Mysterious eye disease ravages domestic finches

Bird lovers who count the birds in the garden feeders and send the results to the scientists closely monitor the disease that halved the number of finches in the house.

The infectious ocular disease that appeared in 1994 has been transformed into a more potent virus and is spreading to new finch species.

Domestic Finches, Purple Finches, Goldfinches, and Evening Grosbeaks are now all affected by "Chaffinch Eye Disease", says the Cornell Lab of Ornithology.

The disease was first reported by amateur ornithologists in 1994. It is caused by a bacterium that has mutated into a stronger form, taking advantage of the fact that infected birds develop partial immunity, but may be infected again if a stronger form of the disease evolves.

The disease brings more mysteries than answers.

First mystery: it is a long-term disease in chickens and turkeys around the world. Birds do not die, but they grow slowly and do not lay as many eggs as healthy birds.

Then, around 1994, he appears in the ancestors of the house. Why? And suddenly, he is deadly for finches (but not for chickens). About half of the North American finch population is gone.

Second mystery: it affects other finches too, but never enough to reduce the population. Goldfinches and grosbeaks can cope with it. And if a scientist takes the bacteria from an infected finch and tries to reinfect a chicken with this sample, it will not cause any disease.

And a third: there is a sudden evolution of a more powerful strain of disease, which ornithologist, Wesley Hochachka of Cornell University, calls "an arms race".

This works in the following way: the original strain makes the birds sick, but many finches survive and their immune system learns to recognize the disease.

The bacterium reacts to the improved immune system, just as germs develop resistance to drugs. He formed a stronger strain and became able to reinfect a bird that fought the weakest form.

In fact, he has already done it twice, first in the East of North America and later in the West.

Neither side has really gained an advantage in the arms race. The birds have improved their immune system and the disease has a stronger attack. In scientific language, he is more virulent.

And although finches are not as common as they used to be, they do not lose numbers either. It's a dead end.

Other bird species do not seem to be bothered at all. "Bacteria seem to come in contact with all kinds of bird species, basically any bird that you would see in your garden or in nearby forests. We are currently detecting signs of "antibodies" against the disease in many species of birds – blackbirds, thrushes and others, but there is no real disease.

It is similar to humans who have antibodies against West Nile virus, which shows that they have been exposed to the virus but have never had the disease.

Hochachka has hints for ornithologists. Regularly clean feeders, especially types with platforms where birds can walk, and install another feeder if any of them becomes too crowded.

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