Infectious laryngotracheitis: an update on current approaches for prevention of an old disease.

Abstract

Infectious laryngotracheitis is a respiratory disease of poultry caused by the Gallid alphaherpesvirus 1, also known as infectious laryngotracheitis virus (ILTV). It is a member of the subfamily of Alphaherpesvirinae (genus Iltovirus), and primarily infects chickens, but infections were also reported in pheasants, peafowls, and turkeys (Winterfield and So, 1968; Crawshaw and Boycott, 1982; Portz et al., 2008). The disease was first described in the United States in 1925 and was thereupon detected in poultry flocks worldwide (May and Tittsler, 1925). It results in substantial economic losses, which are mainly attributed to a drop in egg production in laying hens and reduced weight gain in broilers. Clinical signs vary depending on the infecting virus strain, which also determines the mortality ranging from 5% to 70%. Mild forms include nasal discharge, conjunctivitis, sinusitis, and general fatigue, whereas severe forms manifest themselves in gasping, coughing, expectoration of bloody mucus, and dyspnoea (Jordan, 1966; García et al., 2017). The virus is transmitted in droplets via the ocular and/or the upper respiratory route and the main tissue targeted is the epithelia of the trachea (Jordan et al., 1967). A transovarial or vertical transmission of the virus via egg content has not been demonstrated (Bagust et al., 2000). All ages of chickens are susceptible to ILTV but most ILTV outbreaks were reported for chickens older than 3 wk (Fahey et al., 1983). Upon infection, an incubation period of about 1 to 2 wk follows before first clinical signs get apparent (Jordan, 1966). The incubation time varies according to the virus strain and infectious dose (Oldoni et al., 2009). Furthermore, additional factors like chickens’ immune status and housing conditions (e.g., air quality) can impact the incubation period. During the acute phase of infection, ILTV can invade the trigeminal ganglion where latency can be established (Williams et al., 1992). However, latency is not restricted to the trigeminal ganglion but was also shown to occur in the trachea (Bagust et al., 1986; Hughes, 1987).