Granny, I have copied and pasted some of the links from Mercksmanuels about Coryza and MG
Epidemiology and Transmission-Infectious Coryza
Chronically ill or healthy carrier birds are the reservoir of infection. Chickens of all ages are susceptible, but susceptibility increases with age. The incubation period is 1–3 days, and the disease duration is usually 2–3 wk. Under field conditions, the duration may be longer in the presence of concurrent diseases, eg, mycoplasmosis.
Infected flocks are a constant threat to uninfected flocks. Transmission is by direct contact, airborne droplets, and contamination of drinking water. “All-in/all-out” management has essentially eradicated infectious coryza from many commercial poultry establishments in the USA. Commercial farms that have multiple-age flocks tend to perpetuate the disease.
Clinical Findings for Coryza
In the mildest form of the disease, the only signs may be depression, a serous nasal discharge, and occasionally slight facial swelling. In the more severe form, there is severe swelling of one or both infraorbital sinuses with edema of the surrounding tissue, which may close one or both eyes. In adult birds, especially males, the edema may extend to the intermandibular space and wattles. The swelling usually abates in 10–14 days; however, if secondary infection occurs, swelling can persist for months. There may be varying degrees of rales depending on the extent of infection. Birds may have diarrhea, and feed and water consumption usually is decreased during acute stages of the disease.
Lesions in Coryza
In acute cases, lesions may be limited to the infraorbital sinuses. There is a copious, tenacious, grayish, semifluid exudate. As the disease becomes chronic or other pathogens become involved, the sinus exudate may become consolidated and turn yellowish. Other lesions may include conjunctivitis, tracheitis, bronchitis, and airsacculitis, particularly if other pathogens are involved.
Epidemiology and Transmission-Mycoplasma Gallisepticum, also called CRD
M gallisepticum is transmitted vertically within some eggs (transovarian) from infected breeders to progeny, and horizontally via infectious aerosols and through contamination of feed, water, and the environment, and by human activity on fomites (shoes, equipment, etc). Infection may be latent in some birds for days to months, but when birds are stressed horizontal transmission may occur rapidly via aerosols and the respiratory route, after which infection and clinical disease spread through the flock. Flock-to-flock transmission occurs readily by direct or indirect contact from the movement of birds, people, or fomites from infected to susceptible flocks.
Clinical Findings and Lesions-MG
In chickens, infection may be inapparent or result in varying degrees of respiratory distress, with slight to marked rales, difficulty breathing, coughing, and/or sneezing. Morbidity is high and mortality low in uncomplicated cases. Nasal discharge and conjunctivitis with frothiness about the eyes may be present. The disease is generally more severe in turkeys than in chickens, and swelling of the infraorbital sinuses is common. Feed efficiency and weight gains are reduced. Commercial broiler chickens and market turkeys may suffer high condemnations at processing due to airsacculitis. In laying flocks, birds may fail to reach peak egg production, and the overall production rate is lower than normal.
Uncomplicated
M gallisepticum infections in chickens result in relatively mild catarrhal sinusitis, tracheitis, and airsacculitis.
E coliinfections are often concurrent and result in severe air sac thickening and turbidity, with exudative accumulations, adhesive pericarditis, and fibrinous perihepatitis