Evaluation of the protection elicited by direct and indirect exposure to live attenuated infectious laryngotracheitis virus vaccines against a recent challenge strain from the United States

Abstract

In a recent study , some field strains of infectious laryngotracheitis viruses (ILTV) were characterized as genotypically different (group VI) from ILT vaccine strains. The objective of this study was to evaluate the protection elicited by one chicken embryo origin (CEO) and one tissue culture origin (TCO) vaccine against a field isolate from group VI after direct and indirect exposure to ILTV live attenuated vaccines. In phase 1 of the experiment, non-vaccinated chickens were placed into contact with the eye drop vaccinates for a period of four weeks after vaccination. Transmission of the vaccine virus to these in-contact birds was demonstrated by real time PCR and antibody production, although the in-contact birds did not become protected against disease when subsequently challenged in phase 2 of the experiment. This emphasized the importance of uniform vaccination to obtain adequate protection, both to avoid the occurrence of susceptible chickens, and to minimize the potential for reversion to virulence of live-attenuated vaccines. In phase 2, protection against challenge with a group VI field virus was assessed four weeks after vaccination by scoring clinical signs and mortality, and quantifying weight gain. Sentinel birds were added to the groups one day after challenge to assess shedding of challenge virus, using real time PCR and virus isolation, during the period 2 to 12 days post challenge. The results showed that the CEO and TCO eye dropvaccinated chickens were protected against challenge with the group VI virus, even though it was genetically different from the vaccine strains, and that challenge virus was not transmitted from these protected birds to the sentinels.

Comparison of the replication and transmissibility of an infectious laryngotracheitis virus vaccine delivered via eye-drop or drinking-water

“…Birds that were vaccinated via DW had detectable ILTV for at least up to 28 days. These findings are in contrast with a previous report (Rodriguez-Avila et al, 2007), where ILTV DNA was not detected in the trachea of vaccinated chickens after the first week post vaccination but then was detected again on days 10 and 14 post vaccination. This difference may be due to different sensitivities of the qPCR protocols used in each study, or to variations in replication patterns of the different ILTV vaccine strains.…”

“…A previous report has revealed that, in contrast to vaccination, experimental exposure to ILTV vaccine strains by contact does not necessarily protect birds against challenge with wild-type ILTV (Rodriguez-Avila et al, 2008). Also a field study has postulated that, due to uneven distribution of the vaccine in poultry sheds, DW vaccination does not provide uniform protection against challenge in a flock (Fulton et al, 2000).…”

“…Non-uniform protection in DW vaccination is likely to be caused by suboptimal doses of the vaccine received by each bird, or may be due to some birds failing to drink any vaccine-containing water within the time-frame when viable vaccine is available to them. Importantly, it has been demonstrated that contact exposure to live-attenuated infectious laryngotracheitis virus (ILTV) vaccine strains does not necessarily protect against challenge (Rodriguez-Avila et al, 2008). The possibility of bird-to-bird vaccine transmission and subsequent reversion to virulence (Guy et al, 1991) together with non-uniform protection following DW vaccination (Fulton et al, 2000) may lead to a higher risk of ILT outbreaks occurring within poorly vaccinated flocks.…”

Persistence and spreading of field and vaccine strains of infectious laryngotracheitis virus (ILTV) in vaccinated and unvaccinated geographic regions, in Brazil

“…The presence of the Bastos strain 8 years after the initiation of vaccination in this region demonstrates that the attenuated vaccines were not able to eliminate this strain from the vaccinated region and that field ILTV strains can circulate without the presence of classical clinical signs. It has been suggested that CEO and field viruses can persist on vaccinated farms with chickens that are long-lived and/or of multiple ages, and the absence of clinical signs of ILT could be the outcome of the enhanced biosecurity measures applied and mass vaccination (Graham et al 2000;Dufour-Zavala 2008;Guy and García 2008;Rodriguez-Avila et al 2008;Chin et al 2009). There was a low rate of vaccine virus detection in the Bastos region, despite the massive vaccination program in this region.…”

Comparativein vivosafety and efficacy of a glycoprotein G-deficient candidate vaccine strain of infectious laryngotracheitis virus delivered via eye drop

“…External factors, including conditions at hatching and during rearing, are likely to be responsible for the different initial weights of 3-week-old birds in the different studies. Weight gain has previously been used to measure the pathogenicity of ILTV and to assess protection against challenge Rodriguez-Avila et al, 2008;Oldoni et al, 2009). A recent study, however, found that viral load and clinical signs after challenge were more reliable measures of protection than body weight gain (Johnson et al, 2010).…”