Infectious Laryngotracheitis (ILT) of poultry is a herpesvirus. More specifically it is Gallid herpesvirus I, which is the same family as the Herpes Simplex Virus (HSV) that causes cold sores in humans.
The virus can infect chickens, pheasants, and pea-fowl with generally an acute or sub-acute infection. Being a herpesvirus, there is an obligatory carrier state in animals that have survived the initial infection.
Morbidity in affected animals approaches 100% and mortality may range from 5-50% within a flock. Death occurs due to occlusion of the airway (see photos) at the larynx and proximal trachea. Due to the carrier state and excretion of the virus when stressed, young naïve chickens are most at risk but the virus is not passed into the egg stage of development and so all chickens are born naïve.
Clinical signs of infection include raspy breathing sounds, depression, lethargy, coughing, and death.
Case Study – “Nunga”, Mosquito Creek Rd, Warialda
This was a mixed farming enterprise with cropping and cattle. There was a backyard chicken coup with approximately 25 chickens present of various show breeds. The owner of the flock had an egg incubator and all the young chickens were reared from the egg stage through the incubator. A rooster had been introduced in the last 2 months.
The owner noticed some depressed chooks with dyspnoea and coughing. Most chooks initially seen to be unwell died within 48 hours. The owner contacted a private veterinarian who referred the case to me.
One sick chicken was examined. It appeared to have some caseous plaques on the dorsal palate of the mouth and was dyspnoeic. An autopsy was performed on a deceased chicken. Autopsy results showed a thick caseous plaque covering the entire opening of the larynx. On dissection of this area, there was a large caseous plug distal to the laryngeal opening and some purulent material lining the trachea. The chest and abdominal area appeared normal.
Swabs from the larynx and cloaca, in PBGS (viral transport media), were sent to EMAI for exclusion of Avian Influenza, Newcastle Disease, and ILT. Fixed samples were also sent in but were placed on HOLD. In addition to these tests, the laboratory ran a PCR test for both Mycoplasma and Chlamydia.
The cloacal swabs proved negative for all pathogens. The Laryngeal swabs returned a positive PCR test for ILT and also for Mycoplasma.
The flock were treated on an “as needed” basis. Sick animals were removed and given a dose of 25-50mg/kg doxycycline via the prescription product “Vibravet Paste”. A number of online sources were consulted to identify the therapeutic dose of doxycycline in poultry. Doxycycline has known anti-viral effects on HSV in people and has been used as an aid to treatment of HSV and also as an aid to therapy of herpesvirus infection in cats. It also has activity against Mycoplasmas which may have been beneficial in this case. Another option considered was injectable oxytetracycline as a prophylactic against secondary bacterial pathogens. However, for ease of owner compliance it was concluded that doxycycline paste would be the easiest to administer and may have the most beneficial effects.
Those animals treated with the paste appeared to recover uneventfully from the disease.
Vaccination is readily available to commercial poultry farms and was considered in this case but in the end, sourcing the vaccine in small quantities was difficult and it was decided to treat symptomatically only. Depending on the size that this poultry operation reaches, vaccination may be considered in the future.