BACKGROUND : Florfenicol, a commonly used veterinary antibiotic, was reported to have caused a severe drop in egg
hatchability following its off-label use on a broiler breeder farm in South Africa. According to the pharmacovigilance
report, hatchability dropped by 80 % for up to a week following a five day course at 10 mg/kg (both males and
females treated metaphylactically) to manage an Escherichia coli infection. While mammalian toxicity studies indicate
the potential for early embryonic death in utero or testicular damage, no literature is available on the avian toxicity of
florfenicol. For this study we investigated the effects of florfenicol at various doses from 10 to 90 mg/kg on the egg
hatchability in a breeder flock we kept and established under controlled conditions, with the same cockerels and
hens being exposed in a phased manner.
RESULTS : Following five days of oral exposure, no toxic signs were evident in any of the cockerels or hens treated at
doses up to 90 mg/kg. Treatment of only the cockerels had no effect on egg hatchability, while treatment of only
the hens at doses of 60 and 90 mg/kg resulted in decreased hatchability of 0 % in comparison to 70 % of the control
as early 24 h after treatment. In all cases, decreased hatchability was associated with embryonic death at 5 days of
development. The toxic effects of florfenicol were completely reversible with comparable hatchability being present
by day 4 post-treatment withdrawal. Toxicity correlated with total egg florfenicol concentrations with an LC50 of
CONCLUSION : Florfenicol appears to be toxic to the developing chick embryo at around day 5 of incubation, in the
absence of related toxicity in the hen or cockerel.