In this 90-day study, 60 male Holstein dairy calves were experimentally infected with Giardia duodenalis. Calves were randomly blocked by weight into treatment (
N=30) and placebo (
N=30) groups. Beginning on study Day 0, calves in the treatment group were administered an oral dose of 5mg/kg of
fenbendazole once daily for three consecutive days. Calves in the placebo group received a daily oral treatment of 5 ml of saline for 3 days. These treatments were repeated on Days 30 and 60 of the study. Fecal samples were collected from calves once per week and examined for the presence of Giardia
cysts. Calves were monitored daily for clinical signs of
intestinal disease and all episodes of
diarrhea recorded. Calves were weighed once per week and total feed intake, on a dry matter basis, was calculated daily. Following each treatment, the number of calves shedding Giardia
cysts in the
fenbendazole group was reduced (p<0.001) compared to the saline group. Also, calves in the
fenbendazole group had fewer
cysts (p<0.05) detected in their feces following treatment compared with calves that received saline. Within 2 weeks post treatment, the number of infected animals and fecal Giardia
cysts returned to placebo levels. This pattern of
reinfection was consistent after every treatment period. Calves receiving
fenbendazole had fewer total days with
diarrhea (p<0.01) and the average number of days each calf had
diarrhea was reduced (p<0.05), compared to the placebo group. There were no differences in mean
body weight, average daily gain, or feed intake between the treatment or placebo groups. This study demonstrates that
fenbendazole is an effective treatment for giardiosis, resulting in a clinical benefit and reducing the number of infective
cysts shed by calves. However, this treatment regime had no impact on production parameters and
reinfection occurred rapidly in these calves.