by Whit Gibbons

March 6, 2016

Q: We often hear in the news about people being harmed by contaminated food or water. But other animals, including our family dog, can eat or drink just about anything and apparently be unaffected.

Our dog will quickly snatch up discarded food on the streets, drink water from dirty puddles and even eat wastes of other animals, all of which would surely sicken any human. Yet our dog never catches a cold or the flu. How is it pets and wild animals are immune?

A: A complete answer would require a long article that considered the different evolutionary histories and physiological adaptations among species, the countless types and origins of ailments and diseases as well as our own perceptions about how other animals feel. But here are a few short answers.

First, to assume that other animals do not get sick is incorrect. Dogs and many wild animals can get a host of diseases, including well-known ones such as rabies, distemper and cancer.

Also, dogs can be sick without our knowing it and without complaining. Some may hide away or simply remain immobile, so how would we know if they have a headache? If you slept 12 to 14 hours a day like many dogs, would people really know whether you were sick?

From another perspective, humans sometimes live under what we would consider unsanitary conditions but do not get sick.

Tourists to countries without conventional sanitary conditions often get ill from drinking tap water. Yet the native inhabitants are mostly unaffected because they have become acclimated over their lifetime.

Most people who grew up in a region with constant exposure to unhealthy living conditions probably became sick at some point during childhood but developed immunity to regional bacterial or viral infections.

Regarding recent news about contaminated water, the response to some contaminants - for any animal, including people - cannot be corrected by early exposure.

In Flint, Michigan, where children and adults drank lead-contaminated water, an individual's health would get worse with continued exposure, not better.

Another historic example of heavy metal contamination was the mercury poisoning in Minamata Bay in the mid-20th century when a Japanese chemical company polluted the coastal waters.

Thousands of children and adults, as well as domestic pets and farm animals, died or were seriously impaired by the steady consumption of mercury over the years.

A major factor affecting whether an animal, including dogs and humans, gets sick from certain diseases is simply whether living conditions are such that contagious diseases can spread.

Dogs and most wild animals do not live in high-density situations with others of their species the way people do.

Think how many ailments (colds, flu, intestinal disorders) people get that we can directly attribute to exposure to other humans on airplanes, in crowded stores, or on cruise ships. A family pet is seldom placed in a crowded situation with other dogs.

Many of the common ailments afflicting humans are contracted from other people. Someone who was living alone in a cabin in the woods and never came in contact with other humans would be unlikely to get sick from what we consider everyday transmittable diseases.

Excluding injury, ingestion of something poisonous or genetic propensities toward certain diseases such as muscular dystrophy or ALS, humans isolated from other humans seldom get sick.

Likewise, most dogs live relatively solitary lives and stay healthy, particularly compared to overcrowded livestock.

Such overcrowding has resulted in the extensive use of antibiotics to control contagious diseases in cattle. Dogs in crowded conditions are more likely to get kennel cough.

So, dogs and wild animals are not really any more immune from certain forms of environmental contamination hazards than we are, although natural selection has probably weeded out individuals that took ill and died from eating nasty things.

Sounds like your dog comes from a lineage in which the survivors ate whatever was available when they were puppies.

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