Epsilon Archive for Student Projects

Abstract

Infectious laryngotracheitis (ILT) is a respiratory tract infection of gallinaceous birds, which is
caused by Gallid herpes virus 1 (GaHV-1), also called infectious laryngotracheitis virus
(ILTV). In its most virulent form it can cause acute respiratory symptoms such as marked
dyspnoea, gasping, expectoration of bloody mucus, high mortality (up to 70%) and production
losses, such as decreased growth and egg production.

In Sweden, there are regular outbreaks of ILT in hobby flocks with chickens and occasionally
in other gallinaceous birds, but very few outbreaks have been diagnosed in commercial flocks.
There are vaccines available and the most commonly used vaccine in Sweden is a live
attenuated vaccine of chicken embryo origin (CEO). However, vaccination against ILT is not
routinely practiced in commercial flocks in Sweden but the vaccine is administrated in some
hobby flocks. In several countries where ILT vaccines are used in commercial flocks there have
been outbreaks caused by vaccine-related viruses that have regained virulence though mutation
(so called reversion) or recombined to new viruses, causing clinical outbreaks and production
losses. The aim of this study was to improve the understanding of the epidemiology of ILT in
Swedish chicken hobby flocks, including investigating if ILTVs circulating in Sweden are
related to vaccine viruses. This was done by sequencing parts of the glycoprotein G (gG),
thymidine kinase (TK) and infected cell polypeptide 4 (ICP4) genes from real time PCR ILTV
positive nasal swabs from hobby chickens and comparing them to CEO vaccine strains and
reference strains. These genes have previously been used to differentiate field strains from
vaccine-related ILTVs. The samples originated from hobby chickens submitted for routine
diagnostic necropsy with complaints of respiratory signs to the Swedish National Veterinary
Institute in 2017 and 2018. The samples were also evaluated for co-infection with Avibacterium
paragallinarum (APG), infectious bronchitis virus (IBV) and Mycoplasma gallisepticum
(MG). Based on the three partial gene sequences (gG, TK and ICP4) the results suggested that
the circulating ILTVs in Swedish hobby chickens flocks were related to CEO vaccine virus
strains, including the vaccine used in Sweden for ILT vaccination. Complete genome data from
a subset of the samples showed similar results, however, there were additional sequence
differences in other locations of the genome. Further analyses are needed to determine the level
of similarity to vaccine strains. The source of these viruses is however unknown. It was
demonstrated that coinfections with other respiratory pathogens were common in the
investigated Swedish hobby chicken flocks with ILT outbreaks, with the most common
pathogen in the investigated ILT affected flocks being APG.