Protection Induced in Broiler Chickens following Drinking-Water Delivery of Live Infectious Laryngotracheitis Vaccines against Subsequent Challenge with Recombinant Field Virus

Abstract

Infectious laryngotracheitis virus (ILTV) causes acute upper respiratory tract disease in chickens. Attenuated live ILTV vaccines are often used to help control disease, but these vaccines have well documented limitations, including retention of residual virulence, incomplete protection, transmission of vaccine virus to unvaccinated birds and reversion to high levels of virulence following bird-to-bird passage. Recently, two novel ILTV field strains (class 8 and 9 ILTV viruses) emerged in Australia due to natural recombination between two genotypically distinct commercial ILTV vaccines. These recombinant field strains became dominant field strains in important poultry producing areas. In Victoria, Australia, the recombinant class 9 virus largely displaced the previously predominant class 2 ILTV strain. The ability of ILTV vaccines to protect against challenge with the novel class 9 ILTV strain has not been studied. Here, the protection induced by direct (drinking-water) and indirect (contact) exposure to four different ILTV vaccines against challenge with class 9 ILTV in commercial broilers was studied. The vaccines significantly reduced, but did not prevent, challenge virus replication in vaccinated chickens. Only one vaccine significantly reduced the severity of tracheal pathology after direct drinking-water vaccination. The results indicate that the current vaccines can be used to help control class 9 ILTV, but also indicate that these vaccines have limitations that should be considered when designing and implementing disease control programs.

Determination of the minimum protective dose of a glycoprotein-G-deficient infectious laryngotracheitis virus vaccine delivered via eye-drop to week-old chickens

“…Replication of wild type virus was also greatly reduced in the conjunctival and tracheal mucosa of these birds but was not completely prevented. This is common for ILTV vaccines, which typically induce non-sterile immunity [12, 15, 17, 22, 23]. Increasing the vaccine dose to 10 5.0 PFU did not significantly improve the level of protection compared to that induced by the minimum effective dose, whereas lower doses (10 3.2 PFU or 10 3.5 PFU) failed to fully protect the birds from clinical disease after challenge.…”

“…Most ILTV vaccination and challenge studies assess levels of clinical protection by scoring clinical signs, tracheal pathology, and mortalities [2, 3, 12, 27]. Many also assess weight gain [15, 17, 27]. In this study, weight gain was not measured, as layer type birds do not typically have significant weight gains in short periods of time.…”

“…In this study, weight gain was not measured, as layer type birds do not typically have significant weight gains in short periods of time. Importantly, our study also assessed virus replication after challenge by measuring the viral genome concentration, as have some other vaccine efficacy studies [7, 12, 15, 17]. Vaccines that prevent clinical signs of disease but do not prevent replication of pathogens after challenge have been called ‘leaky’ or ‘imperfect’ vaccines and this is a common feature of ILTV vaccines [27–29].…”

“…Birds that received the ΔgG ILTV vaccine had significantly fewer clinical signs than unvaccinated birds [13]. The suitability of the ΔgG ILTV vaccine strain was then investigated using delivery methods suitable for mass vaccination programs, including delivery via drinking water, eye-drop or in ovo inoculation [12, 15–17]. The results from these studies demonstrated that the ΔgG ILTV vaccine strain has desirable safety and efficacy characteristics.…”

Variability in practices for drinking water vaccination of meat chickens against infectious laryngotracheitis

“…This is likely due to challenges in ensuring that sufficient amounts of the vaccine virus come into contact with respiratory tissues to actually vaccinate the bird (Hilbink et al 1981;Robertson and Egerton 1981;De Wit 2013). Laboratory studies often show successful protection against challenge with field strains of ILT virus with the available vaccines (Arzey and Arzey 2009;Korsa et al 2015) but problems in achieving protection in the field are commonly described (De Wit 2013;Keck 2018). A previous study (Groves et al 2019) conducted in commercial meat chicken flocks in Australia demonstrated marked variation in vaccine virus establishment in respiratory tissues associated with drinking water application factors.…”