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Historically, mycoplasmosis has been a respiratory disease of domestic poultry and captive raised upland gamebirds and waterfowl. Mycoplasmosis was not considered to be infectious to songbirds. During the winter of 1993-94, house finches were reported with severe conjunctivitis infections in the states of Virginia and Maryland. The bacterium (Mycoplasma gallisepticum) was isolated as the causative agent. Within seven years the disease was diagnosed in house finches in nearly all of the states east of the Rocky Mountains. The eastern house finch may be sensitive to this disease because they are an introduced species, highly inbred and therefore less resistant to disease.
The house finch population was introduced to the eastern portion of the U.S. when a flock of caged house finches from California (known as "Hollywood Finches") were sold to people on Long Island, New York in the 1940's. When laws changed and it became illegal to sell and own American songbirds, these captive finches were released. Their descendants now occupy most of the eastern U.S. and are rapidly spreading westward toward the original range of the species.
In domestic poultry and captive raised upland game birds and waterfowl, mycoplasmosis is a chronic respiratory disease affecting the lungs, air sacs and sinuses. The disease was originally identified in domestic turkeys in 1905 and in chickens in the 1930's. Losses in the poultry industries are the result of condemned and downgraded carcasses at slaughter, reduced egg production, poor feed conversion and medication costs. Mycoplasmosis is the most costly disease of domestic birds worldwide.
Mycoplasmosis has been diagnosed in house finches in the U.S. from the east coast to most of the states east of the Rocky Mountains since the winter of 1993-94. The disease has also been identified in several free-flying species: American goldfinch in Georgia, Maryland, South Carolina and Tennessee, purple finch in New York, and evening grosbeak and pine grosbeak in Quebec, Canada. A captive bluejay, which was placed in a cage previously occupied by an infected finch, contracted the disease also. In Michigan, conjunctivitis due to M. gallisepticum has been found in house finches in the southern half of the Lower Peninsula.
Mycoplasmosis is a highly transmissible disease and is transmitted in house finches and other passerine birds via ocular discharge. The disease is most commonly spread at bird feeders and at roost sites. M. gallisepticum does not survive outside the body for any length of time. Transmission usually occurs when large flocks feed or roost closely together and the organism is spread via the eye secretions to neighboring birds. Tube style bird feeders are the most likely type of feeder to allow transmission because eye secretions from a diseased bird can be rubbed on the feeder opening and other birds feeding in that opening can then acquire the bacteria. Eye lesions develop within 12 weeks of exposure. Infected finches are responsible for spreading this disease because they move between bird feeders and to other areas during migration.
No known natural transmission has occurred between house finches and domestic poultry. The disease has been transmitted experimentally to both chickens and turkeys although an extended exposure time to the infected house finches was required. Severe air sac infections developed in both the chickens and turkeys. In poultry, transmission occurs by direct contact, air-borne respiratory droplets, dust (dried fecal material), contaminated equipment and via eggs.
Clinical signs observed in wild avian species with mycoplasmosis are red, swollen eyelids and conjunctival tissue (mucous membrane covering the anterior surface of the eyeball) with a clear ocular discharge. The condition can become more severe resulting in extreme swelling of the eyelid and conjunctiva, crust formation along the eyelid margins resulting in ulcerations on the cornea and purulent discharge and loss of sight. Inflammation of the sinuses may occur resulting in discharge flowing from the nares. The birds may also display wet, matted feathers on the face (especially around the eyes), fluffed body feathers, inactivity (sitting on the ground and remaining at feeders after other birds have left), weight loss, loss of appetite and death due to starvation, exposure or predation.
Domestic poultry display respiratory symptoms such as coughing, sneezing and nasal discharge. They are lethargic, stop feeding and undergo severe weight loss. In turkeys, the sinus under the eye becomes swollen.
Mycoplasmosis in wild songbirds causes conjunctivitis and rhinitis (inflammation of the nasal mucous membrane) with associated hyperplasia (increase in the number of cells) of lymphoid (lymph tissue) and epithelial (skin surface) tissues. Occasionally keratitis (inflammation of the cornea) and tracheitis (inflammation of the lining membrane of the trachea) occurs. Mycoplasmosis in chickens results in airsacculitis and in turkeys can result in sinusitis in the upper form of the disease and airsacculitis in the lower form of the disease. Once domestic birds are infected, they remain carriers for life.
In order to confirm a diagnosis of mycoplasmosis it is necessary to identify the causative organism. The organism can be isolated from the conjunctiva or the infraorbital sinus of sick or fresh dead birds. The organism is one of the most difficult to isolate because it has specific growth requirements, has an intimate dependence on the host species it colonizes, and grows slowly (may take 2 to 3 weeks to grow) on artificial media.
There are other techniques that may be used to confirm the presence of M. gallisepticum: Direct Immunofluorescence on agar media, Polymerase Chain Reaction (antibodies to M. gallisepticum) and Rapid Plate Agglutination (serum antibodies to M. gallisepticum). A study in 2001 found Polymerase Chain Reaction to be more sensitive than culturing in the detection of M. gallisepticum and samples did not need to be as fresh.
There is no recommended treatment of birds exhibiting clinical signs consistent with mycoplasmosis. The concern in prescribing antibiotic treatment of the birds is that it may result in further spread of the disease and an asymptomatic carrier state, with the potential release of birds with organisms that have acquired antimicrobial resistance. This has often resulted from the treatment of domestic poultry. Mycoplasmal conjunctivitis is highly transmissible and it is possible that the infection could be transferred to other bird species housed nearby.
If mycoplasmal conjunctivitis is observed in birds in a residential area, there are various methods of control which need to be instituted immediately. Infected birds should be collected and humanely euthanized. If it is not possible to collect the diseased birds, encourage them to disperse and minimize contact with other birds by eliminating feeding sites. Bird feeders prolong the life of infected birds that otherwise would be unable to feed. Bird feeders need to be cleaned with a 10% bleach solution and not be put back up for at least 2 weeks. Fecal droppings and old wasted or moldy seeds and hulls should be raked and removed from under feeders. Tube feeders should not be used because of the ease with which surfaces can be contaminated by infected birds feeding. The best way to prevent overcrowding birds is to not supplementally feed them. Availability of food will result in birds congregating and make disease transmission easier. Restricting contact between wild birds and domestic poultry by excluding free-flying wild birds from farms is an effective measure and should be part of any farm's biosecurity policy.
Mycoplasmosis is not a threat to human health. In house finches, dramatic drops in populations have been observed within 2-3 years following the beginning of an epidemic. The population stabilized at approximately 40% of its previous abundance. In domestic poultry and captive reared game birds it can be a significant mortality factor.
