Giardia is a flagellate protozoan parasite commonly encountered in small animal veterinary practice. The most common clinical
syndrome associated with Giardia is acute small bowel diarrhea, but in some cases acute large bowel diarrhea, chronic small
or large bowel diarrhea, or rarely acute or chronic vomiting may occur. Studies throughout the world have found infection
rates ranging from 1%-39% in pet and shelter dogs and cats. Recently a study utilizing PCR found 80% of cats in Perth Australia
to be positive. Many of the Giardia infected animals did not have diarrhea. Younger animals had a higher rate of infection.
It appears that various strains posses differing degrees of pathogenicity. Clinical signs may be self-limiting in some patients.
Severe disease may occur in puppies or kittens, animals with other gastrointestinal parasites or diseases, or debilitated
animals, but also can occur in otherwise healthy patients. Giardia cysts are not routinely identified by commonly used fecal
flotation solutions because cysts become shriveled and cannot be identified. In addition, the numbers of cysts shed in the
feces fluctuate over time. Many commonly used anthelmintics are not effective against Giardia. Although the issue is presently
unresolved, some strains of Giardia are a zoonotic threat. This paper will review the important clinical aspects of giardiasis
and will present a practical diagnostic plan and differential diagnosis.
Infection is acquired by ingestion of cysts, only a small number are necessary. Most dogs and cats infected with Giardia remain
asymptomatic. When clinical signs occur, acute small bowel diarrhea is most common. Small bowel diarrhea has the following
characteristics: liquid to semi-formed feces, moderately increased frequency of defecation, and normal to increased quantity
of feces per defecation. The presence of melena (digested blood) is uncommon in cases of giardiasis. Diarrhea usually is self-limiting
in animals that develop clinical signs, and has been described as pale, malodorous, and fatty. Severe diarrhea may be accompanied
by dehydration, lethargy, and anorexia. However, most affected patients remain bright and alert, afebrile, and maintain a
normal appetite. Occasionally acute vomiting may accompany diarrhea. The author has endoscopically observed severe erosion
of the duodenum in some cases that resolved following successful treatment for Giardia. A mild eosinophilia has been demonstrated.
Chronic small bowel diarrhea with weight loss, poor body condition, and intermittent vomiting may also occur. In addition,
the author occasionally has identified Giardia in cases of chronic vomiting. Giardia may be found in dogs and cats that have
other gastrointestinal diseases, especially inflammatory bowel disease. In these cases, the clinical signs and laboratory
findings reflect the underlying disease. In humans, Giardia may mimic inflammatory bowel disease.
Acute or chronic large bowel diarrhea with hematochezia, excess fecal mucus, and tenesmus may occur on occasion. In cases
of large bowel diarrhea, the frequency of defecation is moderately to greatly increased and quantity of feces per defecation
in reduced. Excess fecal mucus is often seen in infected cats.
Differential Diagnosis and Diagnostic Plan
There are many causes of diarrhea in dogs and cats. Common causes for acute diarrhea include the following: Giardia, hookworms,
roundworms, coccidia, dietary indiscretion, foreign body, toxins, drugs, hemorrhagic gastroenteritis (HGE), coronavirus, parvovirus,
and intussusception. A thorough and logical diagnostic plan should be followed to facilitate reaching an accurate diagnosis,
minimizing stress to the patient and expense for the owner.
The initial step in evaluation of cases with acute diarrhea is to distinguish between self-limiting and life-threatening causes.
Most cases are self-limiting and can be diagnosed with a thorough history, careful physical examination, and fecal examination.
Life-threatening cases may be associated with some of the following findings: frequent diarrhea, moderate to severe dehydration
or abdominal pain, frequent vomiting, or systemic signs such as fever, icterus, lymphadenopathy, coughing, nasal discharge,
or dyspnea. Puppies and kittens with severe clinical signs, especially if unvaccinated, should be suspected of having an infectious
A recent dietary change, dietary indiscretion, or administration of medication may be identified in the history and suspected
as the cause of self-limiting diarrhea. If the history does not identify an underlying problem, a fecal examination should
be performed to identify Giardia or other parasites. Appropriate therapy for GI parasites, correction of dietary indiscretion,
discontinuing suspect medications, or feeding a low-fat, highly digestible diet will often resolve clinical signs. Animals
that are mildly dehydrated may require subcutaneous fluid therapy while those with very frequent diarrhea may benefit from
motility modification with narcotics. Failure of the diarrhea to resolve indicates that a more thorough diagnostic approach
should be followed.