While florfenicol is not registered for use in poultry, the product is used on a limited basis in broiler breeders for the treatment of E. coli by means of compounded solutions. While apparently efficacious, one unpublished adverse reaction report from the field suggest that florfenicol may interfere with embryogenesis of the developing egg. With the side effects of the product largely unknown in breeder birds, the aim of this study was to evaluate the effect of florfenicol on egg fertility in a fowl model. In this study 20 week old commercial layer breeder hens (n=30) and cockerels (n=4) in 4 groups were exposed to florfenicol in a phased manner, with the same groupings used for all phases. Prior to each phase, a wash-out period of three weeks was allowed.
In phase 1, only the hens were treated with florfenicol at 0, 10, 20 and 30 mg/kg, respectively, while in phase II, only the cockerels were treated at doses of 0, 30, 60, 90 mg/kg. In phase III only hens again were treated at doses of 0, 30, 60, 90 mg/kg. In all phases, treatments were administered once daily for 5 days directly into the crop. Eggs were collected from all groups on days 0, 2, 4, and 5 of dosing and on days 1, 3, 4, 6, and 8 days post-treatment for incubation. Fertility was evaluated by candling, egg break-outs and number of chicks hatching. In phase III, five hens from each group were slaughtered at 0, 1, 2, 3 and 4 days after drug withdrawal to ascertain the lungs concentrations achieved for the florfenicol, while eggs (n=5) were collected on days 0, 1, 2, 3, and 4 for the same reason. Florfenicol was quantified in the tissues using a validated HPLC method. Doses of 0, 10, 20 or 30 mg/kg of florfenicol had no major effects on the embryo and hatchability. No fertility effects were noted when the cockerels were treated. However, the hens treated at 60 and 90 mg/kg showed signs of embryonic toxicity with a complete absence of hatchability (0%) being evident soon after treatment, which only returned to normal 5 days of treatment cessation. Florfenicol had no other overt toxic effects on the treated birds. The concentration of florfenicol in eggs at the dose of 90 mg/kg was 4.27 μgmg after five days of treatement. Based on the presence or absence of toxicity, the threshold egg concentration for toxicity appears to be 0.6 ug/mg. The safe period for the consumption of eggs after treatment was estimated to be 6 days. In conclusion, florfenicol is toxic to the embryo when the hens are treated with doses of 60 or 90 mg/kg for five consecutive days.