There has been an epidemic of allergic diseases in the last half century. One theory proposed to explain this increase in the prevalence of allergic diseases is that it results from a decrease in the prevalence of childhood exposure to endotoxin[9,16,17] (see below).

In Switzerland, an epidemiologist named Charlotte Braun-Fahrländer has found out the following facts based on her asthma survey[16]: Not only were farming children one-third as allergic as their nonfarming rural counterparts, but the more farming they did, the less allergic they were.  Children from full-time farming families had half the allergies of those from part-time farming families.  And they were one-fourth as likely to have allergies compared with rural children who never farmed.  By 2002, numerous studies from around the world had documented "the farming effect.[20]"

Without living in a farming environment, pet ownership seems to also have similar beneficial effects.  In this article, we will discuss pet ownership and its protective role on our children's health.  Note that both dog contacts and cat contacts may provide health benefits .  But, dog contacts showed a more significant protective role[4].   So, we will focus only on dogs here. 

Dogs—Man's Best Friends

Dogs are domesticated wolves.  They have provided us protection, companionship and hunting assistance since the days of the earliest human settlements. Some recent studies have also shown that pet-ownership to be protective against allergic or infectious disease development.

From one study in Detroit[2,6], researchers examined the effect of having a dog on one indicator of an individual’s tendency to develop allergies: the level of IgE[12] antibodies in the mother’s umbilical cord blood.  They have found that pregnant mothers who lived in houses with dogs tended to have lower levels of IgE antibodies in their cord blood—and such lower levels have been found to be protective when it comes to childhood allergies.

Another study[5] of 965 children aged 4-6 years living in rural or semi-rural South Australia was undertaken to investigate the relationship between pet ownership and gastroenteritis in young children. Scientists have found that living in a household with a dog was associated with a reduced risk of gastroenteritis.

In a birth cohort study[4], 397 children from Finland were followed up from pregnancy onward, and the frequency of respiratory symptoms and infections together with information about dog and cat contacts during the first year of life were reported by using weekly diaries and a questionnaire at the age of 1 year.  Even after adjusting for possible co-factors, children having a dog at home were significantly healthier, had less frequent otitis, and tended to need fewer courses of antibiotics during the study period than children without dog contacts.

It is now widely appreciated that humans did not evolve as a single species, but rather that humans and the microbiomes associated with us have co-evolved as a "super-organism," and that our evolution as a species and the evolution of our associated microbiomes have always been interwined.

-William Parker, Duke University

Endotoxin

Endotoxin is a lipopolysaccharide that forms the outer layer of the cell membrane of all gram-negative bacteria. Importantly, it elicits a strong immune response from the mammalian immune system[16].  

Endotoxin levels vary widely but tend to be highest in environments where there are farm animals such as cows, horses, and pigs, because the fecal flora of larger mammals is a major source of endotoxin. Endotoxin is also found in the dust in houses and outdoors in dirt and can be measured in dust or air.

For people living in urban environments, their fingers rarely sink deeply into the mud.  Scientists have hypothesized that these people are so deeply removed from the diversity of microbial species that their immune systems fail to develop normally.  In such settings, dogs reconnect us to microbial diversity[7], which may be sufficient to bring sense to our immune system .

Dogs bring bacteria in their mouths, on their skin and in their fur, but also  from the dirt around our homes.  The links between our bodies and our dogs’ bodies seem to be direct and intimate. A recent study has found that humans tend to pick up, on their skin, some microbes from their dogs. Dog owners tend to share more microbes with their dogs than with random dogs. Evidence has shown that, at least microbially, dog owners really do become more similar to their dogs over time.

Hygiene Hypothesis[23,27,29]

Hygiene hypothesis [9,16] proposed to explain this increase in the prevalence of allergic diseases (and maybe autoimmune disease too[13]) is that it results from a decrease in the prevalence of childhood exposure to endotoxin.

Bach[10] details a number of potential mechanisms by which the decrease in the frequency of childhood infections and/or exposure to endotoxin might influence the frequency of allergic or autoimmune diseases:

1. Decrease in antigenic stimulation related to the decrease in the frequency of childhood infections has resulted in a decrease in the levels of regulatory cytokines — specifically, interleukin-10 and TGF-b — which act to  down-regulate both Th1-mediated responses and Th2-mediated responses[13,14,19]. 

   • However, it is unclear how interleukin-10 and TGF-b affect T-cell differentiation and regulation to generate a normal, robust, and balanced Th1 and Th2 immune response in the developing child.

2. Stimulation of the innate immune system[18] by endotoxin may be important in the ontogeny of the normal immune system.

   • Beyond the dose, whether exposure to endotoxin is protective or harmful is likely to depend on a complex mixture of the timing of exposure during the life cycle, environmental cofactors, and genetics.

By the late 2000s, a revised model had emerged.  Soon after birth, a wave of autoimmune cells populated the organism.  They helped in defense, anticancer immunity, and tissue repair.  A wave of peacekeeping cells quickly followed these initial pioneers, restraining them and establishing equilibrium.  But keeping the peace in the long run required more suppressor cells.  This secondary squadron emerged only after contact with the outside world—with certain parasites and microbes.  It means that our ability to self-regulated, to maintain homeostasis, was oddly reliant on external stimuli.

