I recently read a few papers (here and here) that concluded that children exposed to peanuts before 1 year of age were less likely to develop peanut allergies later in life than children who were not exposed to peanuts.
Is this backed up by further evidence? If so, is there evidence that exposing children to other potential allergens (eggs, dogs, etc) can reduce the chance of them developing allergies to those things in the future?
It was advised that Peanuts were not to be eaten during pregnancy due to possible allergy issues, but it is no longer the case.
You can eat peanuts or food containing peanuts, such as peanut butter, during pregnancy, unless you're allergic to them or a health professional advises you not to.
You may have heard peanuts should be avoided during pregnancy. This is because the government previously advised women to avoid eating peanuts if there was a history of allergy – such as asthma, eczema, hay fever and food allergy – in their child's immediate family.
This advice has now changed because the latest research has shown no clear evidence that eating peanuts during pregnancy affects the chances of your baby developing a peanut allergy.
For peanut allergies, there is a therapy called Probiotic and Peanut Oral ImmunoTherapy (PPOIT) and a study published in The Lancet Child & Adolescent Health suggests that it can help for upto 4 years (Hsiao et al., 2017)
48 (86%) of 56 eligible participants were enrolled in the follow-up study. Mean time since stopping treatment was 4·2 years in both PPOIT (SD 0·6) and placebo (SD 0·7) participants. Participants from the PPOIT group were significantly more likely than those from the placebo group to have continued eating peanut (16 [67%] of 24 vs one [4%] of 24; absolute difference 63% [95% CI 42–83], p=0·001; number needed to treat 1·6 [95% CI 1·2–2·4]). Four PPOIT-treated participants and six placebo participants reported allergic reactions to peanut after intentional or accidental intake since stopping treatment, but none had anaphylaxis.
As stated in my comment a while ago, there was an article I saw about allergies in general, which stated that there seems to be a rise in the numbers of people suffering from allergies and there is a hypothesis that this is due to the sterility of the environment compared to the past. I cannot find the article I was thinking of, but the linked Wikipedia page provides information on this with citations.
In medicine, the hygiene hypothesis is a hypothesis that states a lack of early childhood exposure to infectious agents, symbiotic microorganisms (such as the gut flora or probiotics), and parasites increases susceptibility to allergic diseases by suppressing the natural development of the immune system. In particular, the lack of exposure is thought to lead to defects in the establishment of immune tolerance.
The hygiene hypothesis has also been called the "biome depletion theory" and the "lost friends theory" [1].
...
Although the idea that exposure to certain infections may decrease the risk of allergy is not new, Strachan was one of the first to formally propose it, in an article published in the British Medical Journal in 1989. [21]
Citations
[1] William Parker (2010-10-13). "Reconstituting the depleted biome to prevent immune disorders". The Evolution & Medicine Review.
[21] Strachan, D. P. (1989). "Hay fever, hygiene, and household size". BMJ. 299 (6710): 1259–60. doi:10.1136/bmj.299.6710.1259. PMC 1838109 Freely accessible. PMID 2513902
The thing with these hypotheses, as indicated in the Wikipedia page, is that
[with reducing modern practices of cleanliness and hygiene, there is] a significant amount of evidence that it would increase the risks of infectious diseases. [8]
[8] Stanwell-Smith R, Bloomfield SF, Rook GA. (2012). "The hygiene hypothesis and its implications for home hygiene, lifestyle and public health". International Scientific Forum on Home Hygiene.
Allergic conditions are caused by inappropriate immunological responses to harmless antigens driven by a TH2-mediated immune response, TH2 cells produce interleukin 4, interleukin 5, interleukin 6, interleukin 13 and predominantly immunoglobulin E. Many bacteria and viruses elicit a TH1-mediated immune response, which down-regulates TH2 responses.
(Source)
Epidemiological data supports the hygiene hypothesis
Studies have shown that various immunological and autoimmune diseases are much less common in the developing world than the industrialized world and that immigrants to the industrialized world from the developing world increasingly develop immunological disorders in relation to the length of time since arrival in the industrialized world.[11] This is true for asthma[40] and other chronic inflammatory disorders.[5]
...
