Mycoplasma gallisepticum (Mg) is a respiratory pathogen of poultry and gamebirds. Outbreaks of upper respiratory disease in pheasants and partridges are often associated with Mg. Experiments with Mg from pheasants have shown that it will infect young pheasants at one-day-old or at 20 weeks of age. However, little is known about mixed infections, although in poultry we know that Mg disease can be exacerbated by respiratory viruses. In adult pheasants, we have shown that Mg in a co-infection with avian pneumovirus causes a more severe disease than Mg alone, despite the fact that disease is not found in birds infected with the virus alone.
Pheasants are reported to be susceptible to coronavirus infections. Coronaviruses isolated from pheasants with kidney problems are different from those isolated from poultry with infectious bronchitis or pheasants with egg production problems.
These viruses replicate in the respiratory tissues (nose, trachea, air-sacs and lungs), oviducts, many parts of the alimentary canal and also in the kidney, causing kidney disease. The virus can usually persist in the bird without a harmful effect until stress brought on by the onset of laying causes the virus to be excreted.
There are many reports in the literature of synergism between Mg and infectious bronchitis virus occurring in respiratory disease of chickens, so the aim of our study was to investigate the possibility of a similar phenomenon occurring in pheasants. For this we used an Mg isolate together with a pheasant coronavirus recovered in 2004 from respiratory disease in pheasants.
We randomly divided 40 adult (eight to 10 months old) hen pheasants into four groups of 10 birds: a control group; one treated with Mg alone; one treated with coronavirus alone; and the fourth treated with a mixed infection of Mg and coronavirus. We subsequently monitored the birds for clinical signs, virus isolation, immune response and pathology.
Our results clearly demonstrated, for the first time, that adult hen pheasants co-infected with Mg and coronavirus developed more severe signs of disease and with a greater morbidity than pheasants with Mg alone (see Figures 1 and 2).
|Figure 1. Mean clinical scores on days 3 to 27 postinfection of four groups of pheasants
Birds infected with both coronavirus and Mg showed most signs of illness.
|Figure 2. Mean post-morten lesion scores among four groups of pheasants
The mixed infection group showed more disease lesions than those infected with Mg alone.
Lesions found in the eyes and sinuses of the infected birds in the Mg and mixed infection groups were probably caused by Mg since such lesions were not reported in pheasants naturally infected with coronaviruses. Such birds reveal visceral gout, urolithiasis and gross swelling and pallor of the kidneys. In our trial, all kidneys examined in the infected birds appeared to be normal.
Based on the results of this work, we were able to set up trials for a vaccine, but as yet these have failed to offer significant protection to birds infected with an Mg/coronavirus co-infection.