Question details: My relative is going to visit us for a week. His father has a very serious allergies towards cats and dogs, but my relative, who is 25, told me, he had a contact with them (quoting “no more than one and half hour a day”) and never something has shown up. What chances are there, thay he may have it?
In other words, could someone with parental history of cat allergy develop cat allergies as an adult, especially if rather than avoiding them, they’ve maintained sporadic contact? Does the scientific literature have an answer for this particular situation? Not clear-cut because allergies are clearly multi-factorial with genetics/family history being only one of many causative factors. Nevertheless in the case of cat allergies, several studies across many countries and age groups cautiously suggest that protection from cat allergy in adulthood shares the following features
- Exposure to cats in the 1st year of life.
- Frequent exposure to cats from a young age.
- Frequent exposure to fairly high levels of environmental sources of cat allergens.
In that regard, someone in contact with cats for ~1.5 hours a day would be exposed to substantial levels of cat allergens. This is also why removal of cats from homes doesn’t necessarily reduce allergic symptoms () for two main reasons,
- Cat allergens are airborne and ubiquitous, i.e., found practically everywhere.
- Cat allergen levels are especially very high in a cat owner’s home.
Ambient cat allergen levels depend greatly on local cat populations, i.e., community levels of cat allergens directly reflect community levels of cat ownership. For e.g., Fel d 1 cat allergen levels are consistently high in house-dust and mattresses of European cat owners (2). High concentrations of cat allergens, specifically Fel d1, have also been found in Swedish schools (3, 4), German schools (), US schools (6), US homes (7), New Zealand school children’s clothing and primary school classrooms (8). Among such studies, cat allergen levels were only substantially lower in special allergen-avoidance Swedish day care centers ( ). Thus, someone in contact with a cat for ~1.5 hours a day is also in contact with substantial amounts of cat allergens that permeate every nook and cranny of the cat owner’s house, i.e., their mattress, clothing, furniture, etc. Higher and more frequent the dose of cat allergens one’s exposed to, more likely to be de-sensitized.
Exposure to cats from early life appears to protect adults from cat allergy. Until the 1990s the prevailing idea was that cat-allergic individuals should practice active avoidance. The narrative started to change when counter-intuitively, a 1999 Swedish study (10) first reported that children raised in a house with a cat were less likely to become allergic to cat dander proteins. Since then this phenomenon has been reported by others and even fine-tuned in terms of details. Typically, such studies test adults for cat allergen-specific immune responses (skin prick tests and blood anti-cat allergen antibodies), and detailed questionnaires gather information on frequency of cat exposure from birth. Multiple such studies across several countries thus confirmed that early life exposure to cats appears to protect adults from cat allergies:
- A study on 13509 adults across 15 countries (Australia, Belgium, France, Germany, Iceland, Ireland, Italy, Netherlands, New Zealand, Norway, Spain, Sweden, Switzerland, UK, US) (11).
- UK studies on 2502 adults (12, 13).
- A US study in Detroit following 270 men and 296 women from birth ( , 15) showed that those who’d had a cat at home during their 1st year of life were significantly less likely to be sensitized to cat allergens at 18 years of age.
- Protective effect of cat ownership is seen even in asthmatics since two French studies on adult asthmatics found earlier the exposure, stronger the protection against cat sensitization in such adults. 187 adult asthmatics and 243 controls in the 1st study ( ), and 89 men and 78 women in the 2nd study ( ).
- In fact, studies, two in the US, one on 226 children (18) and one on 38 adults (19), and one study in Sweden on 412 children (20), suggest exposure to high levels of cat allergen induces de-sensitization types of immune response to them.
- Another study relevant to this question is a US one which asked if decreased cat exposure increased sensitization to cat allergen, a scenario likely when children leave homes with cats and head to cat-free college dorm rooms. Of 97 college students, none developed new-onset sensitization during the course of the year-long study (21). While the study is too small to draw major conclusions, it again suggests an adult who’d been previously exposed to cats but didn’t develop sensitivity/allergy is less likely to do so following a period of reduced exposure. In other words, adults exposed to cats as children are likely to stay stably de-sensitized/tolerant to cat allergens.
- OTOH, an European study of 6292 adults aged 20 to 44 years suggested when an adult keeps a cat for the first time, it can double their risk of cat sensitization, i.e., new-onset cat sensitization, especially if the cat is allowed in the bedroom. However, even in this study, subjects who’d had a cat during childhood seemed protected ( ).
Bottomline, if someone’s been exposed to cats for ~1.5 hours a day frequently, they’ve also been exposed to high levels of cat allergens present everywhere in that cat’s environment, and if they didn’t develop cat allergy yet, they’re unlikely to do so now, especially if they’ve been exposed to cats from the 1st year of life. However, a great deal of this supposition is based on this individual’s accurate observations. With sporadic and short duration exposures to cats, were there never any symptoms at all or were there sometimes mild symptoms, e.g., brief bouts of stuffy/runny nose, sneezing, etc., that the individual just didn’t connect to their cat exposure? This is the key question.
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