Table 1

Study group, authors’ name and year of publicationPopulation and study typeExposure variable(s) and prevalence of pet-keepingOutcome variable(s)Familial allergy stratificationFindings

Wayne County Health, Environment, Allergy and Asthma Longitudinal Study (WHEALS)
Aichbhaumik, 2008 [20]
Unselected birth cohort in southeast Michigan
Recruited women (21–49) in second trimester from Henry Ford Health System (1049 included in analysis)
Pets (cats or dogs) inside home at least 1 hr/day during pregnancyCord blood IgE
(83%)
Detection level ≥ 0.01 IU/mL
Restricted
Excluded those families who selectively avoided pets because of allergy and still found reduced IgE in “low-risk” babies exposed to indoor pets
Infants in pet homes had decreased mean cord blood IgE 0.34 IU/L versus 0.24 IU/L (similar for cats and dogs separately)
Cord IgE higher in atopic/allergic mothers
Serum IgE already higher in mothers with no pets

Prevention of asthma in children (PREVASC) intervention trial, Schönberger, 2005 [21]Control group only of interventional birth cohort (174 included in analysis)Dust samples collected in 4th to 6th months of pregnancy from maternal mattress and living room
Fel d 1 and Can f 1 levels were divided into quartiles
Days 3–5 after birth—heel prick blood IgE (total)
Dichotomized at 0.5 IU/mL
NoProportion elevated IgE increased with increasing HDM but not cat or dog

Copenhagen Prospective Study on Asthma in Childhood (COPSAC) and Manchester Asthma and Allergy Study (MAAS) Bisgaard, 2008 [22]COPSAC: neonates (1 month) of mothers with asthma
Birth cohort (353 included in analysis) MAAS: Unselected birth cohort (412 included in the analysis)
Pet at home
On interview: cat or dog living in house at birth
Age of eczema
COPSAC: physician diagnosed based on Hanifin-Rajka criteria: 1, 6, and 12 months. MAAS: parental report, ISAAC questions at 1 yr
Filaggrin mutations Homozygosity or heterozygosity to 2 (R501X and 2282del4)In both cohorts found:
FLG genotype increased risk of eczema in first year HR 2.26 (95% CI 1.27–4.00)
COPSAC in presence of cat this increased to HR 11.11 (3.79–32.6)
In the filaggrin mutation group the HR for cat exposure was 7.49 (2.37–23.7)
MAAS-3.82 (95% 1.35–10.8) (no increased risk in those without mutation)
In the filaggrin group the HR for cat exposure was 2.47 (1.09–5.62)
Dog exposure protective HR for COPSAC but increased risk for MAAS (not as marked as cat)
In filaggrin loss of function variants there is no increase in eczema without the presence of a cat at birth

*Childhood Origins of Asthma Study (COAST)
University of Wisconsin Gern 2004 [23]
Selected on basis of parental aeroallergen atopy or physician diagnosed asthma or both
families
Birth cohort (285 included in analysis)
Cat or Dog at home at child’s birthAtopic Dermatitis: physician diagnosed, during first year or at 1 year
Sensitization
Allergen-specific IgE values ≥0.35 kU/L at 1 yr
Allergens: egg, milk, peanut, Dust mite (D. pteronyssinus and farinae), and Alternaria
Food Allergy: allergen-specific IgE + historical reports from parents or physician documentation
No
Parental allergy and asthma considered as confounder, not formally modelled for interaction despite finding that fathers with cat allergy less likely to keep cats, and dog ownership more likely if mothers not cat allergic
Dog exposure at birth was associated with reduced allergen sensitization (19 versus 33%, ) and atopic dermatitis at 1 year (30% versus 51%, )
Cat exposure was not associated
No association of cat or dog exposure on food allergy
Postnatal exposure to dogs modified immune development by enhancing IL10 and IL13 responses.
P values 0.12 and 0.08

