Testing for Diarrhea

What causes diarrhea in a pet?

Many different diseases and disorders can cause diarrhea in pets. It can be as simple as a pet having intestinal parasites (worms) or eating spoiled food. However, in many cases, the cause is more difficult to determine. The following are just a few of the many causes of diarrhea:

  • Disorders of the stomach and bowel (gastrointestinal tract) including inflammation, bacterial, viral, or fungal infections, parasites, tumors, an inability to digest and absorb nutrients, sudden changes in diet, food allergies, and eating inappropriate things (dietary indiscretion)
  • Hormonal diseases, such as hypoadrenocorticism (failure of the adrenal glands called Addison's disease) and hyperthyroidism in cats (overactive thyroid gland)
  • Systemic diseases such as kidney failure, liver disease, and pancreatitis
  • Certain drugs and toxins

How can my veterinarian determine the cause of my pet’s diarrhea?

Determining a diagnosis starts with a complete history and physical examination. A pet’s history is the information you give your veterinarian about your pet’s illness, including details about the frequency, urgency, and appearance of the diarrhea. Other helpful information includes whether your pet has eaten anything unusual, such as house plants or table scraps, or whether the pet is showing other signs of illness such as weight loss, poor appetite, or vomiting. To help you put this history together before visiting your veterinarian, please see the handout "Diarrhea Questionnaire and Checklist".

A physical examination involves checking out all parts of the body and may give clues about the cause of diarrhea. For example, a lethargic puppy with a fever and bloody diarrhea may have a parvovirus infection, a cat with thickened bowels may have intestinal cancer, and a dog in good body condition that frequently passes small amounts of diarrhea may have colitis (inflammation of the lower bowel).

Sometimes, a diagnosis can be made based on history and physical examination, but in most cases, additional diagnostic tests will be needed. Your veterinarian may recommend basic screening tests that provide information about your pet's overall health and may provide further clues about the underlying problem.

What screening tests are recommended, and why?

The most commonly recommended screening tests for a pet with diarrhea include a complete blood count (CBC), serum biochemistry profile, urinalysis, and fecal parasite tests. Thyroxine (total T4) concentration is also recommended for middle-aged to older cats. Screening tests may provide clues to the cause of the diarrhea. In addition, they may uncover problems caused by diarrhea, such as electrolyte disturbances and dehydration.

"Screening tests may provide clues to the cause of the diarrhea. In addition, they may uncover problems caused by diarrhea, such as electrolyte disturbances and dehydration."

Complete Blood Count (CBC) is a simple blood test that provides information about the different cell types in blood. These include red blood cells, which carry oxygen to the tissues, white blood cells, which fight infection and respond to inflammation, and platelets, which help the blood clot. The CBC provides details about the number, size, and shape of the various cell types and identifies the presence of abnormal cells in circulation (see handout “Complete Blood Count”).

In a pet with diarrhea, some changes that could be seen on a CBC include:

  • Anemia (a decrease in the number of red blood cells) could be a sign of bleeding in the bowel or a longstanding disorder, such as Addison's disease, liver disease, or cancer.
  • Hemoconcentration (an increase in packed cell volume) indicates dehydration.
  • High numbers of white blood cells often indicate underlying infection or inflammation and may be seen with pancreatitis, liver disease, kidney infection, and some cancers.
  • Low numbers of white blood cells could indicate a viral infection such as distemper or parvovirus in dogs and panleukopenia in cats.
  • Increased numbers of eosinophils (a type of white blood cell) could indicate allergies or parasitic infections, both of which can cause diarrhea.

Serum biochemistry profile looks at many substances, including proteins, enzymes, fats, sugars, hormones, and electrolytes. Measuring the levels of the various substances in the blood provides information about the health of the body’s organs and tissues, such as the liver, kidney, and pancreas. Changes and abnormalities found in the biochemistry profile can help diagnose various diseases and disorders (see handout “Serum Biochemistry”).

