Introduction

This infection, probably the commonest respiratory disease of chickens, was first described in the USA (N. Dakota, 1931). Its affects vary with: the virulence of the virus; the age of the bird; prior vaccination; maternal immunity (young birds); and complicating infections (Mycoplasma, E. coli, Newcastle disease).

Morbidity may vary 50-100% and mortality 0-25%, depending on secondary infections. The cause is a Coronavirus that is antigenically highly variable; new sero-types continue to emerge. About eight sero-groups are recognised by sero-neutralization. Typing by haemagglutination-inhibition is also used. These differences are due to structural differences in the spike proteins (S1 fraction).

Infection is via the conjunctiva or upper respiratory tract with an incubation period of 18-36 hours. The infection is highly contagious and spreads rapidly by contact, fomites or aerosol. Some birds/viral strains can be carriers to 1 year. The virus, which may survive 4 weeks in premises, is sensitive to solvents, heat (56°C for 15 mins), alkalis, disinfectants (Formal 1% for 3 mins). Poor ventilation and high density are predisposing factors.

Signs

• Depression.
• Huddling.
• Loss of appetite.
• Coughing, gasping, dyspnoea.
• Wet litter.
• Diarrhoea.
• Diuresis.

Post-mortem lesions

• Mild to moderate respiratory tract inflammation.
• Tracheal oedema.
• Tracheitis.
• Airsacculitis.
• Caseous plugs in bronchi.
• Kidneys and bronchi may be swollen and they and the ureters may have urates.

Diagnosis

Tentative diagnosis is based on clinical sgns, lesions and serology. Definitive diagnosis is based on viral isolation after 3-5 passages in chick embryo, HA negative, with typical lesions, flourescent antibody positive and ciliostasis in tracheal organ culture.

Serology: HI, Elisa (both group specific), SN (type specific), DID (poor sensitivity, short duration, group specific).

Differentiate from Newcastle disease (lentogenic and mesogenic forms), mycoplasmosis, vaccinal reactions, Avian Influenza and Laryngotracheitis.

Treatment

Sodium salicylate 1gm/litre (acute phase) where permitted - antibiotics to control secondary colibacillosis (q.v.).

Prevention

Live vaccines of appropriate sero-type and attenuation, possible reactions depending on virulence and particle size. Maternal immunity provides protection for 2-3 weeks. Humoral immunity appears 10-14 days post vaccination. Local immunity is first line of defence. Cell-mediated immunity may also be important.