In developed countries where childhood diseases were eliminated, the asthma rate for youth is approximately 10%. In the 19th century, hay-fever, an easily recognisable allergy, was a very rare condition.[42]
Citations
[5] Rook, G. A. W.; Lowry, C. A.; Raison, C. L. (2013). "Microbial 'Old Friends', immunoregulation and stress resilience". Evolution, Medicine, and Public Health. 2013: 46–64. doi:10.1093/emph/eot004
[11] Okada, H.; Kuhn, C.; Feillet, H.; Bach, J. -F. (2010). "The 'hygiene hypothesis' for autoimmune and allergic diseases: An update". Clinical & Experimental Immunology. 160: 1–9. doi:10.1111/j.1365-2249.2010.04139.x
[40] Gibson, Peter G.; Henry, Richard L.; Shah, Smita; Powell, Heather; Wang, He (2003). "Migration to a western country increases asthma symptoms but not eosinophilic airway inflammation". Pediatric Pulmonology. 36 (3): 209–15. doi:10.1002/ppul.10323. PMID 12910582
[42] Blackley CH (1873) Experimental Researches on the Causes and Nature of Catarrhus Aestivus (Hay-fever and Hay-asthma) (Baillière Tindall and Cox, London).
As with Peanut Allergies, Allergen Immunotherapy, also known as desensitisation or hypo-sensitisation, is a medical treatment for some types of allergies.
Immunotherapy involves exposing people to larger and larger amounts of allergen in an attempt to change the immune system's response.[1]
Meta-analyses have found that injections of allergens under the skin are effective in the treatment in allergic rhinitis in children[3][4] and in asthma.[2] The benefits may last for years after treatment is stopped.[5] It is generally safe and effective for allergic rhinitis, allergic conjunctivitis, allergic forms of asthma, and stinging insects.[6] The evidence also supports the use of sublingual immunotherapy against rhinitis and asthma, but it is less strong.[5] In this form the allergen is given under the tongue and people often prefer it to injections.[5] Immunotherapy is not recommended as a stand-alone treatment for asthma.[5]
Citations
[1] "Allergen Immunotherapy". April 22, 2015. Archived from the original on 9 September 2016. Retrieved 15 June 2015.
[2] Abramson, MJ; Puy, RM; Weiner, JM (4 August 2010). "Injection allergen immunotherapy for asthma". The Cochrane Database of Systematic Reviews (8): CD001186. doi:10.1002/14651858.CD001186.pub2. PMID 20687065.
[3] Penagos, M; Compalati, E; Tarantini, F; Baena-Cagnani, R; Huerta, J; Passalacqua, G; Canonica, GW (August 2006). "Efficacy of sublingual immunotherapy in the treatment of allergic rhinitis in pediatric patients 3 to 18 years of age: a meta-analysis of randomized, placebo-controlled, double-blind trials". Annals of Allergy, Asthma & Immunology. 97 (2): 141–8. doi:10.1016/S1081-1206(10)60004-X. PMID 16937742.
[4] Calderon, MA; Alves, B; Jacobson, M; Hurwitz, B; Sheikh, A; Durham, S (24 January 2007). "Allergen injection immunotherapy for seasonal allergic rhinitis". The Cochrane Database of Systematic Reviews (1): CD001936. doi:10.1002/14651858.CD001936.pub2. PMID 17253469.
[5] Canonica GW, Bousquet J, Casale T, Lockey RF, Baena-Cagnani CE, Pawankar R, Potter PC, Bousquet PJ, Cox LS, Durham SR, Nelson HS, Passalacqua G, Ryan DP, Brozek JL, Compalati E, Dahl R, Delgado L, van Wijk RG, Gower RG, Ledford DK, Filho NR, Valovirta EJ, Yusuf OM, Zuberbier T, Akhanda W, Almarales RC, Ansotegui I, Bonifazi F, Ceuppens J, Chivato T, Dimova D, Dumitrascu D, Fontana L, Katelaris CH, Kaulsay R, Kuna P, Larenas-Linnemann D, Manoussakis M, Nekam K, Nunes C, O'Hehir R, Olaguibel JM, Onder NB, Park JW, Priftanji A, Puy R, Sarmiento L, Scadding G, Schmid-Grendelmeier P, Seberova E, Sepiashvili R, Solé D, Togias A, Tomino C, Toskala E, Van Beever H, Vieths S (December 2009). "Sub-lingual immunotherapy: World Allergy Organization Position Paper 2009" (PDF). Allergy. 64 Suppl 91: 1–59. doi:10.1111/j.1398-9995.2009.02309.x. PMID 20041860. Archived from the original (PDF) on 2011-11-12.
