Residue depletion of toltrazuril and its metabolites in egg after an authorised application in hens

Toltrazuril is effective against all coccidial parasites in most animal species. Pharmacological studies showed that toltrazuril was well absorbed in the gastrointestinal tract and rapidly metabolised to toltrazuril sulfoxide and toltrazuril sulfone (ponazuril), the latter being considered as marker residue for toltrazuril residues. The maximum residue limits (MRLs) for toltrazuril sulfone in edible tissues from poultry range from 100 μg/kg in muscle to 600 μg/kg in liver. The substance is currently not allowed for use in poultry producing eggs for human consumption. However, several veterinary medicinal products containing the active substance are authorised in the European Union for use in pullets. According to product literature, withdrawal periods for edible tissues from chickens of 12 up to 18 days as well as restrictions for use before the onset of laying must be observed. Depending on the chicken breed, the start of the laying activity can vary between the 17th and 20th week of age. In monitoring programs, certain cases of residues in eggs were detected although zero tolerance was applicable at that time. The objective of the presented study was to evaluate the residue depletion profiles of toltrazuril and its major metabolites after the treatment of 15-week-old laying hens with an authorised toltrazuril-containing veterinary medicinal product. The pullets were treated on two consecutive days with TOLTRA-K (25 mg/ml) oral solution via drinking water at a dosage of 7 mg of toltrazuril per kg of body weight per day according to the recommended posology. Individual egg samples were collected after the beginning of the laying activity and were analysed in duplicate by LC-MS/MS to determine residues of toltrazuril and its metabolites. Residues of toltrazuril sulfone in egg samples were detected for several weeks and a statistical model was established to determine the time span needed to meet the zero tolerance.