Giardia Infestation in Dogs and Cats
by Chick Newman, Phd, DVM
The Giardia intestinalis trophozoite (vegetative form) is a flagellated (has thin appendages that move, allowing the organism to "swim") protozoal (a class of single-celled organisms) parasite that can infest the intestine of numerous host animals, including humans. Therefore, giardia infestation should be considered zoonotic, with potential to infect human handlers of fecal waste or water contaminated with fecal waste from companion and wild animal hosts! The most common clinical problem associated with giardiasis is diarrhea,
- The life cycle of giardia is illustrated in a separate figure.
- Giardia cysts (7-13 µm) are shed in feces of animals and can live in a moist, cool environment for weeks to months. They are suseptible to drying, and to most commercial disinfectant agents, however
- Viable cysts contaminant in the ground or water are ingested by host animal (including humans)
- The cysts release the motile (flagellated) trophozoites (12-17 µm X 7-10 µm) in the "victim's" small intestine
- these attach to the brush border of the small intestine, suck nutrients, and cause considerable inflammation and damage
- Trophozoites encyst in the ileum (distal small intestine) between 5 and 16 days after infestation and the cysts are then shed in the feces. So...it takes that long after exposure (5-16 days...the "prepatent period") to actually diagnose infestation via examination of feces for giardia cysts. (See below for more about diagnosis)
The most common clinical sign associated with giardiasis in dogs and cats is diarrhea...usually of the small intestine type (episodes of voluminous, greasy, pasty or watery diarrhea). However, many dogs and cats have no symptoms or are intermittently symptomatic. Younger animals are more likely to be symptomatic. Sometimes, giardiases is associated with vomiting with or without diarrhea. On occasion, signs of large bowel diarrhea (frequent urges to defecate, small volumes and mucus or blood in loose feces) may occur.
Direct examination of freshly obtained feces mixed with a little saline solution may reveal the highly motile trophozoites. These are difficult to see in most situations and multiple samples of fresh stool on successive days may be required to make the diagnosis this way. Trichomonas is also a motile protozoan and must be differentiated from giardia trophozoites.
Fecal flotation technique for visualizing giardia cysts is probably the most efficaceous method of diagnosing giardiasis. The recommended flotation procedure involves using a zinc sulfate (ZnSO4) solution prepared to a specific gravity of 1.180. Mixing a sample of feces, then centrifuging the mix for a short time causes the cysts to float to the top of the centrifuge tube (separated by density ...the cysts are less dense than the solution). The sensitivity of this flotation method for detecting giardia cysts is markedly reduced if the centrifugation step is ignored. Unfortunately, centrifugation is not commonly included in the fecal-flotation protocol at many veterinary hospitals. Giardiasis is probably underdiagnosed for this reason. It is recommended that a fresh sample be obtained and analyzed every two days examining at least three samples, if the initial sample is negative. This is because the pre-patent period (time after infection but before shedding) is long and the shedding of cysts may be sporadic especially when the clinical signs wax and wane. Best samples for detection are ones with mucus.
This is a fairly simple way to measure for the presence of giardia trophozoite antigens (proteins) in feces using the ELISA immune detection system. The sensitivity of the commercial test system is unclear; anywhere from 1% to 14% of fecal flotation positive samples were not detected by the ELISA test. On the other hand, the ELISA test was positive in about 10%-15% of fecal flotation- negative samples (about half of these, on repeated samplings, turned out to be fecal flotation-positive). The specificity is apparently high but not yet published; so if this is true, a positive result is likely due to giardiasis. However, the sensitivity is not high and a negative result does not eliminate the possibility of that diagnosis.
Previously invasive tests (e.g. duodenal aspiration) are no longer considered efficacious.
A dose of 22mg/kg of body weight (10mg/kg in cats) twice daily for 5 days was effective in only about two thirds of animals treated for giardiasis. There is, apparently, a growing resistance of giardia to metronidazole.
A dose of 50mg/kg once daily for three days is very effective against giardia. The drug is also non-toxic (safe in pregnant animals too) and efficacious against several other parasites, such as roundworms, hookworms, whipworms, some tapeworms.
A dose of 25mg/kg twice daily for two days is effective in eliminating giardia. However, there are reports of bone marrow depression from use of this drug, so it is not recommended at this time.
A dose of 4mg/kg twice daily for seven days is effective. This medication comes as a suspension (Furoxone®) and is thus convenient to give to small dogs and cats.
The dose of 6.6 mg/kg given twice daily is effective in dogs, but does not eliminate infection in cats. There are minor side effects...mild fever, anorexia and lethargy.
This is part of the deworming product, Drontal Plus® (Bayer Animal Health, Merriam, KS). It is, apparently, converted to fenbendazole after administration, and presumably the giardiacidal activity is related to this conversion. However actual mechanism of action is only suggested, not proven. Various off-label doses have been advocated, from 26.8 to 35.2 mg/kg febantyl daily for 3 to 5 days in dogs to "twice the label dose" in dogs and cats. Drontal Plus also contains praziquantal, and because at high doses this drug can be toxic to cats, the calculated dose must take also, the dose of praziquantal administered into consideration in this species.
The Fort Dodge company offers a vaccine (GiardiaVax®) that is an apparently safe and effective preventative. In research studies, vaccinated dogs challenged with giardia one year after immunization did not develop diarrhea, had few or no cysts and no detectable trophozoites were found in the intestinal tracts.
Giardiasis is a potential risk to humans, through direct contact with their pet or feces. However simple hygiene offers adequate protection. Though cysts can persist in the environment for weeks or months, they are easily killed with most household disinfectants in warm or hot water and by drying. Some general rules: if the pet's coat is even slightly soiled with feces, shampoo immediately. Wash hands frequently after contact with infected pets. Dispose of any feces promptly and safely and disinfect soiled areas of the premesis often.
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