Urbanisation, along with heritage,1,2 has been associated to an increasing prevalence of asthma, allergic rhinitis, and dermatitis around various parts of the world.3,4 Some have argued that increased standards in hygiene and smaller nuclear families have contributed to this pattern.5 Interestingly, this prevalence has not been observed in rural areas; the following rural conditions have been linked to potential protective agents that act against these diseases: a greater number of siblings,1,6 sex,2 breastfeeding,7,8 type of delivery,9–13 interaction with farm animals,14,15 and the intake of unpasteurised cow's milk, amongst other.16–18

The effect that a rural environment might have on the frequency of asthma, allergic rhinitis, and atopic dermatitis has seldom been researched in Mexico,19,20 thus we decided to study the matter, in order to do so, we contrasted the prevalence of these diseases amongst school-aged children that live in an urban setting to those that live in rural regions. Additionally, we attempted to determine which, if any, rural factors act as protective agents against these diseases.

We included a total of 1003 children in this study, 189/1003 (18.8%) lived in a rural area, while 814/1003 (81.2%) lived within an urban sector; we had participation rates of 100% and 96.4%, respectively.

The proportion of participating children was very similar, Table 1. There was no significant difference between a rural environment and an urban one when it came to a family history of either asthma or allergic diseases; similarly, we were unable to link these diseases to parental smoking. Children from a rural environment consumed unpasteurised milk more frequently than children in urban areas (p=0.006). We registered a median of two siblings (with a minimum and a maximum between 0 and 14) per household; children in rural areas had more siblings (p=0.047). Exposure to household pets and farm animals was significantly higher amongst children in rural areas (p<0.001); children in an urban environment had less contact with household dogs (p=0.009).

Approximately one quarter of the children had experienced at least one episode of acute wheezing, but fewer than 10% had had an episode in the last year, Table 2. Interestingly, children in urban areas had fewer wheezing episodes than those in rural zones, a contrast that reached a significant statistical bearing (p=0.029). We did not find any difference between the groups with regards to the prevalence of asthma and its symptoms. Allergic rhinitis was more common amongst rural children; however, there was a statistical difference when regarding one-time episodes of allergic rhinitis (p=0.049). Lastly, dermatitis also proved to be considerably more frequent in rural areas, whether it was a one-time episode or a diagnosis from the previous year (p<0.001 and 0.028, respectively).

In Table 3, we display the factors associated to the prevalence of asthma, allergic rhinitis, and atopic dermatitis, these factors have been adjusted with multivariate analysis. Asthma was associated to males (OR=1.92, p=.007) and a family history of allergic diseases (OR=2.15, p=0.001). Similarly, allergic rhinitis was linked to a family history of allergies (OR=2.58, p=0.002), while a higher number of siblings was found to be a protective factor (OR=0.45, p=0.008) against these two diseases. With regard to atopic dermatitis, we observed that a paternal history of the disease was significantly associated to its prevalence (OR=4.12, p=0.004). Allergic diseases were not associated to: farm animal interaction, the intake of unpasteurised cow's milk, an urban school environment, or the number of siblings. In Table 4, we display the factors that were associated to symptoms of asthma, allergic rhinitis, and atopic dermatitis, one year prior to when our study took place. The only association that had significant statistical relevance was observed between the presence of allergic rhinitis during the previous year and a family history of allergic diseases (OR=2.06, p=0.011).

With the exception of inheritance, we were unable to associate a lower frequency of allergic diseases in children that live in rural areas, however, these children tend to have several siblings, and this does seem to decrease the prevalence of allergic rhinitis.

