What you need to know about allergies and the gut microbiome

Allergic and atopic disease make up a large proportion of the reason patients are brought to vets, particularly if we look at just cases presenting with skin disease. Allergic diseases are always challenging as the reaction to a stimulus can vary hugely in severity, from life threatening swelling within the respiratory tract, to a mild itch on the skin, or a larger amount of wax produced in a dog’s ear. The other big issue with allergic disease is that allergens are everywhere, and some, like grass, pollen or dust mites, are impossible to completely eradicate from an individual’s life. Even food allergies, which tend to be easier to manage using exclusion diets, can be a real challenge, especially in dogs with scavenging tendencies.

Allergic disease has been increasing in prevalence over recent decades, especially in industrialised countries, with the World Health Organisation regarding it as one of the “3 major diseases of the 21st century”^[1]. The response to treatment is variable and sometimes unsuccessful, or requiring the ongoing use of harsh medications, such as steroids. Sufferers, or the owners of suffering dogs, are always on the lookout for ways to help manage symptoms without the need for harsh treatment – so, could the microbiome hold the answer? 

As studies into the microbiome have advanced, we are gaining more insight into how the microbiome, particularly dysbiosis (imbalance), can impact on allergic disease. With food allergies, studies have found that the composition of the intestinal microbiota can directly imprint resistance or susceptibility onto the host. Research in humans has repeatedly shown that those with food allergies have a distinct microbiome compared to those who aren’t affected, with dysbiosis preceding the development of the food allergy.^[2] Changes in the microbiome of babies as early as 3-6 months old can have an impact on later allergy resolution, with those whose microbiome was enriched with Clostridia and Firmicutes after birth leading to resolution of a milk allergy, which persisted in those without these changes. Other studies have found that a gut microbiome enriched with Clostridia protects against sensitisation to food allergies in animals, through regulation of the innate immune system, as well as intestinal permeability, so could have a role in prevention^[1]. This potentially opens up the incentive for people to try to prevent their pets developing allergic disease by working on the microbiome from a very early age. 

Respiratory allergies have also been linked with dysbiosis. Not only has a direct link been found between the gut microbiome and airway inflammation, but children living in rural areas or having frequent contact with animals, and so living with a more diverse microbial exposure, have a lower incidence of asthma and allergic rhinitis, with this exposure positively affecting the lung microbiome[1,3]. Yes, the lung has its own microbiome - bacteria and viruses that live in the mucous layer that lines the airways -  which can be positively affected by external stimuli, as mentioned, or negatively affected, for example by allergens and air pollution. In humans they have found that a healthy lung microbiome is characterised by an abundance of Bacteroidetes, Actinobacteria and Firmicutes, whilst viral respiratory infections are associated with a prevalence of Proteobacteria. Dysbiosis of the gut or lung microbiome can lead to activation of inflammatory pathways, contributes to bronchoconstriction (narrowing of the airways) and dysregulates immune responses leading to increased sensitivity and reactivity to food and respiratory allergens, especially in asthma.^[3]

In a canine atopic skin disease (skin disease caused by inhaled allergens) study, it was demonstrated that antibiotics, gut microbiota and atopic disease were all interrelated. Dogs who had received antibiotics tended to show more severe atopy symptoms, and we know that antibiotics lead to dysbiosis, so we can see there is a direct correlation between the two. They found that atopic dogs showed a microbiome enriched with Escherichia-Shigella. But it is not just the microbiome that causes allergies; interestingly, this same study found that dogs living in an urban environment had an increased incidence of allergic disease, but this did not link directly with the gut microbiome.^[4]

Generally in canine atopic disease, we see a prevalence of skin and gut symptoms over respiratory ones - so more itching and tummy troubles than coughing or sneezing - and as there is strong evidence linking the health of the skin to the gut we know that helping the microbiome can help ameliorate symptoms of atopic skin disease. In humans, they have found that treating atopic skin disease with Lactobacillus plantarum not only improved the gut microbiome but also ameliorated atopic symptoms, whilst Bifidobacterium improved itch levels^[1].

As we explore different disease states, it becomes increasingly clear that the microbiome is connected with all diseases in some way, as it has such a broad impact on the immune system. With a disease like atopy or an allergy, where it is specifically an abnormal immune response causing the disease, we knew it would be linked with the microbiome, but it is fascinating to see quite how directly, and in so many different areas of the body. With levels of atopic disease increasing across all species, hopefully this knowledge of the microbiome and a keenness to improve its health and diversity, will halt or even reverse this change, leading to happier healthier dogs.

References

^[1] Han P, Gu JQ, Li LS, Wang XY, Wang HT, Wang Y, Chang C, Sun JL. The Association Between Intestinal Bacteria and Allergic Diseases-Cause or Consequence? Front Cell Infect Microbiol. 2021 Apr 15;11:650893. doi: 10.3389/fcimb.2021.650893. PMID: 33937097; PMCID: PMC8083053.

^[2] Bunyavanich S, Berin MC. Food allergy and the microbiome: Current understandings and future directions. J Allergy Clin Immunol. 2019 Dec;144(6):1468-1477. doi: 10.1016/j.jaci.2019.10.019. PMID: 31812181; PMCID: PMC6905201.

^[3] Hufnagl K, Pali-Schöll I, Roth-Walter F, Jensen-Jarolim E. Dysbiosis of the gut and lung microbiome has a role in asthma. Semin Immunopathol. 2020 Feb;42(1):75-93. doi: 10.1007/s00281-019-00775-y. Epub 2020 Feb 18. PMID: 32072252; PMCID: PMC7066092.

^[4] Sinkko H, Lehtimäki J, Lohi H, Ruokolainen L, Hielm-Björkman A. Distinct healthy and atopic canine gut microbiota is influenced by diet and antibiotics. R Soc Open Sci. 2023 Apr 26;10(4):221104. doi: 10.1098/rsos.221104. PMID: 37122947; PMCID: PMC10130713.