The Big “D”: Causes of Serious Diarrhea

by Patricia Gail Burnham

A familiar sight; no Big D, thankfully. © Praveen Mutalik

The function of a dog’s lower intestine is to remove water from the digestive contents and return it to the body while the feces are on the way out. If the intestinal contents move through the intestine too quickly, there isn’t time for the fluids to be removed and the result is watery diarrhea. If the lining of the intestine is inflamed, then blood may be flowing from the body to the digestive tract and the result is diarrhea that contains blood. A dog’s body is an energy system in which he takes in food and water, processes them through the digestive tract and then excretes what he doesn’t need. He does need the water. Dog and human bodies are two thirds water. In Star Trek an alien describes human beings as “Ugly bags of mostly water.” This is true of us and of dogs. We both die rather quickly by dehydration so we need to keep all that water inside our bodies. The lower intestine does that by removing the water from the digested food so that the water isn’t lost along with the feces.

Diarrhea is the result of the feces passing through the intestine so quickly that it can’t remove the excess water.

The Main Causes

A bacterial bloom due to a change of diet or eating soil bacteria.

Foreign Bacteria like Salmonella

Protozoa like Coccidia or Giardia

Viruses as in Parvovirus or Distemper or Coronavirus


Autoimmune inflammation of the intestine


The measures to prevent diarrhea depend on which kind you are trying to prevent. So let’s start with the first one.

About Bacterial Bloom

Bacteria are microscopic plants with single cell bodies. They are the oldest, the structurally simplest, and the most abundant form of life on earth. Fossil bacteria have been found in rocks 3.5 billion years old. 4,800 kinds of bacteria have been identified and there are plenty that haven’t been identified yet. We live in a sea of bacteria. There are bacteria on our skin and in our intestines. There are bacteria in the soil. Some soil bacteria have the ability to fix nitrogen and without them plants would not grow. In general bacteria are good but some of them we could do without. A key to bacteria is that bacteria populations are able to reproduce quickly and adapt to changing living conditions.

Healthy intestines contain friendly bacteria that help us and the dogs digest their food. These bacteria populations adapt to the animal’s diet. A dog on a corn-based kibble is going to have a different mix of intestinal bacteria than a dog on a meat and rice diet. But when you change a dog’s diet suddenly, the new food produces a bloom in intestinal bacteria as they try to adapt to the new food. It is this bacterial bloom — the excess numbers of bacteria — that produces diarrhea in response to a change in diet. The narrower the dog’s diet is before the change, the more specialized his bacteria will be and the greater the likelihood is that a change will produce diarrhea. If you feed a dog a varied diet it will support a more varied population of gut bacteria and it will be more adaptable to changes in diet. I don’t mean changing kibbles every week. I feed a basic IAMS kibble with added meats (lamb, beef, chicken, salmon) and vegetables and cooked eggs and some pasta. If you feed a dog straight kibble and want to change his diet then do it slowly by adding small amounts of the new food gradually to give his bacteria time to adapt.

The same also applies to water. On our annual trip to Santa Barbara, I carry as much water from home as I can and buy bottled water if we run out. (I used to work for water districts in Santa Barbara and know what their water is like.)

In addition to native intestinal bacteria, the dog can also get a bloom of soil bacteria. Puppies and adults often like to simply eat garden soil. The average gram of soil contains 2.5 billion bacteria, 400,000 fungi, 50,000 algae, and 30,000 protozoa. The soil bacteria aren’t adapted to living in the dog’s intestine and can have a population bloom in the intestine that produces diarrhea.

In these cases the dogs are happy and playful. They don’t act sick. They just have diarrhea. These respond very well to the antibiotic metronidazole (the generic name for Flagyl). It will often clear up the diarrhea within 12 hours but you need to keep giving it for the recommended number of days to avoid relapses. Kira’s littermates loved to eat the grass and roots and dirt and went though a lot of metronidazole. And Pepto Bismol tablets. One vet recommended that I give liquid Pepto Bismol to an ailing adult greyhound and the result was pink stuff everywhere, on me and the dog and the protective towel but little in the dog. Now they have developed Pepto tablets which are wonderful for dogs with mild cases of the Big D. I guess that a mild case of Big D is little d.

Some antibiotics can kill the intestinal bacteria or unbalance their composition with diarrhea resulting. This is particularly a problem in young puppies. A few tablespoons of live culture yogurt (preferably goat yogurt) added to the dog’s food can help repopulate the good bacteria with live acidopholus. Pasturised yogurt is not helpful here because the bacteria have been killed. You need a yogurt that says “Live cultures” on the package. I give live culture yogurt to any dogs that are on a course of metronidazole or other antibiotics.

