Effect of paracetamol injection in ovo in the course of hatching and thyroid hormone levels in chicken embryos

The aim of the study was to determine the effect of paracetamol (acetaminophen, P) on the course of embryogenesis and thyroid hormone (TH) levels in chicken embryos. Eggs from a broiler breeder flock were incubated under standard conditions. They were candled on the 5 th d of incubation in order to discard unfertilized eggs and those with dead embryos. At the 6 th d of incubation the eggs with living embryos were divided into 3 groups (n=120 in each); two of them were in ovo injected with P (4 mg/egg in 100 µl of aqua pro injectione) or with 100 µl of aqua pro injectione (control C1), while one group was non-injected (control C0). Thyroxine (T4), triiodothyronine (T3), and reverse-T3 (rT3) levels were measured radioimmunologically in samples of blood plasma collected from chick embryos on days: 11, 14, 16, 18, 20 (external piping), and on day 21 of incubation, i.e. just after hatching. The rest of the embryos were used for the investigation of hatchability parameters. In the P-treated group, a significant increase in the mortality of the embryos was found. It was mainly caused by the elevation of dead embryo number on the 6 th and 7 th d of incubation, i.e. just after P injection. In this group, much more damage to the liver and hyperplasia of the stomach were also observed. The time of hatching in P-treated chicks was significantly delayed. In comparison with both control groups, in P-treated embryos T4 was significantly higher on days 14, 16, and 18 of incubation, while it was considerably lower on day 20 and just after hatching. T3 concentrations in P-exposed embryos were significantly lower on day 11 and higher on day 14 and just after hatching. In P-injected embryos, rT3 levels were higher only on day 14. The results obtained indicate that paracetamol exerts unfavourable effects on chick embryogenesis; it decreases hatchability and delays hatching. This effect is probably associated with thyroid hormone alterations following P-treatment in late embryogenesis.