Many cats do love the taste of cow's milk and other dairy products, which prompts their owners to give it as a treat. In cats with very little or no lactase enzymes, the body is unable to digest the milk and instead attempts to expel it from the system. The undigested lactose sugars in the intestinal tract begin to draw all sorts of bacteria. The sugar then ferments, causing adverse reactions by the stomach and gastrointestinal system. This manifests as stomach acid and painful gas in the cat.

Adult cats are naturally 100% carnivorous. While the digestive system of a kitten can handle milk produced by its mother, as the cat grows it loses this ability. This is because the body no longer needs to produce the lactase enzyme, needed to digest lactose sugar in milk. Some adult cats merely slow down production, while others stop it all together. This in itself is not an actual disorder, but rather a natural occurrence in the maturity of a cat's digestive system.

Milk consumption may cause your cat's digestive system to go into distress. This is separate from an allergic reaction, in which the immune system triggers a response within the body when a certain food is consumed. Signs of intolerance include:

The general genetic makeup of cats is what causes an intolerance to lactose. Milk is not a natural food for adult cats. Because it is inappropriate for the species, their bodies do not carry the proper enzymes for milk digestion. The outcome of this intolerance will be seen when the cat ingests milk or other dairy products. The severity of the reaction within the body will vary depending on the amount of lactase enzymes in the cat's digestive tract. Genetic differences from cat to cat will create different levels of intolerance. 

If you become concerned with the severity of symptoms, it is always best to bring the cat to your veterinarian to have it checked. This is especially the case if a long period of diarrhea or vomiting has left your cat showing signs of dehydration. Be sure to inform your vet of any milk or dairy that has been fed to the cat leading up to the adverse symptoms. The vet will then determine the best way to differentiate lactose Intolerance from other, more serious health conditions that exhibit matching reactions from the body. 

A physical examination of the cat will be performed to check for possible underlying issues such as gastrointestinal disease. At this point, tests will also be performed to determine how dehydrated the cat has become. A skin turgor test can reveal dehydration by seeing how long it takes the cat's neck skin to return to position after being gently pulled upward. Capillary refill time will also indicate dehydration, if it takes a lengthened time for the gums to regain normal color after being pressed. The veterinarian will also want to differentiate the episode of intolerance from inflammatory bowel disease (IBD), gastrointestinal cancer, and bacterial infection. A fecal sample may be gathered to test for parasites in the body. Blood work will be run including a complete blood count and a biochemical profile to monitor the overall health of the cat. An abdominal x-ray may be suggested to confirm that there are no abnormalities present.

While long-term treatment simply requires eliminating milk and dairy products from the cat's diet, symptoms may need to be alleviated in the short term to lessen the amount of pain the cat goes through. If the cat has become severely dehydrated, more treatment may be necessary.

If the cat has been vomiting or expelling diarrhea for a lengthened period of time, bodily fluids may need to be replaced intravenously. Hospitalization is required for this process.

If the cat is mildly dehydrated, your vet may administer fluid via a syringe into the loose skin on the back of the cat's neck. You may be given tools for continued at-home subcutaneous fluid administration.

If your cat became dehydrated from symptoms of lactose intolerance, be sure to supply it with fresh water multiple times a day while it recovers. Eliminate all pasteurized milk products from your cat's diet to ensure the episode does not happen again. Lactose-free milk is not an appropriate alternative as it has excess sugar in it. Obesity can have multiple adverse effects on cats, and a high sugar liquid should not be a part of a cat's regular diet.

It should be noted that unpasteurized milk products are less likely to produce gastrointestinal distress, as the bacteria present in the dairy has already broken down much of the lactose. This makes items such as unpasteurized plain yogurt and cheddar or cottage cheeses a better option if your cat simply loves milk-based treats. At the most, these items should be given on occasion as they are not a natural part of a cat's diet.