Giardia are tiny, one-celled protozoal parasites found in the gastrointestinal tract of most domestic and wild mammals, including people. They also infect domestic and wild birds. Giardiasis, sometimes called "beaver fever", is the disease caused by an infection from these parasites. Giardia are found throughout the United States and in many other parts of the world. Infection with Giardia is common, but disease from them is not. Some dogs carry Giardia in their gastrointestinal tracts without showing any clinical signs of discomfort. This is called a "latent" or subclinical infection. However, very young puppies, and dogs with compromised immune systems, can have severe reactions to these parasites, including loss of appetite, malnutrition, diarrhea, weight loss, tiredness, weakness, bloody or pale-colored, greasy, strong-smelling stools and even death. Giardia also pose a risk to human health, especially in immunocompromised people such as the elderly, newborns, pregnant women and people suffering from cancer, AIDS or other serious diseases.
Causes of Giardia Infection
Giardia are found world-wide. They have what is known as a direct life cycle. Dogs become infected when they ingest the cyst (interchangeably called the "oocyst") form of this parasite by drinking contaminated water, eating contaminated food, licking contaminated fur or otherwise coming into contact with contaminated feces in the environment. The cysts lodge in the upper part of the dog's small intestine, called the duodenum. There, each cyst produces several motile larvae, known as trophozoites. Giardia trophozoites reproduce asexually inside the dog's small intestine through an interesting process called "binary fission." Basically, mature trophozoites replicate by splitting into two separate parts, which then each become individual Giardia organisms.
These parasites irritate and damage the dog's intestinal lining, which disrupts digestion, reduces the absorptive surface area, causes abdominal pain and uses up nutrients that are essential to the dog's health. Eventually, before they leave the dog's intestinal tract, the trophozoites encyst (transform into cysts), and are shed intermittently in that form in the infected dog's feces. They rarely are shed on a consistent basis in the stool, which tends to make checking for them in fecal samples a bit of a hit-or-miss exercise. In any event, the cysts are very hardy and can survive for weeks to months free in the environment, especially in cool, moist climates. Oocysts are immediately infective to animals that ingest them. Once that happens, the cycle of Giardia infection begins again.
Prevention of Giardia
Since Giardia are transmitted through direct contact with the cyst form of the parasite in contaminated water, food or feces, the best way to prevent infection is to remove dogs from situations where they can come into contact with contaminated substances. Areas where dogs or other domestic animals defecate should be thoroughly and routinely disinfected, and preferably avoided by dogs that don't normally frequent those areas. Giardia cysts can be inactivated by steam, boiling water and most ammonia-based disinfectant solutions. Regular veterinary check-ups, including fecal examinations, can usually identify Giardia infections. These examinations are especially important at the end of summer, after the hot season.
A Giardia vaccine has been developed for dogs and cats and is available in North America. Unfortunately, according to most reports, this vaccine has not been particularly helpful in preventing or treating Giardia-related disease. It may help to reduce fecal shedding of Giardia cysts, but it has apparently been largely ineffective in preventing or treating actual infection by the parasites.
Giardia are the most common intestinal parasites of people in North America. Whether Giardia can be transmitted from domestic dogs to people, or vice versa, is controversial. However, the organism does not appear to be particularly host-specific. Unless and until medical science proves otherwise, it probably is prudent to assume that an infected dog might pass Giardia to people in the household, and that people may pass the parasites to their pets. Like dogs, people can get Giardia infection directly from contaminated water supplies, usually due to less-than-optimal hygienic conditions (including human sewage effluents). In rural settings, beavers have been blamed for contaminating water supplies with infected feces, leading to the occasional nick-name for this disease, "beaver fever." However, this association is not a significant contribution to Giardiasis among companion animals today.
How Giardia Affect Dogs
Many dogs infected with Giardia have a latent or subclinical infection, which means that they really don't show any noticeable symptoms of illness. However, the parasites eventually damage the lining of the intestines of the animals that they infect, disrupting digestion and using up nutrients that the animals need for normal health. The effects of Giardia are most pronounced in young dogs, old dogs and those with impaired immune systems. The profuse diarrhea that frequently accompanies giardiasis usually also causes severe dehydration. Giardiasis can be fatal, although that is quite uncommon in North America.
Symptoms of Giardia
Giardia can cause a number of gastrointestinal symptoms in domestic dogs. Owners may notice one or more of the following:
Loss of appetite (inappetance; anorexia)
Bloody stools (hematochezia)
Greasy stools (steatorrhea)
Diarrhea (can be acute, chronic or intermittent; often poorly formed or loose stools; often profuse and watery)
Abnormally strong-smelling (malodorous) stools
Abdominal pain (cramping; nausea)
Dehydration (can be severe)
Fever (occasional; not common)
Vomiting (occasional; not common)
Dogs at Increased Risk
Dogs of any age, breed or gender can become infected with Giardia. Young animals, and those confined in close quarters with a number of other animals, seem to be more susceptible to infection, as are dogs whose immune systems are stressed or otherwise compromised.
