Giardia in Dogs: Symptoms and Treatments

By: Laura McLain Madsen, DVM

What is Giardia?

Giardia is dogs is caused by a protozoan (one-celled organism) parasite that can infect various species, including humans. One study found Giardia in 15% of dogs with vomiting and diarrhea in the USA. The prevalence varied by state, ranging from less than 5% in Wyoming to over 30% in New Hampshire. The protozoan can only be seen under a microscope; it is described as looking like a “monkey face,” with flagellae (tail-like appendages) for swimming.

What are the symptoms?

Symptoms of Giardia infection in dogs range from none at all (asymptomatic) to vomiting, diarrhea, loss of appetite, and flatulence (excessive farting). In some cases, Giardia may only be a secondary problem for your dog's health: the symptoms of vomiting or diarrhea might be caused by inflammatory bowel disease or food allergies, with the inflammation from the primary disease allowing Giardia to flourish.

What is the life cycle of Giardia in dogs?

Giardia is shed in the feces of an infected dog. Soon after the organism is passed in the bowel movement, it changes from the active (trophozoite) form to a cyst form. The cyst form is sort of like hibernation: it doesn’t swim and is resistant to environmental factors and chemicals. The inactive cyst form of Giardia can survive for months in the environment, particularly cold or wet areas. If the microscopic cyst is ingested by a second dog (such as by eating feces or drinking contaminated pond water) the cyst will release two new active trophozoite forms. The new trophozoites attach to the intestinal lining and cause damage to the tissue, resulting in diarrhea. The second dog will start shedding Giardia in its feces 5-12 days after infection, perpetuating the life cycle.

Is Giardia contagious to humans?

Giarida in DogsA disease is said to be zoonotic if it can be transmitted from animals to humans. Giardia in dogs may or may not be zoonotic. There are different strains (called assemblages) of Giardia and some affect humans while others do not. Assemblage A infects humans, dogs and cats; assemblage B infects humans and dogs; and assemblages C and D infect only dogs and cats. So depending on which assemblage is involved in a dog’s infection, the organism may or may not be contagious to humans. Testing for the exact assemblage is not commonly done, but fortunately studies have shown that most dogs are infected with a dog-exclusive strain. However, it’s best to be safe, assume that there is some risk, and take steps to prevent spread.

Giardia would be unlikely to infect most adult humans because of their healthy immune systems and good hygiene practices. Risk of being infected with Giardia is higher in small children (because of poor hand washing) and immunocompromised people (like elderly, pregnant, HIV/AIDS, organ transplant recipients, and chemotherapy patients). Everyone in the family should wash their hands after playing with the dog or picking up feces.

Which dogs are at risk?

Giardia infection is common in any situation in which many dogs are concentrated in one area: shelters, breeding kennels, boarding facilities, “doggy day care,” and dog parks. Puppies are more susceptible to infection, as are adult dogs with immune system compromise (like elderly, pregnant, on immunosuppressive medication, or diabetic). It’s estimated that 30-50% of puppies are infected with Giardia. Some dogs may be carriers, meaning they naturally have low numbers of Giardia in their intestines but show no symptoms.

How is Giardia infection diagnosed?

Several tests can diagnose Giardia in dogs but they vary in their effectiveness. A fecal float (a test that involves mixing a stool sample with a liquid, allowing any microscopic parasites to float to the top) occasionally will show Giardia. If Giardia organisms are seen it allows a definitive diagnosis, but not seeing the organisms doesn’t rule out an infection, because the organisms may not be passed in every bowel movement. The reliability of the fecal float can be increased by using a special solution (zinc sulfate) and centrifuging the test tube to enhance floatation of the Giardia organisms. The reliability can also be improved by sampling three different fecal samples from the same dog. More accurate still is an ELISA test, which detects a protein from the Giardia organism. This test is available as an in-house test for veterinarians (IDEXX SNAP® test) or as a test at a referral lab. The ELISA test can remain positive for weeks to months after successful treatment of Giardia infection.

How is Giardia in dogs treated?

A variety of medications have been used to treat Giardia infection in dogs. Unfortunately, no medication is 100% effective, and repeated courses may be necessary. Sometimes the drugs are used in combination to increase their effectiveness. Generally in a multiple-pet household, all the pets should be treated simultaneously. Drugs which may be used to treat a Giardia infection include:

• Fenbendazole (Panacur®), an anti-parasite medication given as a powder or liquid by mouth for 3-7 days. Fenbendazole is very safe; rarely it causes vomiting as a side effect.

• Metronidazole (Flagyl®), an antibiotic tablet given for 7-10 days. Possible side effects of metronidazole include neurologic symptoms (wobbling, head tilt, disorientation, tremors), lethargy, vomiting and diarrhea. It should not be given to pregnant bitches.

• Febantel (Drontal Plus®, a combination of febantel, praziquantel and pyrantel), an anti-parasite tablet given for 3 days. Vomiting and diarrhea are rare side effects. It should not be given to pregnant bitches.

• Azithromycin (Zithromax®), an antibiotic tablet given for 5 days. Vomiting is a rare side effect. It should not be given to pregnant bitches.

• Albendazole (Valbazen®) is a cattle dewormer occasionally used in dogs to treat Giardia. However, it can cause serious bone marrow toxicity so should only be used with great caution.

In rare cases when the Giardia infection is resistant to these common drugs, there are two other medications which can be tried:

• Nitazoxanide (Alinia®), an anti-protozoal drug used to treat Giardia in humans. Studies of nitazoxamide use in dogs have not been done. It should not be given to pregnant bitches.

• Tinidazole (Tindamax®), an anti-protozoal drug used to treat Giardia in humans. It is similar to metronidazole, and can cause neurologic side effects as well as vomiting and diarrhea. It can interact with many other drugs so be sure to tell your veterinarian of any other medications or supplements you give your dog. It should not be given to pregnant bitches.

Can dogs be infected again after treatment?

Re-infection is common and may appear to be a treatment failure. Dogs can develop symptoms again within two weeks of successful treatment—not because the treatment failed but because the dog became infected a second time.

To prevent re-infection, very careful hygiene is important. Remove feces from the yard promptly (wear gloves and wash hands to protect yourself). Bathe the dog every day during treatment with a cleansing dog shampoo to remove any cysts which might be stuck to the fur. Use baby wipes on the dog’s rear end after every bowel movement to remove cysts. Unfortunately, there is no good way to kill Giardia cysts in the grass, other than time and drying out. Make sure the yard has good drainage. Hard surfaces (tile or concrete) should be scrubbed with a detergent and allowed to thoroughly dry.

How can I prevent my dog from getting Giardia?

A vaccine (immunization) against Giardia used to be available but has been taken off the market. Prevent your dog from eating other dogs’ feces. For dogs at risk of Giardia (such as dogs who frequently swim in ponds at dog parks), increasing the fiber content in the diet may help the intestine fight off infection. Probiotics (“good” intestinal bacteria) may also help deter Giardia infection; your veterinarian can recommend a brand.

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