One Health Pharmacotherapy of Zoonotic Giardiasis in Dogs and Cats: A Systematic Review

Introduction: Giardiasis, caused by Giardia lamblia, is one of the most common fecal-oral zoonoses worldwide. Dogs and cats serve as important reservoir hosts and contribute significantly to environmental contamination and potential transmission to humans through the shedding of cysts in their feces. This systematic review aims to evaluate the available evidence on the efficacy of fenbendazole, metronidazole, and newer nitroimidazoles in reducing cyst shedding, with particular emphasis on their potential to mitigate environmental contamination and zoonotic risk within a One Health framework.

Methods: A comprehensive search was conducted using the keywords "Zoonotic giardiasis," "Canine and feline pharmacotherapy," "Environmental cyst shedding reduction," "One Health approach," and "Fenbendazole" in PubMed, Scopus, and Web of Science, with publications from January 2018 to December 2025. After removing duplicates and applying inclusion/exclusion criteria (clinical trials in dogs and cats, cyst shedding quantification, and safety assessment), a total of 36 articles were finally included.

Results: Fenbendazole (50 mg/kg for 3-5 days) remains the first-line treatment in dogs and cats, with 85-90% efficacy in reducing cyst shedding, high safety, and few side effects. Metronidazole (25 mg/kg twice daily for 5-7 days) has 60-75% efficacy, but is associated with neurological complications (ataxia, lethargy) in 10-15% of cases. Newer nitroimidazoles (such as ronidazole) with 75-80% efficacy and shorter treatment duration (3 days) are a good option, but their disadvantages are higher cost and more limited animal studies.

Conclusion: In the context of a One Health system, fenbendazole remains the drug of choice for controlling zoonotic giardiasis in dogs and cats due to its efficacy in reducing cyst shedding, high safety profile, and reasonable cost. Reducing cyst shedding through effective treatment directly reduces environmental contamination (soil, water, parks) and breaks the chain of transmission to humans. Concomitant administration of probiotics and environmental hygiene is an essential complement to this approach.