House finches are songbirds native to the western United States that have been introduced to the eastern U.S. and are now found across North America. In the 1990s, house finches in the eastern U.S. and Canada suffered a major decline due to an eye disease called mycoplasmal conjunctivitis, caused by the bacterium Mycoplasma gallisepticum. This disease causes swelling, irritation, and crusting of the eyes which can lead to blindness and death from starvation or predation. Understanding the impacts and spread of this disease in house finch populations is important for conservation efforts.
What causes mycoplasmal conjunctivitis in house finches?
Mycoplasmal conjunctivitis in house finches is caused by the bacterium Mycoplasma gallisepticum. This bacteria naturally infects and causes disease in poultry but jumped to house finches, likely through contact at bird feeders. M. gallisepticum infects the conjuctiva of the bird’s eye causing swelling, irritation, and discharge. Infected birds have red, runny, crusty eyes. The disease spreads easily between finches at feeders and through contact. While it does not typically kill birds directly, it can lead to blindness and death from starvation or predation.
What was the impact of the epidemic in the 1990s?
When mycoplasmal conjunctivitis first emerged in house finches in the eastern U.S. in the 1990s, it spread rapidly and caused a severe population decline. The house finch population in the eastern U.S. dropped by up to 60% within a few years of the disease’s emergence. The epidemic spread north and south through the eastern U.S. and Canada over several years, transported by migrating birds. The disease continues to cause conjunctivitis outbreaks in house finch populations in eastern North America leading to chronic population impacts.
Can infected birds recover?
Many house finches are able to recover from mild mycoplasmal conjunctivitis infections. However the disease can also lead to chronic severe infections, especially in highly susceptible young birds. Some finches are able to clear the infection completely, especially if it is caught early. Others develop immunity and become asymptomatic carriers of the disease. They can continue to spread M. gallisepticum to other birds even after recovering from the acute phase of infection. The severity of infection and recovery depends on factors like age, nutrition, and genetics. Recovery rates in the wild are hard to estimate.
What helps birds recover?
Several factors influence a house finch’s ability to recover from mycoplasmal conjunctivitis:
Age
Younger birds seem to be more severely affected than older finches and take longer to recover. Older finches likely have stronger immune systems and may have some acquired immunity if they survived past infections.
Nutrition
Well-nourished birds recover better than malnourished ones. Access to ample food resources supports the immune system and healing.
Antibiotics
Antibiotic treatment, in captivity, can help clear mycoplasma infections. However, treating wild birds is not practical.
Genetics
Some finch populations seem genetically more susceptible to severe disease while others show increased resistance. Genetic factors influence immunity.
Rest and shelter
Access to shelter, nests, and roost sites helps reduce stress and supports recovery for infected finches.
Medications
Eye ointments and anti-inflammatory drugs reduce irritation and may help recovery if applied early.
Isolation
Isolating sick finches may reduce exposure and allow recovery without re-infection. This is only possible in zoos or wildlife rehabilitation.
Do finches develop immunity?
House finches are able to develop some immunity to Mycoplasma gallisepticum after recovering from an infection. Both humoral antibody-mediated immunity and cell-mediated immunity have been found in house finches exposed to M. gallisepticum. Finches that survive an epidemic tend to show higher antibody levels indicating increased immunity. However, immunity is imperfect and birds can become reinfected, especially with high exposure pressure at bird feeders. Immunity also declines over time if birds are not continually re-exposed. So while house finches do develop some immunity, it does not make them completely resistant to future infections. The immune response is better described as tolerance rather than true immunity. Finches with higher tolerance tend to experience milder disease when reinfected.
What strategies help finches survive outbreaks?
House finches employ several strategies to try to survive mycoplasmal conjunctivitis outbreaks:
Dispersal
Birds may disperse away from areas with high disease prevalence. This reduces their exposure and allows recovery. However, dispersal can also spread the disease to new areas.
Altered behavior
Sick finches reduce activity and social contact to prevent spreading the disease. Healthy birds may avoid diseased individuals.
Selective feeding
Finches may preferentially feed on sites with lower contamination. Feeder hygiene helps reduce transmission.
Genetic changes
Populations exposed to the disease over many generations show increased genetic resistance. Natural selection favors disease-resistant traits.
Boosted immunity
Surviving infection provides some acquired immunity against future reinfection and disease severity.
What can be done to help house finches recover?
There are a few main interventions that can assist house finch recovery from mycoplasmal conjunctivitis:
Improve feeder hygiene
Frequent cleaning and disinfection of bird feeders reduces M. gallisepticum levels and transmission between birds.
Provide separate feeding stations
Offering multiple dispersed feeding sites limits crowding and contact between sick and healthy birds.
Offer optimal nutrition
Providing nutritious birdseed, nectar, fruits, and greens supports recovery.
Report outbreaks
Tracking outbreaks helps identify impacted populations for research and conservation.
Support wildlife rehabilitators
Rehab centers have resources to treat and isolate sick finches to allow healing.
Remove sources of stress
Reducing factors like predators and competition improves the chance of recovery.
Reduce crowding
Preventing overcrowded feeders minimizes contact and disease spread.
Promote genetic diversity
Supporting finch populations with diverse genetics increases disease resistance.
What is the long-term outlook for house finches and eye disease?
The long-term outlook for house finches impacted by mycoplasmal conjunctivitis is cautiously optimistic. Finch populations exposed to the bacteria over many generations are evolving increased genetic resistance and immunity. Outbreak severity has declined since the initial 1990s epidemic. However, M. gallisepticum is likely now endemic in eastern finch populations and will continue to cause seasonal conjunctivitis outbreaks, especially in young birds. Maintaining diverse finch populations with access to abundant resources will support continued adaptation. Promoting finch health through improved nutrition, nest sites, and reduced stressors will further help finches persist despite the disease. With supportive conservation measures, house finches can recover from and adapt to mycoplasmal conjunctivitis over time.
| Year | Outbreak Severity |
|---|---|
| 1994-1998 | Severe epidemic, high mortality |
| 1999-2005 | Frequent regional outbreaks |
| 2006-2020 | Moderate seasonal outbreaks |
| 2021-present | Mild occasional outbreaks |
Conclusion
The introduction of mycoplasmal conjunctivitis caused dramatic declines in house finch populations across North America in the 1990s. However, finches have begun recovering as they develop immunity, tolerance, and genetic resistance over time. Conservation efforts focused on improving nutrition, reducing stressors, managing outbreaks, and supporting genetic diversity will further aid finch recovery. While mycoplasmal conjunctivitis remains a threat, especially for young birds, cautious optimism remains that house finches will adapt to this challenge, as they have many others throughout their resilient history across North America.
