Giardia in Dogs

What is Giardia?

Giardiasis is an intestinal infection of man and animals caused by a protozoan parasite Giardia intestinalis (also known as Giardia lamblia).  

Giardia is a simple one-celled parasitic species; it is not a "worm", bacteria or virus.  This parasite occurs worldwide, and in North America, it is the most common intestinal parasite in humans.  Outdoor enthusiasts who come down with “beaver fever” have usually inadvertently consumed water contaminated with Giardia.  In humans, symptoms of giardiasis include decreased appetite, vomiting, diarrhea, bloating, and abdominal cramps. In dogs, the most common clinical sign of giardiasis is soft, greasy, rancid-smelling diarrhea that may contain excessive mucus.  The majority of dogs infected with Giardia, however, do not have diarrhea, vomiting or any other signs of illness.

There are two forms of the Giardia organism – a fragile, feeding form that attaches to the surface of the small intestine and then moves from site to site, and a hardy cystic form that is shed in feces, is immediately infective, and can survive several months in the environment – particularly in water and damp environments.

How do dogs get giardiasis?

A dog becomes infected with Giardia when it ingests the parasite in the cyst stage from feces contamination in or on food, water, the environment, or fur. In susceptible dogs, once the cyst passes into the dog's intestines, it goes through transformation to the trophozoite, or feeding, form and attaches to the intestinal wall to feed. If sufficient numbers are present, clinical signs develop reflecting damage to the intestinal wall. Trophozoites reproduce by dividing, and some transform into the cystic form. Eventually, the dog passes cysts in its stool.

Giardia can be transmitted directly via cysts shed by an infected animal. More commonly, there is indirect transmission via contaminated water. 

When Giardia cysts are found in the stool of a healthy adult dog without diarrhea, they are generally considered a transient, insignificant finding. However, in puppies and sick or weak adult dogs, they may cause severe, watery diarrhea that may be fatal if left untreated. The likelihood of developing disease increases when large numbers of cysts are present in the environment from fecal contamination. Giardiasis is a common occurrence in environments that are densely populated, such as kennels, pet stores, or animal shelters. 
 

What are the clinical signs of giardiasis?

The most common clinical sign of giardiasis is sudden-onset foul-smelling diarrhea. The stool may range from soft to watery, often has a greenish tinge to it, occasionally contains blood, and tends to contain excess mucus. Vomiting may occur in some cases. Signs may persist for several weeks and the dog may lose weight and condition.

Diarrhea may be intermittent. Most dogs do not have a fever, but they may be less active. Giardiasis is not usually life threatening unless the dog is very young, very old, or has a compromised immune system.

How is Giardia diagnosed?

A routine flotation test of the stool may fail to detect these tiny cysts, which are shed inconsistently in the feces, and which often require a special zinc sulfate flotation solution for detection. Occasionally, the parasites may be seen on a direct smear of the feces. If your veterinarian suspects giardiasis, a sample of stool will be analyzed for the presence of Giardia-specific antigens (cell proteins). Approximately 70% of Giardia infections will be diagnosed if three stool samples, collected every 2-3 days, are evaluated.  Many cases are presumptively diagnosed on the basis of medical history and clinical signs suggestive of giardiasis.

How is giardiasis treated?

Giardiasis is treated using a combination of medication, plus cleaning and disinfection the dog’s environment, plus bathing the dog to prevent reinfection. The most common drug used to treat Giardia is the antibiotic metronidazole. It is generally given for five to seven days. Another antiparasitic drug, fenbendazole, is suggested as a potentially useful treatment, especially when used in conjunction with metronidazole. Supportive treatment with other drugs may be needed as supplemental therapy if dehydration or severe diarrhea is present. Some dogs may require follow-up tests and treatments based on their condition and the severity of infection.

What is the prognosis for Giardiasis?

The prognosis is good in most cases. Weakened and senior dogs, and those with incompetent immune systems are at increased risk for complications, including death.

Can my dog give a Giardia infection to me or my family?

Giardia can cause diarrhea in humans and can potentially be passed from dogs to humans, but the importance of dogs as a reservoir of human infections appears to be minimal.

Contaminated municipal water supplies are responsible for many outbreaks, and human-to-human transmission is also important.

Regardless of the risk to humans, if your dog is diagnosed with Giardia, environmental disinfection and good personal hygiene are important to prevent accidental spread to humans as well as other animals in the household. In particular, people with compromised immune systems should use extreme care, especially when handling feces or after administering medications.

For environmental disinfection, chlorine bleach at 1:32 or 1:16 dilutions, or 1-2 cups in a gallon of water (60-120 mLs/L) appears to be effective. Quaternary ammonium compounds are also reported to be effective in killing the cysts. Giardia cysts are susceptible to drying so try to keep your dog’s environment as dry as possible. For best results, thoroughly clean the pet's living and sleeping areas and then allow the areas to dry out for several days before reintroducing pets.

Can giardiasis be prevented?

While there is a Giardia vaccine, the results of efficacy studies are mixed.  In some cases, however, the vaccine may be a useful adjunct to medical therapy.