Intestinal obstructions
Dogs and cats don't always think before they eat. "I've seen pets eat gloves, panty hose, corncobs, string, and lots of other indigestible stuff," says Dr. Hibbs. Unfortunately, the inside of your pet's digestive tract is smaller than her mouth, which means that things that slide down the hatch don't always pass through the other end.

You should suspect that your pet has something blocking the digestive tract if she is trying to vomit but isn't bringing much up or if she is straining without success to pass a stool. Her abdomen may be tender, and she may be contorting herself into unusual positions to reduce painful pressure on her insides.

It is not uncommon for foreign objects in the digestive tract to move around, causing the blockages to come and go. Dogs have big stomachs, for example, and a ball or some other object can drift around for a long time without causing symptoms until it blocks the outlet to the intestines. These occasional obstructions are not as serious as complete ones, but in the long run they can still make your pet very sick.

Cats often swallow thread or string, adds Dr. Hibbs. When strings get into the digestive tract, they can wrap around the intestines, sometimes cutting them open.

In most cases, surgery is the only way to remove foreign objects from the digestive tract. "Once the obstruction is out, pets usually get well very quickly," says Dr. Nisson.

Pancreatitis
Another illness that can affect the pancreas is pancreatitis, in which its powerful digestive enzymes, rather than being sent to the intestines, spill inside the pancreas itself. When this happens, they begin digesting tissue in the same way they would pet food.

A number of infections, including feline infectious peritonitis (FIP) and toxoplasmosis, can cause pancreatitis, but most of the time vets don't know what is behind it, says Dr. Hibbs. It is most common in middle-aged, overweight pets, and it often begins right after a fatty meal or snack. Pets with a sudden onset of pancreatitis will vomit violently and have diarrhea, and their bellies may be very tender. Pets with a milder, long-term form of this condition won't appear quite as sick, but they will have similar symptoms for months or even years.

The treatment for pancreatitis depends on how sick your pet already is. If she has a mild case, your vet may recommend putting her on a 24-hour fast, followed by a week of small, bland meals. After that, she should be okay, says Dr. Hibbs. In more serious cases, your pet will need to be hospitalized, put on a fast, and given intravenous fluids and injections of antibiotics.

Parasites
The digestive tracts of dogs and cats are warm, dark, and full of nutrients -- perfect havens for hookworms, tapeworms, giardia, and other parasites.

Worms and Parasites

Pet can often have parasites for years without having any symptoms whatsoever. But when the parasites thrive and multiply, they can cause weight loss, bloody stools, weakness, and fatigue. "They irritate the bowel, so your pet can't absorb the nutrients from her food," says Dr. Nisson. Parasites such as hookworms also suck blood, which can make pets anemic and weak.

Most parasites are highly contagious. The eggs are shed in the stool, which contaminates the ground and water. When your pet drinks contaminated water or runs across grass and then licks her feet, she will ingest the eggs and get infected herself. Tapeworms, which are among the most common parasites, are transmitted differently. They live inside fleas as well as in rabbits, mice, and other rodents. When dogs and cats lick themselves and swallow an infected flea or if they regularly hunt outdoors, they will eventually come down with tapeworms.

Pet supply stores sell a number of medications for controlling parasites, but they are mainly designed to kill roundworms and hookworms and won't touch other parasites, says Dr. Nisson. The only way to be sure that you are using the right medication is to take a fresh stool sample to your vet. Knowing what parasites your pet has will make them much easier to treat.

Parasitic worms can be killed with a drug called fenbendazole (Panacur), which is available by prescription from vets. Non-worm parasites such as coccidia and giardia are treated with antibiotics. Your vet has other medications to choose from as well.

When fighting worms, don't forget about the next generation. It is important to clean up stools in the yard, says Dr. Nisson. It is also a good idea to wash your pet's food and water dishes often and to keep your house and yard as flea-free as possible.

Parvovirus 
If you have ever seen a puppy with full-fledged parvo, you won't soon forget it. This viral infection can make pets miserably ill, causing yellow or white foamy vomit and bloody diarrhea.    Parvovirus

For all its fearsome power, parvovirus hasn't been around very long. Vets believe that a virus that originally caused diarrhea in cats mutated in the late 1970s. It quit bothering cats and began infecting dogs. Any dog can get parvovirus, but it is most common in puppies 16 weeks and younger, probably because their immune systems aren't fully developed. Vets aren't sure why, but certain breeds such as Rottweilers, Doberman pinschers, and English Springer Spaniels tend to get exceptionally ill when they get Parvo.

As with many viral infections, Parvo is highly contagious. Particles of the virus are shed from the body in the stool and are readily passed from dog to dog. It is an extremely serious illness that requires prompt treatment. "Parvo kills a lot of dogs each year, so if you suspect that your dog might have it, see a vet right away," says Dr. Hibbs. 

Dogs with mild cases of Parvovirus are usually treated with antibiotics at home. In more serious cases, dogs may need to be hospitalized in order to receive intravenous fluids and injections of antibiotics. Keep your dog hydrated!

Even though the virus that causes Parvo is very common in the environment, the illness is easy to prevent by getting your dog vaccinated once a year. "This is a bad disease, so avoid it if you can," says Dr. Hibbs.