Giardia is the genus of a protozoan parasite that is infectious to both humans and pets all over the world. Giardia consists of flagellates, which mean they move by means of several whip-like structures called “flagella.” They live as a form called a “trophozoite,” or “troph” for short, in the intestine where it causes diarrhea. In fresh fecal samples, trophozoites can sometimes be captured. They swim around in a jerky fashion characteristic of flagellates and appear as a funny face (the two nuclei form the eyes and median bodies form the mouth).

After a short period of time outside the host’s intestine, the trophozoites round up and form cysts that enable them to survive environmental conditions without a host to protect them. The cyst can be dried out to decontaminate the environment, but if it is cold and wet the cyst can live for many months with two incompletely formed trophozoites inside, ready to infect a new host. Contaminated water is the classical source of a Giardia infection.

After having been swallowed, the cyst shell is digested away, freeing the two trophozoites who go and attach to the intestinal lining. The troph has a structure called a “ventral disc,” which is sort of like a suction cup, and this is used to stay attached to the intestine. If the troph wants to move to another spot, it lifts itself up and swims to a new spot via its flagella (trophs tend to live in different intestinal areas in different host species, depending on the host’s diet). If the host has diarrhea, trophs are shed in the diarrhea, but Giardia may also form cysts within the host in preparation to be shed. Either form can be found in fresh stool.

After infection, it takes 5 to 12 days in dogs or 5 to 16 days in cats for Giardia to be found in the host’s stool. Diarrhea can precede the shedding of the Giardia. Infection is more common in kennel situations where animals are housed in groups.

How Does Giardia Cause Diarrhea?

No one is completely sure but infection seems to cause problems with normal intestinal absorption of vitamins and other nutrients. Diarrhea is generally not bloody. Immune suppressive medications such as corticosteroids can re-activate an old Giardia infection.

Diagnosis Recommended by Our Veterinarians

In the past, diagnosis was difficult. The stool sample being examined needed to be fresh, plus Giardia rarely show up on the usual fecal flotation testing methods used to detect other parasites. Traditionally, a fecal sample is mixed in a salt or sugar solution such that any parasite eggs present will float to the top within 10 to15 minutes.

What has made Giardia testing infinitely easier is the development of a commercial ELISA test kit (similar in format to home pregnancy test kits). A fecal sample is tested immunologically for Giardia proteins. This method has dramatically improved the ability to detect Giardia infections and the test can be completed in just a few minutes while the owner waits.

Giardia shed organisms intermittently and may be difficult to detect. Sometimes pets must be retested in order to find an infection.

Treatment Available for Pets of Alta Loma, Rancho Cucamonga, Upland, Ontario, Claremont, Fontana, and the Inland Empire

A broad spectrum dewormer called fenbendazole (Panacur®) seems to be the most reliable treatment at this time. Metronidazole (Flagyl®) in relatively high doses has been a classical treatment for Giardia but studies show it to only be effective in 67% of cases. The high doses required to treat Giardia also may result in temporary neurologic side effects or upset stomach. Often both medications are used concurrently to enhance efficacy and allow for lower dosages of metronidazole. The ELISA test for Giardia should go negative within 2 weeks of treatment indicating success.

Because cysts can stick to the fur of the infected patient and be a source for re-infection, the positive animal should receive a bath at least every other day to help reduce the chance of reinfection.

Not all patients with Giardia actually have diarrhea but because Giardia is the most common intestinal parasite affecting humans in North America, treatment is generally recommended for the pet testing positive even if no symptoms are being shown.

Treatment generally lasts 1 month and involves two bouts of five day courses of fenbendazole separated by 2 weeks, and a single 8 day treatment with metronidazole.

Pets should be retested by the elisa test two weeks after completion of the last dose of medication.

Available Vaccine?

A Giardia vaccine made by Fort Dodge Animal Health is on the market but it is not intended to prevent infection in the vaccinated animal. Instead the vaccine is licensed as an adjunct to treatment and is used to reduce the shedding of cysts by the vaccinated patient. This would be helpful in a kennel situation that is trying to reduce environmental contamination during an outbreak or where an animal keeps getting reinfected, but it is not helpful to the average dog whose owner wants to simply prevent infection.

The 2006 American Animal Hospital Association Guidelines list this vaccine as “not recommended.”

Environmental Decontamination

The most readily available effective disinfectant is probably bleach diluted 1:32 in water, which in one study required less than one minute of contact to kill Giardia cysts. Organic matter such as dirt or stool is protective to the cyst, so on a concrete surface basic cleaning should be effected prior to disinfection. Animals should be thoroughly bathed before being reintroduced into a “clean” area. A properly chlorinated swimming pool should not be able to become contaminated. As for areas with lawn or plants, decontamination will not be possible without killing the plants and allowing the area to dry out in direct sunlight.

Transmission to Humans:

Giardia can infect humans, cats and dogs (among other animals). The cysts present in the pet’s feces or on their coat, could potentially infect a person if that person were to put their hands in their mouths after coming in contact with the cysts. Small children, who have a tendency to put their hands in their mouths often are at greater risk of infection. In addition, people with compromised immune systems are more likely to get infected as well (HIV positive, people on immune suppressive drugs).

There are 6 different subtypes of Giardia in the US. Recent studies have shown that the Giardia which infect dogs and cats is not very likely to infect people, however, if you have an infected pet, you should be sure to wash your hands after handling the pet until the treatment is completed and the testing shows there are no more cysts.