Infectious laryngotracheitis ( ILT )

• ILT is an acute , highly contagious infection of chickens and pheasants
• Result in severe production losses due to mortality &/ or decreased egg production
• Severe epizootic form of infection are characterized by signs of respiratory depression , gasping, expectoration of bloody mucus and high mortality
• Mild enzootic form : encountered  increasingly in developed poultry industries and manifest variously as mucoid tracheitis, sinusitis, conjunctivitis , general unthriftiness and low mortality
• Distributed world-wide
• The greatest incidence of disease is generally seen in areas of highly intensive poultry production

Route of entry :

• Upper respiratory and ocular routes

Etiology :

                  Gallid Herpesvirus I , commonly known as infectious laryngotracheitis virus (ILTV)

                          [ symmetry : icosahedral ]

Transmission :

• It occurs :
• Actively infected birds
• Mechanical transmission ( contaminated equipment and litter )
• No evidence for vertical transmission

• IP = 6-12 days, following natural infection

     = 2-4 days , following experimental inoculation

• Morbidity = 50-100%
• Mortality = 10-20%
• Recovered and vaccinated birds are long-term carriers ⬎⬐

Pathogenesis :

                                                     Virus        

                          ⬐                                                     ⤵

       Respiratory tract epithelium                          conjunctiva

            ⬇                        ⬇

Larynx , trachea , respiratory sinuses                  swollen watery eyes

             air sacs , lungs

            ⬇

Epithelial damage and hemorrhages

            ⬇

            Nasal discharge , Coughing,

          gasping , bloodstained mucus

• Infectious virus usually is present in tracheal tissues and tracheal secretions for 6-8 days PI
• Extratracheal spread of LTV to trigeminal ganglia after 4-7 days of tracheal exposure
• Trigeminal ganglion – principle site of LTV latency
• Reactivation of latent LTV from trigeminal ganglia , 15 months after vaccination of a flock has been reported from Germany

• Apparently spontaneous outbreaks of ILT can occur in field situation
• Rates of shedding of ILTV into trachea could be significantly increased by the stresses of either onset of lay or mixing with unfamiliar birds

                         ⬇

Latently infected chicken can act as unsuspected     reservoir host and enable ILTV to infect further susceptible chickens

Clinical signs :

a. Sub – acute form :

• Tracheitis
• Conjunctivitis
• Mild rales
• Nasal and ocular discharge

b. Acute form ( severe epizootic form )

• Nasal discharge
• Moist rales
• Gasping ( pump handle respiration )
• Dyspnea
• Expectoration of blood stained mucus
• Decreased productivity

c. Mild enzootic form :

• Reduced egg production
• Watery eyes , conjunctivitis
• Swelling of infra-orbital sinuses
• Mild tracheitis
• Persistent nasal discharge
• Hemorrhagic conjunctivitis

Gross lesions :

• Conjunctiva : edema and congestion
• Infraorbital sinuses : congestion
• Trachea and larynx : blood stained mucus exudates

                                 Mucoid cast ( caseous plug )

• Bronchi : bronchitis
• Air Sacs and lungs : inflammation
• 

Fig : Severe hemorrhagic tracheitis with mucus due to infectious laryngotracheitis virus in a broiler.

Microscopic lesions :

• Tracheal mucosa : loss of goblet cells

                            Infiltration of mucosa with inflammatory cells

• Respiratory epithelium : cell enlarge , loss cilia and become edematous

                                    Syncytia are formed

• Lamina propria : blood vessels within lamina propria may protrude into tracheal lumen
• Blood capillaries : rupture of capillaries
• Intranuclear inclusion bodies are found in epithelial cells by 3 days post infection in tracheal epithelial .

Diagnosis :

• History
• Clinical signs and lesions
• Detection of viral antigen

1. Histopathology
2. Electron microscopy
3. FA test and immunoperoxidase test (IPT)
4. ELISA

• Detection of viral DNA :

1. Dot- blot hybridisation
2. PCR
3. RT- PCR

• Detection of antibody against LTV :

1. Agar gel immunodiffusion (AGID)
2. Virus neutralization
3. Indirect fluorescent
4. Antibody (IFA) test
5. ELISA

Fig : ILT. There is fibrinous exudate in the laryngeal opening and also in the tracheal lumen. The tracheal mucosa is hyperemic. A few small plaques of fibrinous exudate are also on the oral and esophageal mucosa