Foreign Bacteria

In addition to misplaced friendly bacteria there are some types of bacteria that are inherently diarrhea producing. Salmonella is the best known of these bacteria but not the only one. And yes dogs can get food poisoning. I cook everything including eggs and meat. Yes wild animals don’t cook their prey, but their prey aren’t factory farmed and mechanically processed. As scavengers dogs may have better carrion eating abilities than humans do but I wouldn’t bet my dogs health on it. Feed fresh food or refrigerate left overs.

Protozoal Diarrhea

Protozoa are microscopic animals, often single celled. Coccidia or Giardia are a pair of protozoans that we would like to avoid. Coccidia is the reason that my bird feeders are in my front yard where the dogs can’t reach them. Birds carry coccidiosis and a dog with Coccidia is not pretty. They don’t run a fever but they have severe diarrhea. Wildlife carry Giardia and even urban water supplies are sometimes infected. Giardia is hard to find in a fecal sample unless it is really fresh. An antibody test is more reliable. But even if you can’t identify giardia you don’t really have to. The treatment is the same as for bacteria blooms, metronidazole. So for any diarrhea where you haven’t changed the dogs food recently and the diarrhea is the only symptom I would lobby my vet for a course of metronidazole.

Identifying And Treating Bacterial And Protozoal Type Diarrheas

In simple terms bacterial and protozoan diarrheas usually produce a dog that has diarrhea but is happy and eats. The treatment is metronidazole, and if the dog can be reinfected from the soil that treatment may need to be repeated. If the dog is just producing a bacterial bloom in response to a change in diet, then Pepto Bismol may tide him over until his bacteria adapt; but it is better not to change diets abruptly. The bacterial diarrheas are often self limiting and you can wait them out, but why would you want to? The protozoan parasites need metronidazole as soon as possible.

Viral Diarrheas

With Parvovirus, Coronavirus, and Distemper, the dog not only has diarrhea but is very ill. Fever, vomiting, and lethargy and refusal to eat are accompanying symptoms. These diseases are the reason that the onset of diarrhea in the dogs strikes fear into me. Big D could be the first symptom of these life threatening illnesses. My first dog was a Collie puppy claimed out of an animal shelter. He died of distemper in spite of immediate vaccination. It was too late for the vaccination to protect him. He had already been exposed. Twenty years later I had the misfortune of bringing home from some Arizona dog shows in the 1970s the first cases of Coronavirus seen in Santa Barbara.

All you can do with a virus is treat the symptoms. I fed the dogs Pedialyte and baby food with a basting syringe for nearly ten days until they could recover. We went through bottles of Kaopectilin to coat the digestive tract. But nobody died. Parvovirus has a shorter duration but requires more intensive means to avoid dehydration. Intravenous fluids, glucose and, if a lot of blood is being lost, IV bloods are given. Once the virus has run its course the dog is put on a bland diet (baby food, or cooked rice and ground meat, or canned prescription diet ID). Again, Kaolin and Pectin to coat the inflamed intestine is a good idea. It’s available from most large animal suppliers and the Omaha Vaccine mail order company. (800-242-9447)

Antibiotics don’t help against a viral disease. The best protection against these diseases is adequate vaccination. Unfortunately vaccines are not always stored and given properly to racing Greyhounds and outbreaks of distemper do occur on farms and in kennels. I would vaccinate any dog coming into adoption as if it had never been vaccinated. Also a few individual dogs cannot produce antibodies in response to vaccines. This is rare, but there is a small number of dogs that don’t develop immunity. I had a veterinarian once who could not respond to rabies vaccine. So he had to be particularly careful not to be exposed.

Parvovirus, Distemper,And Coronavirus

These can all live in the soil for years. This makes them particularly bad for breeders because an infected litter can contaminate the property and then litters born later can be infected. Or one sick dog can contaminate your back yard and then dogs that move in a year later could be infected. For the virus diseases, prevention is your best protection.

Viral Diarrheas Need Immediate Vet Care And Diagnosis.

The dog’s life is at stake with these. Antibiotics won’t help but fluids, electrolytes, and supportive care may save the dog. And remember the Kao-pectin.

