This study shows that one in five infants have eczema during the first year of life and, as expected, there is a strong association with a family history of eczema. Furthermore, we found beneficial effects from introducing fish before 9 months of age, while short duration of breast feeding, keeping furry pets or the age at which milk or eggs were introduced did not affect the risk.
The KOALA Birth Cohort Study19
found a prevalence of eczema of 22.2% at 1 year. In the BAMSE study, the prevalence of eczema was 25.1% ever before the age of 2.20
Our study confirms the high prevalence of eczema during the first year of life, with a cumulative prevalence of 13.9% at 6 months and 20.9% at 12 months of age.
The prevalence of eczema was estimated using questionnaires sent to the parents, with questions on whether the infant had previous or current eczema at 1 year of age and did not require a doctor's diagnosis. This may lead to uncertainty about the exact prevalence, but as we found a prevalence similar to those in other cohort studies, we believe that the results are valid.
To further evaluate the validity of our study material, we have compared the prevalence of smoking during pregnancy in our study (9.5%) with recent statistics from the National Swedish Board of Health and Welfare which show that 10% of all pregnant women in western Sweden and in Sweden as a whole smoked in 2003.21
Since smoking is associated with several disadvantageous factors, we believe that the data from western Sweden are representative of the population.
The possibility of recall bias is always a limitation in questionnaire studies. However, as food data were collected at 6 and 12 months in this study, we do not think that recall bias is a major problem as the time intervals are relatively short.
Compared with data from Stockholm,22
more homes in western Sweden seem to have furry pets (15.4% vs 20%, respectively). This is probably because the BAMSE study recruited infants from central Stockholm, while our infants were from both urban and rural areas. In the western Sweden study, we found a higher prevalence of furry pets in rural areas.
There is consensus that sensitised children and children with symptoms of allergic disease should not have contact with furry pets.23
On the other hand, opinions differ as to whether furry pets in the local environment of a young child can act as a protective factor and help tolerance to develop,7 9 10
or whether animals are risk factors for the development of allergy in children in high-risk families. In our study, the initial protective effect of keeping a dog disappeared when adjusting for “atopic reasons” (manifest allergy or fear of allergy) for not keeping an animal. The protective effect of having a bird in the home may be due to reverse causation if non-atopic families keep more birds. However, some studies suggest that contact with feathers might prevent atopic disease.24–26
Since birds almost always are kept inside providing continuous indoor exposure, it may be that birds promote exposure to endotoxin. This might explain why we found a protective effect of bird keeping.
Previous primary preventive advice in Sweden recommended the delayed introduction of eggs and fish to infants with strong allergic heredity. Although this advice was abandoned in the autumn of 2002 because supportive evidence was lacking,23
we found that many parents waited to introduce eggs and fish. The results of our study show that outdated primary preventive advice which lacks evidence tends to be still followed by the general population. At the time of the study, allergy prevention advice placed no restrictions on the consumption of eggs or fish either in families at risk or in the rest of the population.
The results suggest that advice previously given at infant welfare clinics concerning the introduction of small portions of cereals and solids at 4 months of age is being followed, while cow’s milk-based formulas and weaning foods are introduced earlier by many parents. The WHO code advocating exclusive breast feeding up to 6 months of age has been recommended in Sweden since 2001. The early steep rise in the curve showing the introduction of cow’s milk () could be explained by the parents (truthfully) counting every small helping of formula, for example during the neonatal period before the firm establishment of breast feeding.
In our study, the strongest risk factor for eczema was a family history of eczema and maternal eczema in particular. Our finding that a family history of eczema is a stronger risk factor than a family history of other atopic diseases is compatible with, for example, the ALSPAC study, where parental eczema was regarded as a better marker than parental asthma or hay fever.27
In fact, a Danish twin study found that genes accounted for 82% of individual susceptibility, while non-shared environmental factors accounted for 18%.28
Recently, it has been demonstrated that loss-of-function mutations in the filaggrin gene predispose to eczema. Filaggrin is a key protein of the epidermis that has an important role in the formation of the skin barrier. Loss of filaggrin function seems to play a role both in the pathogenesis of eczema and the subsequent progression of atopy.29 30
In the multivariable analysis, we found that the introduction of fish before 9 months of age had a protective effect on eczema. This is compatible with the findings of Kull et al
who found that regular fish consumption in the first year of life was associated with a lower risk of eczema at 4 years of age. It has been suggested that a high ratio of omega-6 to omega-3 fatty acids, as is common in western diets, could contribute to the pathogenesis of allergic disease.32
The fact that fish is rich in omega-3 fatty acids could partly explain the effects found in this cohort. However, we found no influence of the type of fish ingested (lean/white or fat/oily) in this study, which is compatible with the CAPS study33
and recent data from Norway.34
The Norwegian study reported beneficial effects on the risk of developing allergic disease if the child ate fish, especially lean fish, but neither the CAPS nor the Norwegian study found that omega-3 fatty acids reduced the risk of allergic disease, including eczema. This makes it somewhat difficult to ascribe the effect to omega-3 fatty acids only.
Dairy products have also been suggested to have beneficial effects on allergic disease. In a study from the Netherlands, it has been shown that milk and butter consumption reduced the risk of asthma.12
However, we were not able to demonstrate any protective effects by dairy products in this study.
The results concerning a possible protective effect on eczema by breast feeding are contradictory. Kull et al35
found a protective effect by breast feeding on eczema at 4 years of age and protective effects have also been reported in other studies.36 37
However, in this study we did not find that breast feeding had any protective effect on the occurrence of eczema. This is in line with the results of Belgian PIPO, the Dutch KOALA and the Australian CAPS studies.9 19 38
What is already known on this topic
- Eczema is common in infants.
- Heredity is a strong risk factor.
What this study adds
- The introduction of fish before 9 months of age reduces the risk of eczema.
- Breast feeding does not provide protection from eczema during the first year of life.
There are also a few studies that claim that prolonged breast feeding can increase the prevalence of eczema.39 40
This may possibly be due to reverse causation, as infants with manifest eczema tend to be breast fed longer than other infants. However, the CAPS study found no significant association between the presence of eczema at 4 weeks and at 3 months and continued breastfeeding beyond those dates. Nor did we find any effects on the prevalence of eczema from parental smoking during or after pregnancy, which is in line with the PIPO study.9
In conclusion, we have found that one in five infants in western Sweden have eczema. In a multivariable analysis, we found large effects from heredity and no effect from the duration of breast feeding or the age at which milk or eggs were introduced, while protective effects from introducing fish before 9 months were found.