The main concern over drinking water is the chlorine. Chlorine kills bacteria, both good and bad. "Good" bacteria are essential for intestinal health; "Bad" bacteria or germs can cause infections if the body’s defenses allow it. If essential bacteria are too diminished, the yeast that is normally there in small amounts will overpopulate the intestines, producing chemicals that create microscopic holes in the intestines that allow relatively large molecules from incompletely digested food to enter the blood.
This is called the "leaky gut" phenomenon. The body’s immune cells don’t recognize these as normal and treats them as invading germs. The immune cells form antibodies against the molecules. Then the immune system may attack any future such molecules. The real danger is that the body will treat any cells that these large molecules associate with as foreign, and attack them, producing autoimmune diseases. Any body tissue could be involved, but most commonly in dogs we see autoimmune diseases of the red blood cells (hemolytic anemia), platelets (thrombocytopenia), thyroid gland (a form of hypothyroidism), joints (arthritis), intestines (inflammatory bowel disease), muscles (myositis), and skin (lupus and pemphigus).
It is especially important to avoid chlorinated water in puppies as their intestines and immune systems are still developing and finding their biological equilibrium. Also, the thyroid gland, which is essential in the control of the body’s immunity, is not fully developed until about 8 months of age.
By the way, chlorine is not the only culprit in causing "leaky gut." I believe that pollution, including some chemicals in processed foods and in anti-parasitic applications, as well as environmental pollutants, kill many intestinal bacteria. Perhaps the worst offenders are medications such as antibiotics and steroids. If your pet must get these drugs, then it is incumbent on you, the owner, to give probiotics to replace the good bacteria (Lactobacillus, Bifidobacterium, etc). Just don’t give the probiotics at the same time as an antibiotic; give them a couple of hours later.
If the water is unchlorinated, as in well water, and other chemicals have not been added, it does not have to be filtered. Letting chlorinated water stand in an open pot or pail for 24 hours will allow the chlorine to "off-gas." Otherwise, the water may be filtered in a reverse osmosis filter or any other filter that is guaranteed to take out the chlorine.
