Oh! Those Dirty Little Kittens!

Oh! Those Dirty Little Kittens!

You may have heard of the hygiene hypothesis. The theory is that exposure to dirt and germs early in life helps to prevent allergies later in life — your immune system needs to be "trained" at an early age to fight off allergens.¹ The idea came about partly from the observation that children who grew up on farms had a much lower incidence of allergies than children who grew up in cleaner environments, without as much exposure to naturally occurring germs. There are several studies that conclude that infants exposed to multiple pets, cats or dogs, have fewer allergies as adults.²
Two recent studies add to this idea. One shows that it may apply to creatures other than humans. The hygiene hypothesis may go far beyond just allergies.
The first involved a study of residents of a Philippine city where levels of sanitation were lower than those generally found in the west. Tests were done when these people reached the age of 20 to measure their blood for CRP, short for C-reactive protein, a marker for chronic inflammation. It was found that the more pathogens they were exposed to before the age of two, the less CRP they had at the age of 20. It appeared that every bout of diarrhea, every period living exposed to animal feces, and being born in the dusty, dirty dry season cut their chances of having a higher CRP levels.
If you don't encounter many pathogens early on, your immune system ends up causing inflammation — a normal reaction to infection — at the wrong times. Chronic inflammation has been implicated in higher levels of heart disease, diabetes and mortality.³ While exposure to all of these pathogens is dangerous, it has the side effect of helping to train the immune system to function correctly.
The second study involved tests with pigs. Piglets were raised in three different environments: outdoors, indoors and isolated. The isolated piglets were also fed antibiotics daily. More than 90% of the gut bacteria of the outdoor pigs were of a family known to limit pathogens such as salmonella and e. coli. This type of gut bacteria was less than 70% for the indoor piglets and about 50% for the isolated piglets. Piglets raised in isolation expressed more genes involved with inflammatory immune responses; those raised outdoors expressed more genes involved with infection fighting T-cells. This study noted that there is evidence that immune responses are linked to organisms in the gut — the "dirty" outdoor piglets had picked up friendly bacteria that had helped them develop robust immune systems.
Maybe all creatures, not just people, need a certain amount of exposure to a natural, dirty environment when young to develop good immune systems that don't turn on them later in life.
I think about the cats I have. I know little about their early kittenhood as they were all adopted from rescue organizations. A few I know were outdoors at least some of the time. A couple I know for certain were strictly indoors from about 2 weeks of age. They had to be bottle fed after their mother was run over by a car. I think about that "clean" upbringing and whether their immune systems may have suffered for it. All in all, being outdoors in the area I live in is probably too dangerous on the macro level, what with coyotes, foxes, hawks, stray dogs and cars, than any benefit they would get on the micro level. But I will keep an eye on them as they get older.
Perhaps someday a pathogen "cocktail" will be developed that would do the job of training immune systems without endangering the infant animal, whether human or feline. In the meantime, maybe letting your toddler make mud cakes in the backyard is a good idea. Perhaps you should let kittens help out in the dirt, under protected supervision of course, so that the macro baddies don't get them.
Margaret Gates is the founder the Feline Nutrition Foundation.
  1. H. Garn and H. Renz, "Epidemiological and Immunological Evidence for the Hygiene Hypothesis," Immunobiology 212, no. 6, April 30, 2007, 441-52.
  2. Dennis R. Ownby, MD, "Multiple Pets May Decrease Children's Allergy Risk," Journal of the American Medical Association, August 28, 2002.
  3. J. Danesh, P. Whincup, M. Walker, L. Lennon, A. Thomson, P. Appleby, J. R. Gallimore, and M. B. Pepys, "Low Grade Inflammation and Coronary Heart Disease: Prospective Study and Updated Meta-Analyses, " British Medical Journal 321, 2000, 199-204.
  A. D. Pradhan, J. E. Manson, N. Rifai, J. E. Buring, and P. M. Ridker, "C-reactive Protein, Interleukin 6, and Risk of Developing Type 2 Diabetes Mellitus," Journal of the American Medical Association 286, 2001, 327-349.
  H. K. Kuo, J. F. Bean, C. J. Yen, and S. G. Leveille, "Linking C-Reactive Protein to Late-life Disability in the National Health and Nutrition Examination Survey (NHANES) 1999-2002," Journal of Gerontology 61, December 7, 2005, 380-87.
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