Chronic respiratory disease (CRD)

Infection with Mycoplasma gallisepticum is characterized by slow onset; CRD is seen in chickens, turkeys, and other birds. Ducks from multispecies mixed populations may be infected. It is now rare in commercial poultry. The route of infection is via the conjunctiva or upper respiratory tract with an incubation period of 6-10 days. Transmission is generally by direct contact with birds, exudates, aerosols, airborne dust and feathers, it may also be transovarian. Recovered birds remain infected for life; subsequent stress may cause recurrence of disease.

The infectious agent survives for short periods although prolonged survival was reported in egg yolk and allantoic fluid, and in lyophilized material as well as on hair and feathers. Intercurrent infections with respiratory viruses (IB, ND, and ILT), E. coli, Pasteurella spp. Hemophilus and extreme environmental conditions are predisposing factors for clinical disease.

Symptoms and lesions: Clinically, the birds show coughing, nasal and ocular discharge, and thereafter the batches undergo poor productivity, unable to pickup bodyweight, leg weakness, reduced hatchability and hatching of weaklings. The classical picture presents airsacculitis with caseous plugs on lungs, air sacs; pericarditis, catarrhal inflammation of nasal passages, sinuses, trachea and bronchi. Occasionally arthritis, tenosynovitis and salpingitis in chickens are also noticed.

Diagnosis: History, lesions, isolation and identification of organisms, demonstration of specific DNA by PCR are normally used for diagnosis of Mycoplasmosis. Culture requires inoculation in specific pathogen-free embryos or more commonly in Mycoplasma broth followed by plating out on Mycoplasma agar containing sterols and horse serum. Serum agglutination is the standard screening test used to detect suspect flocks.

Prevention and control: Mycoplasma infection is important for trade in birds or hatching eggs. Purchase of uninfected chicks, all-in/all-out production systems, biosecurity, and culling of infected birds with routine serological monitoring are the means of disease control.