And yes, the protozoan diarrheas can be passed from dog to owner and back again. Remember your mother’s advice and wash those hands before handling your or your dog’s food. Especially wash your hands between handling pet birds and your or your dog’s food. I once visited a Greyhound kennel that also raised livestock. I helped them bottle feed some cute baby lambs that they had just bought at an auction. The lambs had severe diarrhea. In the next week, both my aged parents, most of my dogs, and I came down with diarrhea. Then I read a clipping about an infectious diarrhea that could be passed from livestock to humans. It was too late. Wash those hands, especially if either you or the dogs have “The Big D.”


Fortunately cancer is a fairly rare cause of diarrhea but tumors can attack the digestive system. An endoscopic exam and biopsy may be needed to diagnose them. If they are found early enough they could be removed surgically since there is enough intestine to snip some of it out surgically. The problem is that the tumors are likely to have spread before you realize that there is a problem.

We spent seven months trying to diagnose one of my favorite Greyhound’s mysterious and varied symptoms before she liberated a pot roast, took it onto the guest room bed and ate several pounds of beef before tossing the neck bones to the other dogs. They carried them out to where I was mowing the back lawn and when I found them carrying around fresh cow vertebrae I knew something was amiss. I wasn’t surprised when Love developed diarrhea but when it didn’t clear up in a few days I took her to the vet. He did an endoscopic exam and a biopsy that revealed widespread sarcomas in her digestive tract. An endoscopic exam involves anesthetizing the dog and then running a fiber optic tube into the digestive tract to look at the inside of the dog.

Stress-induced Diarrhea

When we are under stress our bodies produces adrenaline and other chemicals to prepare us for flight or a fight. These can inflame the lining of the intestine. So instead of defecating because we are about to be eaten by a sabre-toothed tiger, now we get diarrhea because there is a final exam looming before us.

Stress affects dogs the same way. My first Collie would respond to training sessions by throwing up. His stress was attacking the upper end of his digestive system. Other dogs respond to stress by irritating the lower end of the digestive system and having diarrhea. Stress produces diarrhea by putting unwelcome hormones and chemicals into the dog’s intestine. Stress can also trigger an allergic response. (My training methods now are a lot less stressful for the dogs. Now they want to work and are indignant if they have to watch me work somebody other than them.)

Allergic Inflammation of the Intestine Triggered by Stress

Or an allergy can be produced without stress simply by exposure to the allergen. Allergies are what happens when the body produces antibodies to everyday substances or to itself. The body is supposed to be producing antibodies against foreign invaders like viruses and bacteria. When it produces antibodies to harmless substances, and those antibodies produce a histamine reaction which causes tissue inflammation, that inflammation is an allergic reaction. Allergies are triggered by exposure to allergen which are either foreign substances like food or pollen, or may be the body’s own tissues. When the body is producing antibodies to its own tissues it is called an autoimmune response. (Auto means “self” so it is an immune response to the bodies own self.)

Allergic Reactions can Target Any Part of the Body

When inflammation occurs in the brain, seizures may result. But the skin and the digestive tract are the most common sites for canine allergies. These are the areas where the body comes in contact with the outside world. I know you think of your intestinal tract as being inside you, but actually it is a tube that connects your mouth with your other end. There is daylight on both ends. Nutrients don’t really enter your body until they pass through the intestinal wall. Our bodies are like a doughnut with a hole in the center. Food is just on its way through the doughnut hole, and we snatch out nutrients as it passes through.

When the skin is the target of an allergy, a rash occurs. When an allergic reaction targets the intestine, the result is inflammation, diarrhea and sometimes ulceration. This is a subject I have a lot of experience with since my family is riddled with allergies, and my sister nearly died of allergy caused ulcerative colitis. (As a result my mother gave each of us kids a medical textbook on food allergies and their treatment. It has even proved helpful for the dogs.)

A Short Course in Allergies

Allergies are the result of the body’s immune system producing antibodies in response to exposure to antigens. Any of the four types of antigens can produce inflammation in the digestive tract with resulting diarrhea. However because of the direct contact between food and the digestive tract food allergies are the most frequent source of intestinal allergies.

Similarly contact allergies usually occur on the skin. Inhalant allergies generally affect lungs first. But while all three of the above have their favorite locations, they can cross over and produce symptoms anywhere in the body. And autoimmune responses can occur anywhere.

