Temporal variation of ILTV and MDV viral genome in dust samples after vaccination in a layer flock

Abstract

An experiment with two phases, a pullet raising phase and a laying phase, was conducted to monitor the temporal variation of infectious laryngotracheitis virus (ILTV) and Marek's disease virus (MDV) vaccinal load in dust samples. These were collected weekly by settle plate and scrapings from the wall and horizontal surfaces from placement at day old to 40 weeks of age. The chickens were vaccinated against MDV (Rispens) at day old and with ILTV in water at 6 weeks (A20 strain) and by eye drop at 12 weeks of age (SA2 strain). The genome copy (GC) number of ILTV and Rispens in the dust samples were measured by using qPCR. ILTV was detected in dust 4 weeks after the first ILTV vaccination and viral load peaked 2 weeks after the second vaccination. ILTV became undetectable by 8 weeks after the 2nd vaccination apart from one positive sample at week 26. MDV was detected in dust one week after vaccination, peaked 3 weeks post vaccination and was detectable at a high level until week 17, when levels started to decline. There was no difference between settle plate and scraped samples in ILTV GC but, for MDV, higher GC were found in scraped samples later in the experiment probably reflecting accumulation of old dust. The findings suggest that the settle plate method better reflects the current level of vaccine virus in dust while the scrape method likely represents a cumulative and historical record of shedding over a period of time. Assessment of viral GC in a dust sample post vaccination is a promising candidate for a practical, routine method of indirectly assessing vaccine virus take for these viruses in commercial layer flocks, consistent with findings for ILTV vaccine take in broilers (Ahaduzzaman et al., 2019).