Of the 23 species of Mycoplasma bacteria known to infect birds, Mycoplasma gallisepticum (MG) is the most significant to wild species. Until recently, MG was only known to cause disease in domestic poultry and occasionally wild turkeys. In the winter of 1994 there was an outbreak of mycoplasmal conjunctivitis caused by MG in house finches in the mid-Atlantic United States. The strain of MG isolated from the house finches differed from the strain that causes disease in poultry, so the source of the outbreak is currently unknown.
House finches were introduced to the eastern United States in the 1940’s when caged finches from California were released on Long Island, New York. As a result, the eastern house finch population is highly inbred, which has made the birds less resistant to disease. Since the emergence of mycoplasmal conjunctivitis, house finch populations have experienced major declines in the eastern United States.
MG was once considered a pathogen primarily of domestic chickens, where it can cause major economic loss, wild and domestic turkeys, and captive game birds. MG emerged as a new infectious agent of songbirds in the 1990’s and is now known to infect house finches, American goldfinches, purple finches, pine grosbeaks, and evening grosbeaks. MG is not known to infect humans.
House finches have been reported with conjunctivitis caused by MG in every state in their eastern population range. The disease in finches spread up and down the east coast and continued westward, and has recently been found as far west as Montana. Mycoplasmal conjunctivitis has also been reported in finches in Quebec and Ontario, Canada. MG rarely causes disease in wild turkeys in the United States; sporadic cases have been reported in Georgia, California, Colorado, and Texas. MG occurs worldwide in domestic poultry and other captive birds.
MG is transmitted primarily via direct contact with ocular discharge from infected birds. The disease is highly transmissible, especially when birds feed or roost together in high densities. Crowded bird feeders provide the ideal siutation for MG transmission. The bacteria can also be passed from infected parents to their young. Mycoplasmosis can be transmitted over short distances when secretions are aerosolized. Mycoplasma bacteria are relatively fragile and cannot survive for long periods of time outside of the host. However, the bacteria can remain infective for short periods in dust, litter, feathers, and on bird feeders, meaning the disease can be transferred indirectly by contact with contaminated materials. Humans can also transfer the bacteria to new areas on boots and clothing.
House finches with mycoplasmal conjunctivitis will exhibit swelling around the eyes, crusty eyelids, and watery ocular and/or nasal discharge. Extreme swelling and crusting can lead to impaired vision and at times blindness. In severe cases, birds may become debilitated, depressed, lose body condition, and die. Some birds can act as carriers of MG while showing no clinical signs of the disease. Wild turkeys infected with MG often experience swelling of the sinuses around the eyes, which may impair vision. Turkeys also may exhibit sneezing, coughing, nasal discharge, difficulty breathing, depression, and weight loss.
Mycoplasmosis is diagnosed based on clinical signs and the isolation of M. gallisepticum by culture or other laboratory tests.
Treatment of wild birds with MG is not recommended. Although antibiotics may clear clinical signs, birds can become asymptomatic carriers that can spread the bacteria to new locations.
Management efforts to control mycoplasmal conjunctivitis in finches are focused on transmission prevention. Bird feeders and baths should be kept clean and spaced far enough apart to prevent crowding. Only clean, fresh feed should be provided at feeders. Tube-style feeders seem to be particularly problematic in MG transmission. During outbreaks of mycoplasmosis, bird feeding should be discontinued to eliminate this source of transmission. Wild turkeys can acquire MG from infected domestic poultry flocks, though transmission from wild birds to domestic poultry has never been reported. Wild birds should be prevented from coming into contact with domestic flocks. Wild turkeys transferred from one state to another in restocking efforts are screened for MG to avoid spreading avian mycoplasmosis. Wildlife rehabilitators should avoid taking in birds suspected of having mycoplasmal conjunctivitis because the disease is very difficult to treat and could infect other patients.