Table 1.

Articles examining the association between early exposure to cats and/or dogs and allergy and asthma in children.

Reference	Study Group	Study Type	Aim	Age of Outcome Assessment	Key Results
[22]	In the European Union Child Cohort Network, information was collected from 77,434 mother–child pairs from 9 birth cohorts.	Meta-analysis	To investigate whether early ownership of cats and dogs is associated with asthma in school-aged children.	The outcomes were evaluated in school-age children employing the International Study of Asthma and Allergy in Childhood (ISAAC) questionnaire.	No significant correlation was found between owning cats or dogs and the likelihood of developing asthma. The duration and the degree of ownership did not significantly affect these associations. Likewise, owning cats or dogs did not show a significant association with allergic sensitization specific to cats or dogs. However, there was a strong relation between allergic sensitization specific to cats or dogs and the presence of asthma in school-age children.
[23]	Data from 1231 children born in 1996–1997 at Östersund Hospital in Sweden were collected.	Observational study	To determine the relationship between dog and cat ownership during and after the first year of life and the likelihood of sensitization and allergy symptoms at age 13.	At 13 years of age, diagnoses of dog and cat allergies and asthma were made based on parental reports of allergic symptoms in a sample size of 834 individuals.	Keeping a dog or a cat during the first 12 months of life was found to decrease the risk of sensitization to dog or cat allergens, respectively, as well as sensitization to birch and other allergens (Timothy, Cat, Birch, Horse, Dog, Soy, Wheat, Fish, Egg, Milk). Furthermore, owning a cat during and after the first year of life decreased the likelihood of developing cat allergies and hay fever. Dog ownership beyond the first year of life had no effect on allergic symptoms, however, having a dog at home during the first 12 months of life lowered the likelihood of developing dog and cat allergies.
[24]	A randomized trial was conducted on 1303 infants aged three months to evaluate the prevention of food allergy. A survey was administered to determine pet ownership, and the patients were assessed for AD at the time of enrolment.	Randomized control study	To research how having a pet in the house might safeguard against food allergies.	Analyses of serum and skin were performed at 3 months, 1, and 3 years to evaluate sensitization to food and aeroallergens. Additionally, food allergy status was determined through double-blind, placebo-controlled food challenges conducted between 1 and 3 years of age.	Infants who lived with dogs exhibited a 90% lower likelihood of developing a food allergy. Notably, none of the 49 newborns who had at least two dogs in their household experienced food allergies, raising the possibility of a dose–response link. Specifically, for each additional dog owned, the odds of developing a food allergy decreased. However, no significant association was observed between owning dogs or cats and the development of AD.
[25]	Data from Japan Environment and Children’s Study, including 97,413 mother–child dyads, were evaluated.	Prospective study	To investigate the impact of pet exposure on the likelihood of developing food allergies.	The prevalence of food allergy in pre-schoolers was assessed based on questionnaires.	Exposure to dogs or cats during fetal development or the first years of life reduced the chance of developing food allergies up to the age of 3. Specifically, dog ownership was associated with a lower risk of developing egg, milk, and nut allergies, while cat ownership was linked with a lower risk of developing wheat and egg allergies.
[26]	Data from the Polish Mother and Child Cohort (REPRO_PL) were gathered.	Prospective study	The objective of the study was to investigate if early-life lifestyle variables and indoor allergen exposure affected a child’s chance of developing asthma, AD, and allergic rhinitis by the age of 10.	Data were collected at 10 years of age, using questionnaires during interviews.	Among the children included in the study, cat and dog allergens were the most commonly detected allergens in their homes, present in all of the households. A higher concentration of the cat allergen Fel d1 in house dust was found to be significantly associated with an increased risk of developing asthma at the age of 10 years. However, no significant associations were observed between allergen concentrations and the development of AD or allergic rhinitis. For AD and allergic rhinitis, no significant associations were observed.
[27]	The Cohort for Childhood Origin of Asthma and Allergic Diseases was used to provide information.	Prospective study	To examine the relationship between dog ownership at any point between pregnancy and the age of 1 year and sensitivity to airborne allergens at ages 3 and 7, bronchial hyperresponsiveness (BHR), and asthma at age 7.	At 36 weeks of gestation, at delivery, at 6 months, at 1 year, and then once a year, regular follow-up visits for medical examinations and self-report questionnaires were carried out. At 3 and 7 years old, BHR and asthma were evaluated, as well as sensitization to aeroallergens.	No significant differences in sensitization to dogs at ages 3 and 7 were observed between children who owned dogs and those who did not. However, owning a dog during early life was found to reduce the risk of sensitization to aeroallergens at age 7. On the other hand, dog ownership was associated with a significant increase in the risk of nonatopic BHR. Furthermore, dog ownership was linked to the development of nonatopic asthma at 7 years of age.
[28]	Parents of children aged 3 to 6 years (N = 3606) reported data on home environment and symptoms using ISAAC questionnaire in Taiyuan. The survey collected information on symptoms experienced during the first 2 years of life and symptoms reported within the past year.	Retrospective study	The study aimed to examine the reported appearance of childhood wheeze, rhinitis, and eczema symptoms concerning the prenatal, perinatal, and postnatal home environment.	Information on symptoms experienced during the first 2 years of life and symptoms reported within the past year were collected in preschool children.	The prevalence of wheezing and eczema was increased in households with dogs.
