What Are the Best Drugs for Giardia spp. Infection?

Giardia spp. have specific antimicrobial sensitivity patterns like bacteria and so it is currently impossible to predict which anti-Giardia drug will be effective. Because G. canis and G. felis can be difficult to cultivate, there is little in vitro susceptibility test result information available. While there have been multiple drugs used for the treatment of giardiasis in dogs and cats, there are few studies that utilized dose titrations and evaluation of drugs in experimentally infected animals. In most studies, fecal samples were only assessed for short periods of time after treatment and immune suppression was not induced to evaluate whether infection was eliminated or merely suppressed. Infection with Giardia does not appear to cause permanent immunity and so reinfection can occur, a finding that also hampers assessment of treatment studies. Treatment options currently available or used historically include metronidazole, tinidazole, ipronidazole, ronidazole, fenbendazole, albendazole, pyrantel/praziquantel/febantel, quinacrine, and furazolidone (Table 1). Newer drugs being studied include paromomycin and nitazoxanide.

If spore-forming rods, morphologically consistent with Clostridium perfringens are concurrently detected with Giardia, use of metronidazole is indicated as this drug is an antibiotic. If there is clinical evidence to suggest concurrent infection with a nematode, fenbendazole or febantel are indicated. Many clinicians currently utilize fenbendazole once daily for 5 days as initial therapy. Some clinicians currently recommend the combination of metronidazole and fenbendazole. Others only resort to combination therapy if there is evidence of a persistent infection not cleared by monotherapy.

What Are the Goals of Giardia spp. Treatment?

The primary goal of treatment is to stop diarrhea. Because healthy pets are not considered human health risks, elimination of infection (which is difficult) is a secondary goal. Giardia spp. can have resistant patterns and so if the first drug fails to clear the infection (cysts or antigen) or resolve the diarrhea, a second drug from an alternate class is indicated. The addition of fiber to the diet may help control clinical signs of giardiasis in some animals by helping with bacterial overgrowth or by inhibiting organism attachment to microvilli. Feeding a restricted fat diets may also be effective. Immunotherapy with the Giardia vaccine has aided in the elimination of cyst shedding and diarrhea in some infected dogs. However, in a controlled study in 16 experimentally infected cats, vaccination as immunotherapy was ineffective with one strain of Giardia. In addition, the both Giardia vaccines has been discontinued. Probiotic administration may also be beneficial in some animals. In one study, bathing the dog on the last day of treatment was a beneficial adjunct therapy. In dogs and cats with persistent diarrhea and Giardia spp. infection, a more extensive workup to attempt to diagnose other underlying diseases is indicated if several therapeutic trials fail. Common underlying disorders include cryptosporidiosis, T. foetus in cats, inflammatory bowel disease, bacterial overgrowth, exocrine pancreatic insufficiency, and immunodeficiencies.

Should Healthy Dogs and Cats with Giardia Infection be Treated?

Healthy pets are not considered significant human health risks by the Centers for Disease Control (www.cdc.gov/hiv/pubs/brochure/oi_pets.htm). However, because clinical signs induced by Giardia spp. can be intermittent and since some Giardia spp. may be zoonotic, treatment of healthy infected animals should be considered with each owner. Treatment of healthy animals is controversial because all of the drugs can potentially cause side-effects, animals with normal stools are not considered human health risks, treatment is unlikely to eliminate infection, and re-infection can occur within days. For example, in a recent study of naturally infected dogs, > 65% of treated dogs were still Giardia infected when rechecked 34 days after treatment. If treatment deemed indicated by the clinician and owner, many clinicians currently recommend that a 5 day course of fenbendazole be administered for apparently healthy dogs and cats that test positive for Giardia. The AAFP Advisory Panel on Zoonoses recommends attempting to remove the source of infection during the treatment period and performing a fecal centrifugal flotation after Giardia treatment one time, within 2–4 weeks after the end of the treatment period (www.catvets.com), even if centrifugal flotation was negative while the antigen test was positive when used to establish the initial diagnosis. If the animal is healthy and negative for cysts, retesting is not indicated again until the next scheduled fecal flotation. Currently it is not recommended for any of the Giardia antigen tests to be used as a recheck test in the early post treatment phase. As discussed, it is currently unknown how long Giardia antigens will persist in feces after successful treatment.

What Should I Do with Dogs or Cats That Have Normal Stool and Are Giardia Antigen Positive, Giardia Cyst Negative?

This issue continues to be a dilemma and a source of ongoing discussion among parasitologists and clinicians alike. These animals have either a low grade infection or a low percentage of animals (approximately 2–5%) have false positive antigen test results. To further evaluate for cyst shedding, the veterinarian can perform an IFA test or 2 additional fecal flotations (3 negative centrifugal flotation assays run within 5 days is considered adequate to rule-out a Giardia infection in both animals and humans); if these other test results are negative, the antigen test was likely falsely positive. If cysts are still identified after fecal centrifugal flotation performed 2–4 weeks after appropriate administration of a drug with anti-Giardia activity, then a second course of therapy may be indicated using a drug from an alternate class. Some clinicians and parasitologists recommend the combination of metronidazole and fenbendazole for 5 days and then further treatment and testing are suspended. It is not recommended that apparently well animals be treated beyond two courses of therapy. No further diagnostics are indicated until GI signs occur or the animal is again due for routine fecal screening (once to twice annually as a minimum for all dogs and cats).

What Can I Do To Prevent Re-infection with Giardia spp.?

Prevention involves boiling or filtering of water collected from the environment prior to drinking and disinfection of premises contaminated with infected feces with steam cleaning or quaternary ammonium compounds (1 minute contact time). Transport hosts should be controlled and treatment and bathing of all animals in the environment could be considered. Feces from infected animals should be removed from the environment promptly. To date, no study has shown the Giardia spp. vaccines licensed for dogs and cats to have lessened Giardia spp. infections in the field and so both vaccines have been discontinued.

Table 1. Drugs used for the treatment of Giardia spp. infections.

C = canine; F = feline; B = canine and feline