If we really wanted to fend off allergic disease, the emphasis on the avoidance of allergens is somewhat misguided. Yes, if you're allergic to dust mites, you should avoid them. But if you wished to prevent allergic sensitization altogether, you needed to intercede earlier.  The coup de grâce of allergy prevention consists of teaching the immune system tolerance from an early age.  How early it should be?  The earlier the exposure began, the better. Children who accompanied their parents to the stables during the first year of life had less allergy compared with children who began working on a farm at school age.  As matter of fact, even prenatal exposure counts.  Indeed, you could predict a child's odds of developing allergy by measuring the endotoxin in her mother's mattress[6,8, 16].

Warnings

As shown above, recent researches lend support to the health benefits of pet ownership. However, this must be weighed against the potential negative consequences, such as dog bites, particularly for children at young ages. Finally, Karen DeMuth, MD, MPH, has warned that[15]:

"Certain people who have a dog in the house are protected against infections and allergies, but some are not. This is not a one-size-fits-all kind of thing."

References

1. Are Cats or Dogs More Protective For Children’s Health?

2. Dogs Make Me (and You) Wild: Ten Effects of Dogs on Dog People

3. History of respiratory infections in the first 12 yr among children from a birth cohort

4. Respiratory Tract Illnesses During the First Year of Life: Effect of Dog and Cat Contacts

5. Does dog or cat ownership lead to increased gastroenteritis in young children in South Australia? 

6. Aichbhaumik N, Zoratti EM, Strickler R, Wegienka G, Ownby DR, Havstad S, et al. Prenatal exposure to household pets influences fetal immunoglobulin E production. Clin Exp Allergy 2008;38:1787-94.

7. K. E. Fujimura, C. C. Johnson, D. R. Ownby et al., “Man’s best friend? the effect of pet ownership on house dust microbial communities,” Journal of Allergy and Clinical Immunology, vol. 126, no. 2, pp. 410.e3–412.e3, 2010.

8. Perinatal Pet Exposure, Faecal Microbiota, and Wheezy Bronchitis: Is There a Connection?

9. EAT DIRT — THE HYGIENE HYPOTHESIS AND ALLERGIC DISEASES

10. Bach J-F. The effect of infections on susceptibility to autoimmune and allergic diseases. N Engl J Med 2002;347:911-20.

11. Braun-Fahrländer C, Riedler J, Herz U, et al. Environmental exposure to endotoxin and its relation to asthma in school-age children. N Engl J Med 2002;347:869-77.

    • A subject's environmental exposure to endotoxin may have a crucial role in the development of tolerance to ubiquitous allergens found in natural environments.

12. Immunoglobulin E

    • IgE plays an essential role in type I hypersensitivity, which manifests various allergic diseases, such as allergic asthma, allergic rhinitis, food allergy, and some types of chronic urticaria and atopic dermatitis.

13. T helper cells

14. Can Parasites Heal the Gut? (Travel and Health)

15. Many Babies Healthier in Homes With Dogs (WebMD)

16. An Epidemic of Absence — A New of Understanding Allergies and Autoi mune Diseases (excellent book)

    • Kids in the inner city are exposed to a lot of more dander from rodents and cockroaches than those living in better-off neighborhoods, but not more microbes.  This explains why kids living in the inner city have more asthma than other urban kids.  Like everyone else, they experience the same lack of a good immune education while also encountering more aggravation.

17. ASTHMA/ALLERGY EPIDEMIC: IS CLEANLINESS THE CULPRIT? 

18. Role of Phagocytes in Innate or Nonspecific Immunity (Kahn Academy)

19. Limitations to the Th1/Th2 model

20. Amish Have Fewer Allergies Due to ‘Farm Effect’ (ABC News)

    • The Amish who live and work with animals on farms in northern Indiana, have some of the lowest rates of allergies and asthma in the westernized world, perhaps because of the so-called “farm effect,” according to an international study.

21. Amish children living in northern Indiana have a very low prevalence of allergic sensitization (The Journal of Allergy and Clinical Immunology)

22. Exploring The Invisible Universe That Lives On Us — And In Us

23. Graham A. W. Rook. Hygiene hypothesis and autoimmune diseases.  Clin Rev Allerg Immu 2012 Feb; 42(1):5-15.

24. Dog Ownership Benefits Families of Children with Autism, MU Researcher Finds

25. Does Dirt Make You Happy?

26. Effects of early-life exposure to allergens and bacteria on recurrent wheeze and atopy in urban children

    • These findings suggest that concomitant exposure to high levels of certain allergens and bacteria in early life might be beneficial.

27. The Epithelial Gatekeeper Against Food Allergy

    • Intestinal epithelial barriers play a crucial role in the maintenance of gut homeostasis by limiting penetration of luminal bacteria and dietary allergens, yet allowing antigen sampling via the follicle-associated epithelium for generation of tolerance.

28. Pollen Allergy (video; good)

29. Barnyard Dust Offers a Clue to Stopping Asthma in Children (important)

30. The Friend Who Keeps You Young

31. Justin Sonnenburg on "The Good Gut"

32. Gluten-Free Diet: Myths and Facts (Travel to Health)

33. Asthma Associated with Decreased Sepsis-Related Mortality

    • Asthma’s complex immunopathology involves several major mechanisms.  Read this article for more details.