In Mexico, few publications have presented findings similar to ours, in fact, there have been reports of a decreased frequency of asthma amongst children in rural areas19 and a 50% reduction in the prevalence of nasal symptoms.20 These results are similar to those reached by other global studies on the matter. In Latin America, a study from Brazil reported a lower prevalence of asthma and allergic rhinitis amongst children in rural areas.25 Bolivian children in urban sectors had a higher prevalence of wheezing episodes, symptoms of sever rhino-conjunctivitis, and severe dermatitis26; in the east, a Chinese study found that adolescents living in rural areas had a lower frequency of asthma and wheezing episodes during the previous year.14 Nonetheless, not all of the compiled studies have reached this conclusion, for example, in Turkey there were no differences between an urban setting and a rural area regarding the prevalence of asthma, allergic rhinitis, and atopic dermatitis27; in The Republic of Macedonia, a population-based study showed no significant statistical differences between these two settings once they were adjusted by the confounder factors28; on the same note, a study in Portugal also reported that there was no disparity between these environments and the prevalence of asthma or its symptoms.29 Our findings are consistent with the latter studies, this may be a result of the fact that rural children spend much of their time in urban settings nowadays, another contributing factor might be that rural families have gradually been moving to cities or begun to modify their lifestyles to more urban tendencies. Due to these contrasting results, we inquired about each child's interaction with farm animals or household pets, as well as their intake of unpasteurised cow's milk, factors which proved to be far more prevalent in the lives of children in rural settings. In the light of the inconsistent data regarding this matter, it is necessary to continue studying the effect that a rural environment might have on the origin of allergic diseases.

Breastfeeding has been researched in various studies that concern its effects on the development of allergic diseases, and the compiled data is still very contradictory. Our multivariate models did not find any connection between breastfeeding and allergic diseases, which is consistent to what other studies in western Mexico have reported.7,30 Contrastingly, a recent meta-analysis reported that breast milk acts as a protective agent against allergic diseases.8

Like breastfeeding, studies that regard caesarean births and their potential association to allergic diseases have produced inconsistent results. In a Korean study, they reached findings similar to ours; that is to say, they found no difference between the prevalence of allergic diseases or the sensitisation of allergic diseases and birth methods.12 Recently in Korea, Yu et al.13 reported a correlation between atopic dermatitis and caesarean births, particularly when the operation was performed under emergency circumstances9; a meta-analysis showed a 20% increased risk of asthma in children that were delivered via caesarean section,10 additionally, another study reported that children born in this manner are at a greater risk of developing immune system diseases, such as: asthma, ulcerative colitis, celiac disease, and juvenile idiopathic arthritis.11

Moreover, our study has found that a higher number of siblings reduces the prevalence of allergic rhinitis, which was also reported in Germany.1 Contrastingly, the third phase of ISAAC has established that having more siblings is also associated to a higher prevalence in allergic rhino-conjunctivitis, asthma, and atopic dermatitis.6

A very important factor that has grasped the attention of several studies concerns the intake of unpasteurised cow's milk and its association to a higher prevalence of allergic diseases. In our study, we did not find evidence to support this notion, despite the fact that children in rural settings drink unpasteurised cow's milk more regularly than children in urban areas. These findings contrast starkly with other studies in which it has been reported that the early ingestion of this type of milk reduces the risk of asthma, atopy and allergic dermatitis,16,17 regardless of a child's living environment. Studies undertaken in Europe have reported a decreased prevalence in allergic diseases and a reduced risk to allergen sensitisation amongst children that drink unpasteurised cow's milk.15,18 It is argued that by drinking cow's milk, children ingest a considerable amount of bacteria that affect how the immune system is regulated, thus, incorporating unpasteurised cow's milk into a child's diet might be considered a preventative measure.

The following limitations should be taken into consideration when interpreting our results. First, we only had a short amount of time to analyse the extent of the exposure that children had to the conditions that have been associated as protective agents within a rural environment against the development of allergic diseases; second, these diagnostics were not clinically corroborated; third, it is possible that parents have criteria that differ from our own when interpreting wheezing episodes; four, we did not differentiate between the type of medical treatments that each child had access to; five, the only nutritional aspects that we analysed regarded breastfeeding and the intake of unpasteurised cow's milk; lastly, we did not factor in whether caesarean sections had been planned or preformed under emergency circumstances.

In this study, we were unable to uphold the hypothesis that a rural environment offers protective advantages against the prevalence of allergic diseases. However, we did observe that children with a higher number of siblings might have a decreased prevalence of allergic rhinitis. With regard to atopy, a family history of the disease has a significant bearing on its frequency; notwithstanding, further research is required to elucidate on these matters.

The authors have no funding or conflicts of interest to disclose.

None.