Darrell W. Trampel, D.V.M., PhD.
Iowa State University
January 6, 2014
Each year, chicken owners contact me to discuss a serious disease in their chickens characterized by difficult breathing, loud wheezing, and gurgling. Affected chickens often stand with heads and necks extended as they attempt to inhale. Many owners report that their sick chickens are “gaping” with beaks wide open as they attempt to breathe. When owners of backyard flocks are asked if new birds were added to their flocks during the weeks preceding onset of illness, they usually answer “yes.” Owners of exhibition chickens frequently confirm that their birds have been to a fair, show, or swap meet in the recent past. Almost always, new chickens added to the flock and other chickens displayed by other owners at exhibitions and sales were clinically normal and showed no signs of disease. The flock history described above is typical for chickens experiencing infectious laryngotracheitis (ILT).
ILT is caused by a herpesvirus. The time between exposure and onset of disease (incubation period) ranges from 6 to 14 days. Disease within a flock may last for 2 or more weeks. Nearly 100% of chickens in a susceptible flock may become ill and mortality may reach 70% or more. In addition to the symptoms described above, chickens with the severe form of the disease may violently shake their head and expectorate blood‑tinged mucus on walls. The inner surface of the eyelids may be infected (conjuncitivits) and yellow exudates may accumulate beneath the eye lids. Chickens may die suddenly with few previous signs of disease and egg production may decline by 10‑20%. The trachea may contain blood, or if the chicken survives longer, a yellow mat of fibrin closely adhered to the inner surface of the trachea. In many cases, the yellow exudate dislodges from the tracheal wall and causes a plug which obstructs the opening to the trachea (glottis) and the bird dies of asphyxiation.
Sources of infection include direct or indirect exposure to acutely affected birds and recently recovered carriers. ILT virus replicates in the trachea for the first 7 days of an infection and virus shed in respiratory secretions from infected chickens heavily contaminate the environment. Movement of contaminated litter, crates and coops, trucks, egg flats, and equipment can spread this virus from pen to pen, house to house, and farm to farm. People can carry ILT virus on their hands, clothing and shoes, and are major contributors to transferring this disease from one location to another. Wild free-flying birds, rodents, and insects are not infected themselves, but can act as mechanical carriers in dissemination of this virus. Up to 50% of chickens that recover from this disease may be carriers because of the establishment of a latent infection and can shed ILT virus for up to 16 months. Latent virus is found in the trigeminal ganglia where the ILT genome is integrated into the host DNA which allows the ILTV to evade the immune system. Reactivation of latent virus and subsequent re-excretion occurs following stress associated with rehousing, mixing with unfamiliar birds, onset of egg production, and molting. Egg transmission does not occur.
Prevention is of paramount importance because there is no specific treatment. Antibiotics are sometimes used to prevent secondary bacterial infections, such as E. coli, but have no effect on viruses. Backyard flocks kept for meat or egg production should be kept isolated. Common mistakes, such as introducing recently vaccinated birds or adding birds obtained from a friend or auction should be avoided. These chickens may appear normal but are potential ILT carriers. Owners should never visit neighboring poultry flocks, especially those suspected of having a contagious disease. Avoid borrowing equipment from other flock owners. All used equipment brought onto a premises should be cleaned and disinfected prior to use. Chickens should be transported in plastic crates or carriers which can be cleaned and disinfected. ILT virus is readily killed by common disinfectants containing glutaraldehyde or quaternary ammonium. However, ILT virus is resistant to disinfectants in the presence of organic matter, so thorough cleaning prior to disinfection is essential.
Vaccination early in an outbreak is feasible because of the long incubation period and relatively slow spread through a flock. Two types of attenuated live vaccines are available, chicken embryo-origin (CEO) or tissue culture-origin (TCO) vaccines. Immunity may develop by 3-4 days and persists for 15 – 20 weeks. Vaccination by eyedrop provides more uniform flock protection than water or spray administration. CEO and TCO vaccines may regain virulence as a result of passages in poorly vaccinated flocks or after reactivation from latency. Outbreaks of ILT associated with prior use of tissue culture origin TCO vaccine are infrequent, but have been associated with CEO vaccines on multiple occasions. CEO and TCO vaccines can be transmitted from vaccinated to unvaccinated chickens and may establish latency in apparently healthy chickens. Consequently, vaccination is recommended only where the disease is endemic. CEO and TCO vaccines transmitted to unvaccinated chickens do not protect contact-infected chickens. In recent years, recombinant vaccines have become commercially available that are produced with a herpesvirus of turkeys (HVT) or fowl pox vector. Recombinant vaccines do not replicate in the trachea and do not completely prevent replication of field strains in the trachea, so vaccinated chickens can shed field strains of ILTV. Vaccination of backyard flocks is necessary only in areas where ILT is common. In such flocks, all birds must be vaccinated. Vaccination of exhibition fowl is strongly recommended when ILT is prevalent in an area.