JPC SYSTEMIC PATHOLOGY
Signalment: Adult Barnvelder hen, Gallus gallus, avian
HISTORY (JPC #4057682): Hens in a “backyard” flock developed respiratory distress following close contact with ducks overnight in a very small poorly ventilated accommodation. The ducks and geese remain unaffected. Clinical signs included anorexia, depression, mouth breathing, nasal discharge, coughing and gasping.
HISTOPATHOLOGIC DESCRIPTION: Trachea: The architecture of the tracheal mucosa and submucosa is diffusely irregular, eroded or ulcerated, with epithelium lost and replaced by an extensive inflammatory infiltrate composed of large numbers of lymphocytes and macrophages admixed with fewer plasma cells, heterophils, fibrin and hemorrhage. The inflammatory infiltrate occasionally extends into the underlying adventitia and adjacent parenchyma. Within the mucosa, respiratory epithelial cells fuse and contain up to thirteen nuclei (viral syncytial cells). Viral syncytial cell nuclei often contain a single round eosinophilic intranuclear viral inclusion body that is 3-6 um in diameter, surrounded by a clear halo, and peripheralizes the chromatin. Overlying the mucosa is an exudate composed of eosinophilic cellular and karryorhectic debris (necrosis) admixed with hemorrhage, eosinophilic fibrillar material (fibrin), previously described inflammatory cells, viral syncytial cells, and small colonies of cocci (diphtheritic membrane).
MORPHOLOGIC DIAGNOSIS: Trachea: Tracheitis, acute, fibrinous and necrotizing, severe, with epithelial viral syncytial cells and intranuclear viral inclusion bodies, chicken, Barnvelder, avian.
ETIOLOGIC DIAGNOSIS: Alphaherpesviral tracheitis
CAUSE: Avian infectious laryngotracheitis virus (Gallid herpesvirus type 1)
CONDITION: Infectious Laryngotracheitis; Fowl Diphtheria
- Infectious laryngotracheitis (ILT) is an acute, highly contagious disease of chickens characterized by respiratory distress, coughing, expectoration of bloody, mucoid exudate and high mortality
- Causative agent is Gallid herpesvirus type 1 (GaHV-1), genus Iltovirus, subfamily alphaherpesviridae
- One of the three most important respiratory diseases of chickens (others include infectious bronchitis and Newcastle disease)
- Outbreaks range from highly pathogenic epizootic forms to mild enzootic disease
- Primarily a disease of chickens but may occur in pheasants and peafowls
- Affects birds of all ages but it is most common in 4-18 month olds
- High morbidity (90‑100%) in all age groups, variable mortality (10-70%, average 10-20%) depending upon the pathogenicity of the strain
- Spreads via aerosolization of respiratory secretions
- Virions enter through upper respiratory membranes, conjunctiva or may be ingested; spread is from cell to cell with no detectable viremia
- Incubation period is generally 6‑12 days
- Recovered birds may become inapparent carriers and harbor virus in the upper respiratory epithelium for months, creating an endemic condition within flocks
- The virus can become latent in the trigeminal nerve ganglion with subsequent reactivation in immunocompromised animals
- Cell-mediated immunity is the key to disease resistance
- Outbreaks of acute disease have occurred in broilers as a result of reversion to virulence of live-attenuated chicken embryo origin vaccine virus strains
TYPICAL CLINICAL FINDINGS:
- Severe dyspnea; bloody discharge from beak and nostrils; weakness; cyanosis; loss of weight; decreased egg production; characteristic head shaking with coughing
- Mild cases have conjunctivitis, swollen infraorbital and nasal sinuses, congestion of mucous membranes and decreased egg production
- Death due to suffocation or other secondary bacterial or fungal infection
TYPICAL GROSS FINDINGS:
- Severe laryngotracheitis with necrosis, hemorrhage, ulceration and occlusive pseudomembranous, caseous or blood casts or plugs in trachea
