A novel HVT-based recombinant vaccine to control infectious laryngotracheitis and Marek's disease in chickens
Infectious laryngotracheitis (ILT) is a serious, costly and highly contagious respiratory disease in chickens. Modified live vaccines are widely used to prevent ILT outbreaks, but have several drawbacks such as the spread of the vaccine virus to non-vaccinates.
Innovax-ILT is a relatively new, commercially available recombinant vaccine based on the herpesvirus of turkeys (HVT), which has already been used extensively for preventing Marek’s disease.
Controlled studies with the vaccine administered either in ovo or subcutaneously to chicks show that Innovax-ILT protected 97% of chicks from ILT. The vaccine was also found safe to use and did not spread from vaccinates to nonvaccinates. A study in progress is expected to show that Innovax-ILT induces lifelong immunity.
Infectious laryngotracheitis (ILT) is an acute, highly contagious respiratory disease of chickens caused by an alphaherpesvirus. It is an economically important disease of poultry that may cause severe losses in meat and egg production.
Currently, ILT vaccines based on modified live viruses are widely used to prevent disease outbreaks. Although they are highly efficacious, attenuated live ILT vaccines have often been associated with a variety of adverse effects, including spread of the vaccine virus to nonvaccinates, increased virulence as a result of bird-to-bird passage and occurrence of latently infected carriers.
Vaccines based on the herpesvirus of turkeys (HVT) have already been extensively used for the prevention of Marek’s disease. Here, a commercially available, recombinant HVT vaccine developed to help prevent Marek’s disease as well as ILT is described. The vaccine expresses two protective glycoprotein genes of ILT virus (Innovax-ILT).
The safety profile of Innovax-ILT has been shown to be identical to the safety of commonly used HVT vaccine strains.
Innovax-ILT was also genetically stable in more than ten serial cell culture passages and remained fully infectious in chickens. However, Innovax-ILT could not be isolated from sentinel chickens during a contact exposure period of four weeks, demonstrating that it does not spread horizontally from vaccinated to susceptible chickens.
A single in ovo administration of the vaccine protected 97% of the chickens against ILT challenge for at least 10 weeks (Figures 1 & 2) — the longest period of time tested in the study after in ovo administration.
In addition, a single subcutaneous administration protected 97% of vaccinated chickens against an ILT challenge that caused clinical signs or death in more than 90% of the control chickens (Figures 1 & 2).
Because the vaccine persists without being shed, it is expected to induce life-long immunity against ILT and experiments to confirm at least one year duration of immunity are in progress.
No statistically significant difference in Marek’s protection provided by Innovax- ILT and the parent HVT strain was observed, indicating that insertion of foreign sequences into the HVT genome did not compromise the ability of HVT to protect against Marek's disease.
Overall, the safety, efficacy, ease of administration, the expected ability to induce long-lasting immunity and its non-spreading nature are some of the features that make Innovax-ILT an ideal vaccine to replace current modified live ILT vaccines and avoid the drawbacks associated with them. Field experience and additional laboratory data with Innovax-ILT since its recent introduction in the USA will be discussed.