, formed by two 18-20 kDa heterodimers. 

“Each subunit has two polypeptide chains, Ch1 and Ch2, encoded by separate genes,” Leroy explained. “This dual-gene structure is particularly interesting, as it offers potential avenues for gene therapy.”

In one study, using CRISPR-Cas9 gene editing to disrupt the Fel d 1 chain 2, investigators produced a homozygous cat, Alsik, with near elimination of measurable Fel d 1 and a clone that retained the same hypoallergenic profile. Single guide RNAs were designed using CHOPCHOP and CRISPR RGEN; guide C2‑1 was validated by a T7 endonuclease I assay and, together with Cas9 messenger RNA, was microinjected into one-cell embryos. Edits were confirmed by T7 endonuclease I and Sanger sequencing, and enzyme-linked immunosorbent assay showed a 98.6% reduction in saliva and a 99.2% reduction in fur compared to the wild type, with heterozygous cats also showing a reduction.

Sequencing of the four predicted off-target‑ sites revealed no edits, although no genome-wide assessment was reported, and human allergenicity was not tested. This first proof of concept for hypoallergenic cats proposes extending editing to chain 1 to remove Fel d 1 entirely and collaborating to assess human allergenicity, while noting the unclear biological role of Fel d 1, the possible immune effects in cats, and ethical considerations.

The experts declared no conflicts of interest.

This story was translated from Medscape’s French edition.