- In milder forms there is sinusitis, conjunctivitis, congestion of mucous membranes and seromucoid discharge
- The lungs and air sacs are rarely affected; recent report of esophageal and pharyngeal lesions
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Intranuclear inclusion bodies (Cowdry Type A) in tracheal and conjunctival epithelial cells early in infection (1-5 days); difficult to find in more chronic cases due to desquamation of epithelium
- Multinucleate syncytial cells formed by tracheal mucosa epithelial cells (also respiratory epithelium lining parabronchi and air sacs, if affected)
- Necrosis and loss of epithelium with ulceration and attenuation of remaining cells; loss of goblet cells; formation of diphtheritic (fibrinonecrotic) membrane
- Hemorrhage may occur in cases of severe epithelial destruction and desquamation due to exposure and rupture of trachea blood capillaries
- Lymphocytic and histiocytic inflammation within the mucosa and submucosa
- Virulent strains may cause severe suppurative bronchopneumonia and air sacculitis
- Viral capsids with icosahedral symmetry form in the nucleus and bud through the nuclear membrane to acquire the cell derived envelope
- Viral particles migrate through endoplasmic reticulum and form large masses within intracytoplasmic vacuoles
- Viral particles are released upon cell lysis or exocytosis of vacuolar membrane
ADDITIONAL DIAGNOSTIC TESTS:
- Virus isolation, qPCR, IHC
- Serology (ELISA) may be used to evaluate seroconversion induced by vaccination, but is not a suitable diagnostic tool as it cannot differentiate antibody responses of infected carriers from vaccinated birds
- Newcastle Disease (Paramyxovirus) – high mortality; surface epithelium seldom lost
- Fowl Pox (Avian pox virus) - intracytoplasmic inclusion bodies (Bollinger bodies), no syncytial cells, proliferative epithelium, tracheal diphtheritic lesions
- Infectious Bronchitis (Coronavirus) - no inclusion bodies; affects lower respiratory tract; nephrotic strains may also affect kidneys
- Avian Influenza (Orthomyxovirus, Type A Influenza virus) - non-respiratory organs also involved
- Mycoplasmosis ( gallisepticum)- severe air sacculitis; no pseudomembranes; lack of extensive epithelial necrosis
- Infectious Coryza (Avibacterium paragallinarum) - early stages with facial edema; usually involve lower respiratory tract and other organs
- Fowl Cholera (Pasteurella multocida) - high mortality; respiratory signs; diarrhea
- Trichomonas gallinae (frounce, canker) - caseous material in the trachea and esophagus; flagellates visible with Giemsa staining
- Vitamin A deficiency - lesions primarily in the mouth and tongue
Other affected galliformes include: Indian peafowl, guineafowl, pheasants and turkeys
Other Herpesviral Respiratory Diseases:
- Infectious Bovine Rhinotracheitis (Bovine herpesvirus 1, alphaherpesvirus)
- Caprine Herpesvirus (Caprine herpesvirus 1, alphaherpesvirus)
- Pseudorabies (Suid herpesvirus 1, alphaherpesvirus)
- Canine Herpesvirus (Canid herpesvirus 1, alphaherpesvirus)
- Feline Herpesvirus (Felid herpesvirus 1, alphaherpesvirus)
- Pacheco’s disease (Psittacid herpesvirus 1, alphaherpesvirus)
- Porcine Cytomegalovirus (Suid herpesvirus 2, betaherpesvirus)
- Malignant Catarrhal Fever (Alcelaphine herpesvirus 1 or Ovine herpesvirus 2, gammaherpesvirus)
- Equid Herpesvirus 1 (alphaherpesvirus)
- Equine Nodular Pulmonary Fibrosis (Equid herpesvirus 5, gammaherpesvirus)
- Proliferative tracheitis in parrots (Amazon tracheitis virus, alphaherpesvirus)
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- Sary K, Chénier S, Gagnon CA, et al. Esophagitis and pharyngitis associated with avian infectious laryngotracheitis in backyard chickens: two cases. Avian Dis. 2017;61(2):255-260.