Drug therapy
While dietary manipulation is often used, most veterinary clinicians also prescribe drugs. In the last several years, several
studies have been published concerning the treatment of feline giardiasis; the following are the protocols recommended by
our laboratory.
In several studies, metronidazole effectively controlled diarrhea and eliminated Giardia species cyst shedding in naturally and experimentally infected cats.40,60-62
Metronidazole may also help correct secondary bacterial overgrowth. Thus, if fecal cytology is consistent with concurrent
bacterial disease, we generally prescribe metronidazole at the maximal dosage of 25 mg/kg given every 12 hours for seven days.62 If the owner can afford the formulation fee, we use metronidazole benzoate in tuna flavor.62 The USP formulation is routinely available in the United States but induces salivation and inappetence when administered
in some cats.63 The protozoal toxicity of metronidazole is from short-lived intermediates or free radicals that produce damage by interacting
with DNA and possibly other molecules.64
Gastrointestinal or central nervous system toxicosis has been associated with metronidazole administration in some kittens
at either an excessive dose or from a probable cumulative neurotoxic effect.65
Benzimidazoles may have an anti-Giardia species effect by interacting with the colchicine site in tubulin in the microtubules, resulting in the disruption of assembly
and disassembly.66 Selective toxicity is achieved because the drug is minimally absorbed from the host intestine.66 If the clinical history and laboratory findings are most consistent with concurrent Giardia species and nematode infections, we generally prescribe fenbendazole at a dosage of 50 mg/kg given orally every 24 hours
for at least five days. Fenbendazole was shown to be safe when administered to healthy, adult, nonpregnant cats at a dose
five times higher than the approved dose in wild felids and dogs.67 The suggested dosage for fenbendazole for the treatment of giardiasis is 50 mg/kg given orally every 24 hours for three
to five days.68 However, when fenbendazole was administered in cats concurrently infected with Giardia species and C. parvum, only four of eight cats stopped shedding Giardia species cysts.37 Albendazole has been used successfully to treat giardiasis in dog studies but can cause bone marrow suppression in cats
and dogs. Thus, we do not currently prescribe it to cats or dogs.69
Febantel is a benzimidazole found in a combination product containing febantel, pyrantel, and praziquantel (Drontal Plus—Bayer
Animal Health). When we administered the product to adult cats empirically at a dosage of two small dog tablets per cat (about
50 mg/kg febantel) orally for five days, decreases in cyst shedding by experimentally infected cats were noted.70 Additionally, four of the six treated cats had no evidence of cysts even after the administration of glucocorticoids in
an attempt to induce immunosuppression. Febantel is, in part, metabolized to fenbendazole, which likely explains its benefit
for giardiasis.71
Whether this drug is superior to fenbendazole is unknown.
We have prescribed paromomycin or nitazoxanide in some cats with naturally occurring, resistant giardiasis by using the protocols
described for cryptosporidiosis. However, none of the data are controlled, so whether these drugs are reasonable first-choice
drugs is debatable.
Vaccination
The Giardia species vaccine used as a preventive in dogs was apparently successful in eliminating cyst shedding and diarrhea in a group
of naturally infected dogs.72 However, when we infected 16 cats with Giardia species and then administered three doses of a commercially available feline Giardia species vaccine (Fel-O-Vax Giardia—Fort Dodge Animal Health) to eight kittens, we could not detect a difference in cyst shedding between vaccinates and controls,
suggesting that the vaccine was an ineffective therapy.73
However, only one Giardia isolate was used, so whether the vaccine is an effective immunotherapy in naturally infected cats is unknown.
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