Giardiasis in dogs: fenbendazole vs metronidazole for clearing infection and resolving diarrhoea — POSSIBLE (the treatment made no difference)

POSSIBLE (the treatment made no difference): In the one randomised head-to-head field trial, fenbendazole and metronidazole did not differ significantly in clearing Giardia infection in dogs, and the trial reports no by-group comparison of diarrhoea resolution. Ciuca 2021 (PMID: 33842570; 24 owned dogs naturally infected with Giardia, living in home conditions in the Campania region of southern Italy, randomly allocated to two groups of 12 with similar cyst loads) gave fenbendazole 50 mg/kg orally once a day for 5 consecutive days or metronidazole 25 mg/kg orally twice a day for 5 consecutive days (PMC full-text methods; the abstract's '50 mg/kg, once a day' wording for metronidazole is superseded by the full text, which governs), retreating dogs still shedding cysts at Day 7, 14 or 21 with the same drug until negative, and re-examining all dogs at Day 50. Efficacy (reduction in mean cysts per gram, PMC full-text table and results text, which govern over the abstract's differing fenbendazole series) was, for fenbendazole vs metronidazole: 80.9% vs 70.8% at Day 7, 100% vs 99.0% at Day 14, 97.0% vs 100% at Day 21, and 95.0% vs 97.1% at Day 50, with no statistically significant difference between the drugs (P = 0.686). After the first course, 4/12 fenbendazole dogs (33.3%) and 5/12 metronidazole dogs (41.7%) were still shedding cysts at Day 7, and at Day 50 two dogs (one from each group) were still shedding. On diarrhoea, the full text records only that at baseline faeces were unformed to diarrhoeic in dogs of both groups and gives a combined clinical-score distribution; it reports no diarrhoea-resolution outcome by treatment group, so this shelf cannot say either drug resolves diarrhoea more reliably. Design class (b): randomisation is established by the carded record (random allocation is stated in both the abstract and the full text) answering this shelf's own comparison, but blinding is not established — no blinding is stated anywhere in the record — so the class is not (a). The stamp is POSSIBLE, not LIKELY, because the whole comparative base is 24 dogs in one home-conditions trial in which later time points reflect retreatment of still-positive dogs, reinfection from the home environment cannot be separated from treatment failure (the authors' own caveat), and cyst-count efficacy is a mean-reduction measure, not a per-dog cure rate. Tier C is not engaged: no mortality or survival claim is made. One-home cross-point: metronidazole vs placebo in acute (non-Giardia) diarrhoea is shelf #163's home — a different condition — and none of its records is re-carded here. A further head-to-head study cited by Ciuca 2021 (Faure 2018, a field study of a metronidazole-based oral suspension vs fenbendazole in dogs with giardiasis) has no PubMed record in the searches run at drafting, so under the standing rule it is noted qualitatively here only, with no figures. Drafted Nov 10, 2026 (VetMed build, topic #410); figures from the abstract and the PMC full text, fetched and verified at drafting.