We’ve acquired a puppy this year, which has been very exciting for us. However, owning a pet is not a new thing to us. We’ve had pets in our family even before we had children and we feel our children benefit from having animals around them in many ways.
Has anyone heard of the Hygiene Hypothesis?
This was first suggested by Strachan in 1989 and since then it has been supported by epidemiological studies.
[Strachan DP. Hay fever, hygiene, and household size. BMJ 1989; 299: 1259-60.]
[von Mutius E and Vercelli D. Farm living: effects on childhood asthma and allergy. Nature Reviews Immunol 2010; 10: 861-68.]
It suggests that modern infants living in developed countries with higher hygiene standards and smaller family sizes have less stimulation of their immune system in early life. These children then do not develop a so-called, tolerant immune system and are therefore more prone to developing allergic disease.
This sounds barmy!
There is some sense behind it all. Allergic disease is mediated by a group of antibodies called IgE antibodies and everyone possesses these. However, in allergic disease (and some other conditions), IgE antibodies are formed to things which do not bother others, such as tree and grass pollens or cow’s milk and egg protein. Traditionally, IgE antibodies are the antibodies that fight off parasitic infection, something that is common in developing countries where allergic disease is rare. It is thought that, with modern hygiene practises our anti-parasitic defense mechanisms are now redundant and allergic disease happens in those circumstances.
[Bruschi F, Araujo, MI, Harnett W et al. Allergy and Parasites. J Parasitol Res 2013; http://dx.doi.org/10/1155/2013/502562]
I’m getting confused! So, we all should get a parasite?
Well no. To continue the story, supportive evidence for the Hygiene Hypothesis all started with evidence looking at children living on farms who are exposed to a wide variety of microbes (bacteria and other organisms). The more diverse the exposure to these microbes, the less the risk of asthma.
[Ege MJ, Mayer M, Normand AC et al. Exposure to environmental micro-organisms and childhood asthma. New Engl J Med 2011; 364, 701-709.]
However, further studies looking at what part of living on a farm is contributing to this protective effect showed that farmers’ children have less asthma and hayfever symptoms than other rural children. This protective effect was mainly due to consumption of unpasteurised milk and if this was taken into account there was also less allergy, less hayfever and atopic eczema.
[Perkin MR & Strachan DP. Which aspects of the farming lifestyle explain the inverse association with childhood allergy? J Allergy Clin Immunol 2006; 117: 1374-81.]
So, where does it all tie in with getting a puppy?
Not surprisingly, therefore, there have been numerous studies looking at children with asthma, hayfever and atopic eczema to investigate the role of pet ownership on these diseases.
Current guidelines in Australia, USA, the UK and the Global Initiative for Asthma agree that there is currently insufficient evidence to provide any recommendations in relation to pet-keeping in early life and the development of asthma and allergic disease.
[Prescott SL, Tang MLK. The Australasian Society of Clinical Immunology and Allergy position statement: summary of allergy prevention in children. Med J Aust 2005; 182: 464-7.
National Asthma Education and Prevention Program, Expert Panel Report 3 (EPR-3): guidelines for the diagnosis and management of asthma-summary report 2007. J Allergy Clin Immunol 2007; 120: S94-138.
Douglas G, Higgins B, Barnes N et al. British Guideline on the management of asthma: a national clinical guideline. Thorax 2008; 63: iv1-iv121.
Bateman ED, Hurd SS, Barnes PJ et al. Global strategy for asthma management and prevention: GINA executive summary. Eur Resp J 2008; 31: 143-178.]
That doesn’t sound good!
However, there are several recent papers showing beneficial effects of pet ownership. You have to bear in mind that in order for guidelines to be made, there needs to be a sufficient body of evidence supporting them. Just because there is not enough evidence doesn’t mean that something doesn’t work; just that there have not been enough large, well-conducted studies out there to prove it so.
What is the evidence then?
The earlier papers showed that dog exposure was protective against the development of eczema and that cat exposure at birth was associated with an increased risk in the development of eczema in children who have a mutation in the filaggrin gene (see previous blog on this).
[Bisgaard H, Simpson A, Palmer CNA et al. Gene-environment interaction in the onset of eczema in infancy: Filaggrin loss-of-function mutations enhanced by neonatal cat exposure. PLoSOne 2008; 5: e131.]
One review article looking at 9 studies with a total population of 6,948 children showed that exposure to dogs or cat and/or dog in the perinatal period (classified as 20 weeks prior to birth and up to 4 weeks after), and a critical time in the maturation of the infant immune system, may reduce the development of allergic disease in children without a family history of allergy. Interestingly, 2 of the studies looked at high-risk children (family history of allergic disease) and found an increased risk of allergic outcomes in the presence of dog or cat exposure at birth.
[Lodge CJ, Allen KJ, Lowe AJ et al. Perinatal cat and dog exposure and the risk of asthma and allergy in the urban environment: a systematic review of longitudinal studies. Clin Dev Immunol 2012; doi: 10.1155/2012/176484]
Another study looking at pet ownership in infancy (first 2 years of life) showed that pet ownership of cats, dogs, birds, rodents did not appear to either increase or reduce the risk of asthma or hayfever symptoms in children aged 6-10years.
[Lødrup Carlsen KC, Roll S, Carlsen K-H et al. Does pet ownership in infancy lead to asthma or allergy at school age? Pooled analysis of individual participant data from 11 European birth cohorts. PLoSOne 2012; 7: e43214.]
This has been corroborated by another study looking at cat exposure and daycare in children with atopic eczema and the development of asthma in these children. Dog and mouse exposure also protected but not to a significant level.
[Gaffin JM, Spergel JM, Boguniewicz M et al. Effect of cat and daycare exposures on the risk of asthma in children with atopic dermatitis. Allergy Asthma Proc 2012; 33: 282-8.]
The most recent paper looked at perinatal pet ownership and subsequent wheezy bronchitis, together with analysis of microbes in the gut of these infants. 129 infants were included in the study and were followed up for 24 months. None of the 30 infants exposed to pets suffered from wheezy bronchitis by 24 months whereas 15% of the nonexposed infants did. Also interestingly, the gut microbial counts were higher in samples from non-wheezing infants with pet exposure compared to those in wheezing infants without pet exposure.
[Nermes M, Niinivirta K, Nylund L et al. Perinatal pet exposure, faecal microbiota and wheezy bronchitis: Is there a connection? Allergy 2013; http://dx.doi.org/10.1155/2013/827934.]
So where does that leave us?
If you are a family at high risk of allergic disease, then there is still insufficient evidence to give recommendations either way. However, it seems that if you are a family at low risk of allergic disease, there is no reason at all to not have a pet if you wish to have one.
Dr Sandy Flann, Consultant Dermatologist