[6] Rank, MA; Li, JT (September 2007). "Allergen immunotherapy". Mayo Clinic Proceedings. 82 (9): 1119–23. doi:10.4065/82.9.1119. PMID 17803880.
Hsiao, K. et al. (2017) Long-term clinical and immunological effects of probiotic and peanut oral immunotherapy after treatment cessation: 4-year follow-up of a randomised, double-blind, placebo-controlled trial. The Lancet Child & Adolescent Health, 1(2): pp. 97—105
DOI: 10.1016/S2352-4642(17)30041-X
The second paper you mention is the LEAP trial. This went beyond the previous advice which said that children were less likely to develop allergies if exposed to peanuts early; they had enough evidence to make a positive recommendation:
Existing guidelines pertaining to the early introduction of complementary foods have indicated that the introduction of highly allergenic foods, such as peanut, need not be delayed past 4 or 6 months of life.
...
There is now scientific evidence (Level 1 evidence from a randomized controlled trial) that healthcare providers should recommend introducing peanut-containing products into the diets of “high-risk” infants early on in life (between 4 and 11 months of age) in countries where peanut allergy is prevalent because delaying the introduction of peanut can be associated with an increased risk of peanut allergy.
in 2017 this was codified into a recommendation by the National Institutes of Health. From the addendum for parents:
Guideline 1 recommends that if your infant has severe eczema, egg allergy, or both (conditions that increase the risk of peanut allergy), he or she should have peanut-containing foods introduced into the diet as early as 4 to 6 months of age.
Guideline 2 suggests that if your infant has mild to moderate eczema, he or she may have peanut-containing foods introduced into the diet around 6 months of age to reduce the risk of developing peanut allergy.
Guideline 3 suggests that if your infant has no eczema or any food allergy, you can freely introduce peanut-containing foods into his or her diet.
Though it is suggested to "[c]heck with your infant’s healthcare provider before feeding your infant peanut-containing foods."
These recommendations were contemporaneously reported in mainstream press.
This is tricker to answer as there hasn't been a single focused study with evidence comparable to LEAP.
One study looked at 514 children between birth and 24 months, and concluded that those who are exposed to "spores and pollen in the first 3 months of life are at increased risk of early wheezing".
Similarly, there may be some correlation between early sensitivity to house dust mite (HDM) & grass pollen and month of birth due to seasonal variations. Asthma itself seems to vary by month of birth.
However, it is important to note that these relations don't necessarily imply that early exposure causes the sensitisation- as ever, correlation does not equate to causation.
A 2017 review on the subject of allergy development (PDF) is more equivocal:
It is becoming increasingly clear that antigen exposures during the first few years of life are unique in their capacity to either elicit allergic sensitization or result in tolerogenic responses.
It has been proposed that cutaneous exposure (through the skin) is more likely to result in sensitisation; whereas oral exposure is more associated with a development of tolerance.
Interestingly, it would seem that cutaneous exposure to peanuts -- for example peanut dust entering via break in skin -- can drive intestinal sensitivity and perhaps prevent oral tolerance.
While not technically in the scope of your question, there is interest in what effect prenatal exposure -- positive and negative -- can have on allergy development.
Evidence here is inconclusive as well. Some studies show a benefit to early exposure, while others link it with the development of an allergy. A 2009 review discusses the evidence.
Over the past decades, numerous studies have investigated the role of pet exposures in relation to atopy, but the published findings have been inconsistent and sometimes conflicting. Although there is growing evidence that exposure to pets may protect against development of atopic outcomes, the protective effects have not been replicated in all studies.
Since then, there has been further evidence on both sides, for example a large Swedish study which showed that animal exposure could be protective against asthma development (reported in mainstream press); wile another study from Finland concluded that pet exposure was associated with a higher incidence of pet allergy.
