Diarrhea is not a disease itself but rather a symptom that something is not right with your dog's health or gastrointestinal system. Diarrhea is the passing of unformed, frequent, and increased volume of stool.

Many dogs, like humans, may occasionally suffer from an acute bout of diarrhea.
Acute means coming on suddenly and lasting no longer than a few days.
If your dog is well and has no other symptoms other than a frequent loose bowel motion there is probably no cause for immediate alarm and you can treat it at home.
There are many causes for diarrhea, for example your dog may have eaten something disagreeable. Other causes could include a sudden change in diet, food allergies, worm infestation or viral or bacterial infection.

If your dog has any of the following symptoms
in conjunction with diarrhea seek veterinary advice

Fever
Pain
Vomiting
Blood in the diarrhea
Lethargy or depression
Weight Loss
Dehydration
Loss of appetite
Foul smelling diarrhea
Any other sign of illness

What you can do at home for diarrhea

Withhold food (but not water) for 24 hours (12 hours for young puppies) to give your dog's digestive system a rest. Then feed him a bland diet of 50% boiled rice and 50% chicken for the next two to three days.
If the diarrhea doesn't resolve seek advice from your vet.

Watch out for dehydration

A lot of water is lost from the dog's system when passing frequent watery motions so watch your dog for symptoms of dehydration. Sticky or dry gums can often indicate dehydration.

Encourage him to drink plenty of water or Lectade may be given. Lectade, an oral re-hydration therapy for cats and dogs can be used to reverse the effects of dehydration and loss of electrolytes following diarrhea.
In the first 24 hours this should be given at the rate of 8-30mls every half hour by mouth. For example a small toy dog would require 8mls and a large dog such as a german shepherd would require 30mls.

Diet can cause diarrhea in dogs

If the diarrhea has come on suddenly consider if you have made any changes to his diet. Often cheaper brand dog foods can cause diarrhea as can a sudden change in diet. Feed your dog a good high quality diet.
Cheap dog food may ultimately cost you more with higher vet bills. My advice is to feed your dog the best possible food that you can afford. Cheaper dog foods are bulked out with vegetables and carbohydrates which pass straight through your dog.

Rule of thumb: If you put in rubbish, rubbish will come out.

Diarrhea can also be caused by dairy intake in dogs due to lactose intolerance. Dogs and puppies do not need to be given milk to drink.

Don't feed dogs people food. Especially spicy food.

Worms can cause diarrhea in dogs

Worms, such as roundworm, hookworm, whipworm and tapeworm can cause diarrhea in dogs so ensure they are given de-worming medication regularly.

Viral Infections that cause Diarrhea in dogs and puppies

Parvovirus
Parvovirus enteritis is a highly contagious, viral disease which is often fatal. Symptoms include vomiting, diarrhea, dehydration, dark or bloody feces, and in severe cases, fever and lowered white blood cell counts.

Distemper
Canine Distemper is also a highly contagious viral disease which is often fatal. Diarrhea is one of the many symptoms.
Bacterial Infections that cause Diarrhea in puppies and dogs

Bacterial infections such as E. coli, Campylobacter jejuni and Salmonella varieties can cause severe abdominal pain often accompanied with bloody diarrhea

Coccidiosis

Diarrhea is the main sign of coccidiosis. Blood and mucous in the diarrhea is a common symptom.

Your vet may prescribe Sulfa antibiotics. This will not kill the organism but will keep it in check until your puppy develops a good level of immunity.

Giardia

Symptoms in puppies and dogs include diarrhea with mucus and having a pale greasy appearance. The diarrhea may be greenish or yellowish. There may also be blood in the diarrhea There is usually a very bad odor to the diarrhea. Other symptoms are weight loss, abdominal pain and vomiting.

Cryptosporidia

Cryptosporidia is a single cell parasite which can cause diarrhea in dogs and puppies.

Pumpkin for Diarrhea and Constipation

It's quite remarkable that fresh or canned pureed pumpkin (if using canned: do not use the pie filling variety but the 100% natural canned pumpkin) can work wonders for both the occasional bout of diarrhea or for the opposite, constipation in dogs. It has been reported that it firms up dog's loose stools or diarrhea within a few hours. How much pumpkin should you give? It depends on the size of the dog but as a rule of thumb a couple of teaspoons daily for a small dog or a couple of tablespoons for a large dog. You may have to disguise it in you dog's favorite food.

Chronic Diarrhea

If your dog has chronic diarrhea (Chronic means continuing for a long time, lingering or persistent) then medical advice should be sought. Always check with your vet if diarrhea is severe or persistent. Diarrhea can be life threatening.

Some Possible Causes of Diarrhea in Dogs and Puppies

Diet

Drinking Milk (Lactose Intolerance)

Eating Garbage

Infection (viral or bacterial)

Worms

Parasites such as giardia & coccidia

Bowel Disease

Poisoning

Cancer

Blockage (foreign object)

Pancreatitis

Kidney disease

Liver disease

Intussusception (telescoping of the bowel on itself)

Hormone Imbalance

Exocrine Pancreatic Insufficiency


As you can see there are many causes of diarrhea in dogs and puppies. If diarrhea persists you should seek advice from your vet.