In a pet with diarrhea, some changes that could be seen on a serum biochemistry profile include:

  • Abnormally high levels of liver enzymes are associated with liver damage or dysfunction.
  • Increased kidney values - BUN (blood urea nitrogen) and creatinine suggest kidney failure.
  • Low albumin (blood protein) suggests a pet may be losing excessive amounts of protein through the gastrointestinal tract. 
  • Elevated lipase levels (an enzyme from the pancreas) are sometimes seen in pets with pancreatitis.
  • Pancreatic specific lipase (PSL), spec fPL, or cPL may be included in the biochemistry profile to assess the health of the pancreas accurately. 
  • Altered electrolytes (salts and minerals found in the blood), especially sodium and potassium, are often seen with kidney disease and Addison's disease and can also indicate electrolyte disturbance caused by diarrhea.

Urinalysis is a simple test that analyses urine's physical and chemical composition. It measures how well the kidneys are working, identifies inflammation and infection in the urinary system, and helps to detect diabetes and other metabolic disturbances. Urinalysis is essential for adequately interpreting the serum biochemistry profile and should be done at the same time as blood testing.

In a pet with diarrhea, some changes that could be seen on a urinalysis include:

  • Low specific gravity indicates the pet is passing dilute (pale watery urine) urine. If serum BUN and creatinine are elevated at the same time, kidney disease or possibly kidney failure is present.
  • High specific gravity indicates that the pet is passing highly concentrated urine (dark yellow). It may be a sign of dehydration caused by fluid loss in diarrhea.
  • Bilirubin (a pigment in the urine), especially in cats, is a sign of liver disease.

Testing for intestinal parasites that can cause diarrhea, especially in puppies and kittens. No test can detect all intestinal parasites, and several different tests may be used. The most straightforward test is fecal flotation, in which a small amount of fresh stool is prepared and examined microscopically for the presence of parasites or parasite eggs. Multiple fecal flotations may be needed to detect infection. A simple version of the fecal flotation can be done in the veterinary clinic, or a sample can be sent to the laboratory for a more sensitive test that also screens for parasite DNA.

"Multiple fecal flotations may be needed to detect infection."

Some intestinal parasites, such as Cryptosporidium, Toxoplasma, Giardia, and Tritrichomonas, are challenging to detect with flotation methods. Specialized testing, including blood tests, may need to be done. Samples may be sent to a veterinary referral laboratory for these tests.

Serum thyroxine (total T4) is used to diagnose hyperthyroidism in cats. Hyperthyroidism is caused by an overactive thyroid gland and is common in middle-aged to older cats. The gland produces excessive thyroid hormones, substantially increasing the body’s metabolic rate and often causes vomiting and diarrhea.

Do all these screening tests need to be performed if my pet has diarrhea?

If your pet is bright and alert and the physical examination is normal, screening tests may not be immediately recommended. This is especially true if there is an obvious reason for the diarrhea, such as a sudden change in diet or a history of eating table scraps. However, screening tests are strongly recommended if diarrhea continues or reoccurs or your pet shows other signs of illness, such as fever or abdominal pain.

"...screening tests are strongly recommended if diarrhea continues or reoccurs or your pet shows other signs of illness..."

What additional tests might be recommended for a pet with diarrhea?

Depending on the results of a pet’s history, physical examination, and screening tests, many additional tests might be recommended. If the diarrhea seems to be caused by problems that do not involve the gastrointestinal tract, such as liver disease, kidney failure, Addison’s disease, or heart disease, specific testing for those conditions would be recommended.

If screening tests suggest the problem involves only the gastrointestinal tract, further testing will focus on stomach and bowel disorders. Some examples of testing that might be recommended include:

  • Fecal culture to detect specific bacteria known to cause diarrhea
  • Fecal antigen to detect parasites, such as Giardia and whipworm, that are sometimes difficult to detect with other methods
  • Fecal cytology (direct fecal smear evaluation) to look for specific bacteria called Clostridia, other organisms that cause bacteria, and inflammatory and neoplastic cells
  • Fecal DNA testing to detect viral, bacterial, and parasitic infections that can cause diarrhea
  • Radiographs (X-rays) or ultrasound to examine abdominal organs and the digestive system
  • Endoscopy or exploratory surgery to look at the bowel directly to find abnormalities and possibly take biopsy samples
  • Trypsin-like immunoreactivity (TLI) to detect exocrine pancreatic insufficiency, a disorder in which food is not digested properly, resulting in diarrhea (most common in German Shepherds)
  • Elimination diet trials to assess for a food hypersensitivity (like an allergy)

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