The ‘hygiene hypothesis' continues to be controversial, with evidence for and against early exposure to infection conferring protection from allergic disease. Domestic and farm animals are potent allergens in susceptible children, so an important issue to settle is whether such exposure increases or decreases the risk of asthma.

The Nordic countries benefit from having numerous vast databases covering entire populations, which can be linked using a common identification. Researchers in Sweden used national databases for diagnoses, prescribing, dog ownership and farm work to address this question (Fall T, et al. JAMA Pediatr …