Air sac disease: the disease, a possible vaccine and treatment
By Dr Colin Walker BSc, BVSc, MRCVS, MACVSc
Mycoplasmas are a group of microorganisms that infect birds, animals and people. There are many different types, with the different types affecting different species of animals and causing different clinical problems. For example, Mycoplasma pneumoniae infects the lungs of humans, Mycoplasma synoviae can infect the joints and respiratory system of chickens, while Mycoplasma gallisepticum is associated with respiratory infection in chickens, pigeons, turkeys and other birds. Different types of Mycoplasma vary in their ability to cause disease. Some cause severe disease with significant mortality rates while others do not cause disease at all and are considered to be normal inhabitants of, for example, the respiratory system. A birdís response after exposure to Mycoplasma is very much modified by a number of factors, including the birdís age at exposure, nutritional status, general health and genetic make-up, etc.
In many species of birds, including pigeons and chickens, the upper respiratory tract is invaded and colonized by mycoplasma in early life. Once they are there, they are essentially there forever. Antibiotic courses will reduce their number but not eliminate them. The significance of disease-causing mycoplasma is that, if the birds become run down so that their ability to resist disease is reduced, the mycoplasma will take advantage of this, increase in number and start to cause disease. Typically, in this situation, the mycoplasmas inflame the membranes lining the respiratory tract, causing some primary interference with infected pigeonsí racing performance but more importantly the inflamed membranes become vulnerable to infection with other secondary agents such as bacteria, Chlamydia and yeasts.
In chickens, there is a serious and common disease called Chronic Respiratory Disease (CRD). CRD is a primary mycoplasma infection with a secondary Chlamydial and bacterial (usually E. coli) infection. A vaccine is available to prevent chickens from catching CRD. Vaccines are live Mycoplams strains that can colonise the respiratory tract but have been modified so that they cannot cause disease. Drops of vaccine are placed in young chickens eyes and the hope is that these non-harmful Mycoplasma strains will colonise the respiratory system before the chicken has been exposed to any disease-causing strains. Once the vaccine strains have colonized the respiratory tract, they then exclude other strains, making it impossible for harmful strains to colonise.
In pigeons, usually at a young age, their respiratory systems are colonized by potentially harmful Mycoplasmas. The mycoplasmas then sit there and in many well-managed lofts donít cause clinical disease. If there are loft factors that interfere with the young maturing pigeonís ability to resist disease, such as overcrowding, poor hygiene, damp conditions, untreated parasitic disease or poor nutrition, the Mycoplasmas will flare, causing signs associated with respiratory tract infection, such as watery squinty eyes, dirty ceres, a croaking sound when breathing, general weight loss, lethargy, green mucoid droppings and sometimes death. Antibiotics given at these times will improve the birdsí health but do not eliminate the Mycoplasmas. Because of the inherent stresses of racing Ė time away from the loft, altered feeding and rest patterns, exertion, exposure to predation, etc, Mycoplasmas can flare in race birds. Although race birds can show symptoms similar to youngsters, signs are usually modified and more subtle because the birds are older and have more mature and functional immune systems. Racing pigeons with active Mycoplasma infection, with or without secondary Chlamydial or bacterial infections, are often described as having Ďair sac diseaseí. In these older birds, the Mycoplasmas inflame the lining of the respiratory tract, including the sinuses, windpipe, lungs and air sacs. This interferes with their breathing. Often in race-aged birds, the only signs observed are increased panting or fatigue after moderate exercise or sometimes just poor results or heavier losses.
Obviously, any disease that saps energy can cause similar symptoms and so it is important for fanciers not to jump to conclusions and say that their birds have air sac disease. Such fanciers find themselves giving antibiotics to no effect. Birds with Coccidia, worms and wet canker will naturally also prematurely fatigue.
And so, how does the racing fancier know if his birds have air sac disease? Unfortunately, there is no quick and easy test. Mucous can be taken from the throat and any Mycoplasma present cultured. There is no point in the lab just growing Mycoplasma unless they can actually identify the type (i.e. the species). They may just be the normal non-harmful variety and are not causing disease. Even if harmful ones are grown, it takes several days and is quite expensive. Also, just because they are there, they may not be causing disease. Pigeons can be autopsied and tissue samples collected for histology (microscopic examination). If this identifies inflammation, particularly if harmful Mycoplasmas are also grown, this confirms the diagnosis. But again, this is time-consuming and expensive. Microscopically examining mucous from the throat will sometimes identify inflammatory cells and secondary bacterial infection, suggestive of a Mycoplasma infection. Microscopic examination of the droppings can identify altered bacterial populations associated with stress. These changes occur with any debilitating condition and are not specific for air sac disease but do at least indicate that the pigeons are run down and possibly sick.
Often during the racing season, when a diagnosis needs to be made and treatment initiated fairly promptly, a tentative diagnosis is based on the signs the birds are showing, inflammatory changes visible on microscopic examination of a throat swab, altered bacterial patterns on microscopic examination of fresh droppings, a failure to demonstrate other conditions like wet canker and coccidia that cause similar symptoms and sometimes a response to treatment.
It is always a matter of correlating the degree of diagnostic effort with the severity of the problem and if the loft is experiencing ongoing problems, swabs should be collected for Mycoplasma culture and ideally a bird, showing symptoms that are representative of the problems in the flock, being submitted for autopsy and histology.
In the longer term, it would be really great if vaccines similar to those available in chickens became available for pigeons. Nestlings could have vaccine drops containing non-harmful Mycoplasma placed in their eyes. These would then colonise their respiratory tracts, preventing infection with disease-causing strains.
Currently, control focuses on avoiding the factors that trigger Mycoplasma flare-ups. During racing, however, this is pretty well impossible and most fanciers can expect flare ups of Mycoplasma -based respiratory infection during the season. Around the world, antibiotic blends containing tylosin (effective against Mycoplasma), doxycycline (the antibiotic of choice for Chlamydia) and sometime spiromycin (also called suanovil, effective against a range of bacteria) are prescribed by veterinarians as required to manage these flare-ups.