Abstract
Background:
Phytobiotics are natural plant-derived products that have gained recognition as superior alternatives to antibiotics that provide benefits without the risks of antibiotic resistance and residues.

Aim:
This study investigated the efficacy of phytobiotics supplemented in drinking water on the immune response and survival rate of broiler chickens against challenge from the Avian Avulavirus 1.

Methods:
200 one-day-old broiler chickens were selected. At first day, 20 chickens were sacrificed, and their blood sample was collected to estimate the maternal antibody titer against Avian Avulavirus 1 (Newcastle disease virus) using the indirect enzyme-linked immunosorbent assay (ELISA). The remaining 180 chicks were allocated into a completely randomized design with three replicates (n = 15) of four groups (n = 45). Chickens in the first group were vaccinated twice against Newcastle disease virus (NDV) by live attenuated vaccine on days 9 and 18 only, without any additives. Chickens in the second and third groups were vaccinated twice against NDV on days 9 and 18, respectively, in addition to 0.3 ml/ l of coriander oil and thyme oil added to their drinking water, from day 1 to the last day of the experiment. The chickens of the fourth group were left unvaccinated and without supplements as a control group. Ten blood samples were taken from each group on day 23 to measure the serum antibody titer against NDV using an ELISA test. On day 25, all chickens in all groups were challenged with a virulent Newcastle disease virus. Chickens in all groups were monitored for 10 days after the challenge to record clinical signs, postmortem lesions, and mortality rate.

Results:
There was a significant difference (p ≤ 0.05) between the treatment groups (G1, G2, and G3) and the control group (G4) in the mean antibody titer against NDV at day 23 of the chicks life. In contrast, there was a significant difference (p ≤ 0.05) between the groups that were vaccinated against NDV and supplemented with phytobiots in drinking water (G2 and G3) and the G1 group that was vaccinated against NDV only without any supplement. In addition, a higher survival rate after NDV challenge was recorded in the phytobiotic supplemented groups along with fewer of clinical signs, gross lesions, and mortality rates.

Conclusion:
The study concluded that adding a phytobiotic (coriander or thyme oil) to the drinking water had a positive effect on the humoral immune response and survival rate of vaccinated chickens against NDV infection. This study recommends adding coriander oil or thyme oil to the drinking water of broiler chickens due to their positive effects on the immunity and health of the birds.

Key words: Phytobiotic; Avulavirus 1; Humoral immune response; Challenge; Broiler.