The Antigens can Reach the Body in Four Ways:

1. Antigens can be inhaled (pollen, dust, mites, molds, perfume, smoke.)

2. Antigens can be eaten (food allergies)

3. Antigens may be applied to the skin (contact allergies-dips, shampoos, grass, cosmetics, flea stuff, fabric softeners)

4. Antigens may be the body’s own tissues (autoimmune response)

Inhalant Allergies

Inhalant allergies are tough to treat. You can eliminate perfume and smoke exposure, but pollen, dust and molds are everywhere. Human patients go to the extreme of staying indoors with electrostatic air cleaners. With dogs we eliminate the smoke and perfume and then treat the resulting allergies with antihistamines and steroids. The same applies to autoimmune responses. But before you can diagnose an allergy as autoimmune you need to eliminate all the other possible antigens. And it is virtually impossible to eliminate all the possible allergens. I believe that autoimmune response is over diagnosed by vets who lack the persistence to identify the actual allergen. Airborne and contact allergens can be identified by scratch tests.

Food Allergies

Food allergies can only be identified by the use of an elimination diet. So here is the basic elimination diet with credit to Dr. Rowe’s “Food Allergy-Its Manifestations and Control and the Elimination Diets.” The bad news is that the elimination diets are home cooked. The good news is that they are simple. The basic diet is a half-pound of cooked meat combined with two cups of cooked cereal product. I use cooked lamb and white rice. But if you have a dog that has been fed lamb and rice kibble it may have developed antibodies to lamb and rice and you will need other meat and cereal sources. Rabbit and barley, beef and pasta, chicken and corn meal. For this reason I don’t feed my dogs lamb and rice kibbles. I reserve the real lamb and rice for allergy and convalescent meals.

First, I cook the rice according to the package directions. Then I sauté the ground meat and add the cooked rice, stirring so the rice grains are coated with fat from the meat. Let it cool and it is ready to serve. The basic diet is fed for three weeks and by the end of that time you should see a dramatic improvement in the symptoms. Once the symptoms clear up you know you have a food allergy. Then you get to test for additional foods by adding them to the diet one at a time every two weeks. If the dog stays symptom free for two weeks then you have another acceptable food. If he develops symptoms you discontinue the test food and go back to the basic diet until he is cleared up and ready for another test food.

It could be the Additives

Sometimes what a dog is allergic to is not the basic food, (corn, chicken, beef, etc.) in commercial dog food but the additives of which there are many. Just read the list on your dog food bag. If that is the case, then the dog will stay healthy on any combination of home cooked foods but get sick once you put him back on commercial kibble.

Feeding Dogs to Reduce Additive Allergies

Sometimes keeping dogs away from commercial foods will enable them to reduce their antibodies to that antigen to the point where they can tolerate commercial food again. My first dog was an allergic Collie who needed to have his food rotated every six months. For six months I would feed him beef and rice, and for the next six months would feed him beef and kibble. If we didn’t change his food source periodically he would start throwing up. So we rotated his food on a six month schedule for twelve years and he stayed healthy. He also had topical allergies to soaps. But, once I learned to wash him monthly in plain water, he lived the last half of his life without skin eczema. As a first dog he fit right into my family and shared our allergies.

Reintroducing Kibble

More recently I kept a Greyhound on home cooked lamb and rice for two years and was then able to reintroduce her to kibble without symptoms. If you keep the antigen away from the dog long enough the antibodies die out and you can reintroduce the food later. But long enough is like six months to several years.

Why Cook for my Dog?

I have had folks say, “Gee, I don’t cook for my family, I certainly can’t cook for the dog.” But cooking for the dog is a lot easier and cheaper than treating the results of an uncontrolled food allergy. Vomiting and diarrhea aren’t fun. And while I have only been talking about gastrointestinal allergies here, allergies can also cause inflammation of the skin, bladder, ears, eyes, and lungs, and in the brain they can cause epilepsy.

With autoimmune disorders and inhalant allergies that we can’t control the only choice is to treat them with drugs. But with food allergies the best approach is to eliminate the offending foods through the use of the elimination test diet. You can buy a lot of ground lamb and rice for the price of one major vet visit.

Bio – Gail Burnham has written hundreds of magazine articles, plus two dog training books illustrated with greyhounds, Playtraining Your Dog in 1981 and Treats, Play, Love: Make Dog Training Fun For You and Your Best Friend in 2008. She contributed regularly to Celebrating Greyhounds the first five years, most notably the Kira series which was a finalist in the Dog Writer’s Association writing contest. She won a DWAA award for the poem, The Red Bitch’s Hunt, which was part of a series of poems about Coventry and the Red Bitch. Currently she is retired, still writing about dogs, and is is living a very active life with the descendants of the original Tiger.

CG F,W 98



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