[29]	The study involved the participation of children aged 3 to 6 years from daycare centers in seven Chinese cities, with a total sample size of 39,782 children. Parents of children completed a questionnaire regarding the home environment and their children’s health, which included questions about rhinoconjunctivitis and wheezing in the presence of furry pets, as well as diagnosed rhinitis. Additionally, data from monitoring stations were used.	Retrospective study	To investigate how early-life exposure to hairy pets affects a child’s respiratory symptoms.	Outcomes were evaluated in preschool children aged 3 to 6 years.	Children who lived in rural or suburban regions and whose mothers worked as farmers during pregnancy had lower rates of other diagnosed rhinitis (unrelated to furry pets). Both perinatal and postnatal ownership of dogs and cats were associated with symptoms related to furry pets and diagnosed rhinitis specifically related to furry pets. In contrast, owning cats and dogs appeared to have a protective effect against other forms of diagnosed rhinitis.
[30]	Data from Shanghai Allergy Cohort were collected. Information on furry pet ownership was collected through questionnaires.	Prospective study	To research how having pets early in life affects children’s pet AD and sensitization.	Pet sensitization and AD were diagnosed at 5 years old.	Domestic pet ownership during infancy and preschool years was positively linked with a higher risk of dog sensitization. A greater risk of AD at age 5 was inversely related to pet ownership during infancy.
[31]	Information from 3781 children, including in the Finnish Type 1 Diabetes Prediction and Prevention (DIPP) Nutrition Study, was collected at age 5 using ISAAC questionnaire. Data on allergic disease and asthma and exposure to pets during the first 12 months of life were gathered.	Retrospective study	To examine whether exposure to farm and indoor pets in infancy affects the chance of developing asthma and allergies by age 5.	Asthma and allergy were assessed by age 5.	Asthma, allergic rhinitis, and atopic sensitization risk were found to be inversely correlated with the presence of dogs in the home. On the other hand, owning a cat was linked to a lower chance of developing atopic eczema.
[32]	From birth through to age 2 years, 108 mother–child couples included in the Kingston Allergy Birth Cohort were monitored.	Prospective study	To assess the impact of exposure to seven air pollution variables during pregnancy and the first 2 years of life on allergy sensitization.	A skin prick test (SPT) was performed on the 2-year-old children.	Exposure to cats during the 6-month period resulted in a significant increase in the OR of having a positive SPT result. No statistically significant associations between a positive SPT and dog exposure were found.
[33]	539 mother–child pairs included in the study were part of the Polish Mother and Child Cohort (REPRO_PL), a multicentre prospective cohort established in 2007. Mothers in each trimester of pregnancy and 1 year after childbirth completed a questionnaire on animal exposure.	Prospective study	To evaluate the association between prenatal and postnatal exposure to pet ownership and the development of AD, food allergy, and wheezing in children at the ages of 1 and 2.	Children’s health status was assessed at around one year and two years of age.	Keeping a dog at home before and during pregnancy decreased the risk of food allergy in the first year of life. On the other hand, keeping any animal other than a dog (cat, hamster, guinea pig, or rabbit) before and during pregnancy increased the risk of food allergy in the first year of life for children.
[34]	Records from 23585 Swedish children born from 2001 to 2004 were collected from national register.	Retrospective study	To evaluate if dog housekeeping during the first 12 months of life and different dog characteristics reduce the risk of asthma among school-age children.	The outcomes were evaluated at age 6.	Living with female dogs during the first year of life was associated with a lower risk of developing asthma compared to male dogs. Additionally, children who had 2 or more dogs had a decreased chance of asthma compared to those with only one dog.
[35]	Data from 756 children, aged 6–7, about exposure to dogs and cats and allergic diseases were collected with the ISAAC questionnaire.	Cross-sectional retrospective study	To analyze the influence of early exposure to dogs or cats on the prevalence of asthma and allergy among school-aged children.	Data regarding the prevalence of allergic diseases were collected at the age of 6–7 years old.	Exposure to dogs, whether indoors or outdoors, showed a significant association with a reduced prevalence of AD. On the other hand, exposure to outdoor cats was linked to nocturnal coughing and ongoing rhinitis.
[36]	Parental reports of 7360 children from birth to 8 years old in China were analyzed, using the ISAAC questionnaire.	Retrospective study	To find the relationship between pet keeping (cat, dog, rodent, bird, or fish) and asthma and allergy in infancy and childhood.	ISAAC questionnaire was used to assess the association between allergy and early-life pet exposure in children aged 0–8 years old.	Keeping pets, especially cats, was a notable risk factor for diagnosed asthma and AD. Exposure to a pet in early childhood notably heightened the risk of experiencing current wheeze, current dry cough, and diagnosed rhinitis. Even after accounting for avoidance behavior, the adverse impact of pet keeping on children’s health became even more evident. A dose–response relationship was observed between pet keeping and the prevalence of current wheeze and eczema.
[37]	Data from 442 children belonging to a birth cohort of high-risk inner-city children were collected to examine the effects of prenatal and early-life factors on the risk of asthma in school-age individuals.	Prospective study	To discover early-life environmental risk factors for asthma in children.	Data were analyzed at 7 years of age.	In the first 3 years of life, a decreased asthma risk was linked to higher levels of cockroach, mouse, and cat allergens in household dust.
[38]	Two Sweden cohorts were examined (n = 1029, n = 249). The first cohort consisted of 7- to 8-year-old children. The second cohort was a birth cohort. Data regarding asthma and allergy were collected through validated questionnaires in school-age children.	Cross-sectional and retrospective study	To determine if keeping cats and dogs throughout the first year of life was associated in a dose-dependent manner.	The onset of allergy and asthma disease was evaluated in school-age children (7–9 years old)	Between individuals without pets and those with five or more pets, the prevalence of allergy drastically dropped from 49 to 0%. Similarly, the prevalence of allergy in the last year decreased from 32% to zero with an increasing number of pets.