*COAST
Bufford 2008 [24]
Same cohort with followup at 3 years
Birth cohort (275 included in analysis)
Cat or Dog at home at child’s birthAtopic dermatitis or wheezing in the past yearNoDog at birth associated with current AD OR 0.35 (0.15–0.83) at 3 years
Current wheeze at 3 OR 0.49 (0.25–0.95)
No association with cats

Mothers of German Nationality Pohlabeln 2008 [25]Unselected mothers from 5 hospitals in 3 cities in northwest Germany
Birth cohort
(1881 included in analysis)
Cats and dogs at home at birthISAAC questions
Physician diagnosed eczema or itchy rash for more than 6 months or cracked earlobes
Physician diagnosed asthma or chronic bronchitis
Physician diagnosed hay fever
All these were considered as ever rather than current
Allergic symptoms: any of eczema/asthma/hay fever defined above
All Stratified by first degree relatives with a history of allergic disease (reported history of asthma, eczema or hay fever in parents or siblings)Newborns without a family history of allergic disease had a lower prevalence of asthma and eczema at age 2 when their families kept a dog. OR 0.52 (0.33–0.83)
The risk was modestly elevated in allergic families who kept a dog in the allergic OR 1.43 (0.95–2.15)
No associations found for cats or other pets

Prevention and Incidence of Asthma and Mite allergy (PIAMA) Kerkhof 2005 [26]Selected: allergy high-risk birth cohort
1327 children of allergic mothers
2819 children of nonallergic mothers (1027 included in neonatal analysis, 492 included in 12 month, 682 included in 4 year)
Pets at home in last trimester of pregnancyTotal IgE from heel prick neonatal screening
Specific IgE at 12 months and at 4 years (HDM, cat, dog grass, milk, egg)
Restricted
Stratified by “allergy status” of mother for reporting demographics but not analysed separately
Also, excluded those families with pet avoidance or familial allergy—no change in associations (low-risk group only)
Dogs during pregnancy had less risk of high IgE at birth OR 0.5 (0.2–1.0)
Cats during pregnancy and less cat sensitization at 12 months OR 0.6 (0.4–1.0)

Oslo birth cohort Nafstad 2001 [27]Unselected 3754 children
Birth cohort (2531 included in analysis)
Pets in home at birthSurvey assessed bronchial obstruction, asthma, allergic rhinitis, and infantile eczema Stratified by parental atopy but found no change (parental atopy based on questionnaire concerning asthma and hay fever)Less odds
Asthma 4 yrs 0.7 (0.5–1.1)
Allergic rhinitis 4 yrs 0.6 (0.4–1.0)
Infantile eczema 6 months 0.7 (0.5–0.9)

Tucson Children’s Respiratory Study (TCRS) Remes 2001 [28]Unselected birth cohort—1246 healthy babies (1076 included in frequent wheeze analysis; SPT analysis included 737 (6 yrs), and 613 (11 yrs); IgE analysis included 829 (9 months), 534 (6 yrs), and 462 (11 yrs))Cat or dog at birthFrequent wheezing: >3 episodes of wheeze in the last year (from 1–13 years) SPT at 6 and 11yrs
Serum IgE at 9 months and 6 and 11 yrs (alternaria, HDM mix, Bermuda grass, careless weed, mesquite tree, mulberry tree, olive tree)
Stratified by parental history of asthma (asthma diagnosed in either parent)Dogs at birth associated with less risk of developing frequent wheeze—seen only in children whose parents did not have asthma HR 0.47 (0.31–0.72)
There was no increased risk in the parental asthma group
No association with cats.
Both nonatopic and atopic children with dogs at birth also had a reduced risk of wheeze. HR 0.47 (0.24–0.91) and
HR 0.56 (0.32–0.98)
Neither dog nor cat at birth associated with SPT positivity or IgE

*These articles are both from the same study group.
Abbreviations: HDM: house dust mite; IgE: immunoglobulin E; SPT: skin prick Test.