How Giardia are Diagnosed
The disease caused by infection with Giardia is not difficult for veterinarians to diagnose. Examination of a dog's feces can reveal the trophozoite and/or cyst forms of the organism. Giardia can also be identified through more advanced, specialized tests. Most veterinarians evaluating a dog with diarrhea, abdominal pain and other signs of gastrointestinal upset will take a blood sample and perform a complete blood count and a serum biochemistry panel. They will also probably take a urine sample for a urinalysis. The results of these tests typically will be inconclusive if the cause of the dog's discomfort is giardiasis, without some other contributing condition. However, they are useful diagnostic tools to evaluate the dog's overall health.
The initial database also will typically include an examination of the dog's feces. There are several different techniques that can be used to identify parasites in feces. Fecal suspension involves mixing a small amount of fresh feces with saline, and looking at drops of the mixture on a glass side covered with a thin glass coverslip. The goal here is to try to identify the trophozoite form of the parasite. Fecal floatation is a bit more involved, but not too much so. It involves mixing a fresh stool sample with zinc sulphate or another special substance, spinning it down in a centrifuge so that the cyst forms to float to the surface of a test tube and then looking at the top part of the sample under a microscope. This is usually done several times over a series of days. Diagnosis can also be made by finding the parasite or its cysts by examining smears of fresh stool under a microscope. Fecal smears are simple to make from a rectal swab.
Because Giardia cysts are only shed intermittently and not constantly, a Giardia-free fecal examination does not necessarily rule out giardiasis. Two or three negative fecal tests, done at least several days apart, usually are conducted before a Giardia infection is ruled out. Of course, a positive fecal examination is usually diagnostic of the parasite, depending upon the skill of the person looking at the sample.
A test kit is available for in-clinic screening for Giardia. Advanced testing can be done by either direct immunofluorescent assay (IFA) or enzyme-linked immunosorbent assay (ELISA), both of which are done on fecal samples. Fecal polymerase chain reaction (PCR) testing is also available, but is not normally used for routine diagnosis. Veterinarians can also take a physical sample of the upper small intestinal lining via endoscopy. This involves using a wand-like instrument with a camera on the end to go into the small intestine, visualize its lining and snip and retract a small piece for submission to a diagnostic laboratory. However, this procedure is usually unnecessary to diagnose Giardia; it is a bit of an overkill in terms of time, invasiveness and expense.
Owners should consult with their veterinarian about diagnosing and treating their dogs for any infection with this or any other gastrointestinal parasites.
Any of these diagnostic methods can produce false positives (a positive test result in a dog that is free from infection) or false negatives (a negative test result in a dog that is infected with Giardia). False negatives are far more common in dogs with giardiasis than are false positive test results.
Giardia is treatable, usually on an outpatient basis. Unfortunately, the effectiveness of treatment is evaluated based on the presence or absence of cysts in the affected dog's feces, which does not necessarily equate with whether the parasites are present inside of the dog. A dog can remain internally infected, without shedding the cyst form of the organism at the time the fecal sample is taken. In addition, infection can recur after the parasite is initially eliminated from a dog's gastrointestinal tract. In other words, the first infection does not provide any immunity or protection against later infections.
The goals of treating giardiasis are to eliminate the shedding of infective cysts, eliminate the symptoms of the illness (diarrhea, abdominal pain, weight loss, etc.), and return the dog to a comfortable, pain-free quality of life.
The antibiotic drug metronidazole (Flagyl), and the anti-parasitic drug fenbendazole, are among the most frequently-used treatments for giardiasis in dogs. Other de-worming medications are available as well, includeing combinations of febantel, praziquantel and pyrantel pamoate, among others. While these drugs treat the parasitic infection, they also can cause of harmful side effects, such as liver damage. Many veterinarians will not prescribe metronidazole or fenbendazole unless the dog is showing moderate to severe symptoms of giardiasis. Metronidazole can cause physical malformations in developing embryos, so it should not be given to pregnant bitches. The same is true for a related medication, albendazole. Metronidazole also reportedly has a very bitter taste, making it difficult for some owners to administer. Some of these drugs reportedly also can cause acute anorexia, depression and vomiting.
Even with treatment, it is possible that only the cystic form of Giardia has been removed from the feces, while the infective trophozoite form in the dog's small intestine remains. In other words, fecal tests for Giardia can be negative, but the parasites still can live inside the dog's gastrointestinal tract, making those dogs a source of potential infection for other animals, and possibly for people. Giardia is rarely deadly in otherwise healthy dogs. Dogs infected with this parasite typically have flu-like symptoms that eventually resolve.