Giardia are protozoa (one-celled organisms) that live in the small intestine of dogs and cats. Giardia are found throughout the United States and in many other parts of the world. Infection with Giardia is called 'giardiasis.'
There are many things we do not know about this parasite. Experts do not agree on how many species of Giardia there are and which ones affect which animals. Veterinarians do not even agree on how common Giardia infections are and when they should be treated. Generally, it is believed that infection with Giardia is common but disease is rare. There is much about the life cycle we do not know either.
A dog becomes infected by eating the cyst form of the parasite. In the small intestine, the cyst opens and releases an active form called a trophozoite. These have flagella, hair-like structures that whip back and forth allowing them to move around. They attach to the intestinal wall and reproduce by dividing in two. After an unknown number of divisions, this form develops a wall around itself (encysts) and is passed in the feces. The Giardia in the feces can contaminate the environment and water and infect other animals and people.
Signs of Giardia
Most infections with Giardia are asymptomatic. In the rare cases in which disease occurs, younger animals are usually affected, and the usual sign is diarrhea. The diarrhea may be acute, intermittent, or chronic. Usually the infected animals will not lose their appetite, but they may lose weight. The feces are often abnormal, being pale, having a bad odor, and appearing greasy. In the intestine, Giardia prevents proper absorption of nutrients, damages the delicate intestinal lining, and interferes with digestion.
Giardiasis is very difficult to diagnose because the protozoa are so small and are not passed with every stool. Tests on serial stool samples (one stool sample every day for three days) are often required to find the organism. Special diagnostic procedures, beyond a routine fecal examination, are necessary to identify Giardia. The procedures we use to identify roundworms and hookworms kill the active form of Giardia and concentrate the cyst form.
To see the active form, a small amount of stool may be mixed with water on a microscope slide and examined under high magnification. Because these forms have flagella, you can see them move around on the slide. The active forms are more commonly found in loose stools. If you ever have the opportunity to see the active form of Giardia under the microscope, take it! It is an interesting-looking creature. It is pear-shaped and its anatomy makes it look like a cartoon face, with eyes (which often look crossed), nose, and mouth. Once you see it, you will not forget it.
Cysts are more commonly found in firm stools. Special solutions are used to separate the cysts from the rest of the stool. The portion of the solution that would contain the cysts is then examined microscopically.
In spring, 2004, a diagnostic test using ELISA technology became available. This test uses a very small fecal sample, and can be performed in 8 minutes in a veterinarian's office. It is much more accurate than a fecal examination.
Now we come to how to interpret the test results. It can be a dilemma for your veterinarian. What you see (or do not see) is not always a correct indication of what you have. A negative test may mean the animal is not infected. However, few, if any, laboratory tests are 100% accurate. Negative test results can also occur in some infected animals. If a negative test occurs, your veterinarian will often suggest repeating the test.
What about a positive test? That should not be hard to interpret, right? Wrong. Giardia can be found in many dogs with and without diarrhea. If we find Giardia, is it the cause of the diarrhea or is it just coincidence we found it? The animal could actually have diarrhea caused by a bacterial infection, and we just happened to find the Giardia. Test results always need to be interpreted in light of the signs, symptoms, and medical history.
Fenbendazole is an antiparasitic drug that kills some intestinal worms and can help control giardia. It may be used alone or with metronidazole. Metronidazole can kill some types of bacteria that could cause diarrhea. So if the diarrhea was caused by bacteria, and not Giardia, the bacteria can be killed and the symptoms eliminated. Unfortunately, metronidazole has some drawbacks. It has been found to be only 60-70% effective in eliminating Giardia from infected dogs, and probably is not 100% effective in cats, either. It can be toxic to the liver in some animals. It is suspected of being a teratogen (an agent that causes physical defects in the developing embryo), so it should not be used in pregnant animals. Finally, it has a very bitter taste and many animals resent taking it €“ especially cats.
Quinacrine hydrochloride has been used in the past, but is not very effective and can cause side effects such as lethargy, vomiting, anorexia, and fever.
But now we come to yet another unknown. It is possible these treatments only remove the cysts from the feces but do not kill all the Giardia in the intestine. This means even though the fecal exams after treatment may be negative, the organism is still present in the intestine. This is especially true of the older treatments. So treated animals could still be a source of infection for others.
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