A study of 20000 children found having a dog before the age of 3 cuts the chance of asthma by 40%.
But having a cat in those early years increases the risk by half, the Imperial College London researchers found.
Study leader Silvia Colicino believes this is because dogs are dirtier than cats, so they expose children to bacteria.
This helps prime the immune system, triggering the production of antibodies that protect against allergies. Cats tend to be much cleaner and less affectionate, so do not pass on this bacteria.
Cats’ fur also carries tiny antigens - particles that trigger a response in the immune system - that can lead to asthma.
Colicino said: “Pre-school children who live in a dirty environment with a high level of bacteria are less likely to develop allergies. Your body has to react to the bacteria by generating antibodies which are good at protecting against asthma. Dogs are dirtier than cats.
“They tend to live outside and they carry high levels of bacteria. Early contact of children with dogs, especially in the first year of life, seems to protect against asthma.”
She said cats were more likely to be kept indoors, so the antigens in their fur circulated in the house.
She said: "Our research showed there were many factors which can predict whether children will go on to have asthma by the age of 20."
Dr Erika Kennington, of Asthma UK, said: “It’s important to remember that all animals produce dander, urine and saliva that can also trigger asthma symptoms.”
- Daily Mail