One great thing about the gorillas’ outdoor exhibit at the Pittsburgh Zoo is that it attracts loads of birds. The babbling stream, the clean pool, and extensive lawn are what bring them. Mostly it’s sparrows, but I have also seen starlings, robins, blue jays, indigo buntings, grackles, cardinals, mourning doves, red-winged blackbirds, and the occasional peregrine falcon flyby.
One morning, after letting the gorillas into their outdoor exhibit, I noticed a ragged looking sparrow hopping around the yard looking unable to fly. I assumed that one of the gorillas had injured it and it was suffering from a broken wing. This assumption has precedent: about ten years ago Mimbo, Mrithi’s father (five years dead, R.I.P.) caught a peafowl that had strayed into their exhibit and had broken its wing. After conferring with Roseann, we decided to leave it be and hope for the best. The next day we found it dead in the moat. That was the end of that, or so we thought.
A few days later we saw another sparrow, ragged and damaged. The following day we found another dead bird. Were the gorillas maiming the birds for sport? No; it soon became obvious that the malady had nothing to do with gorillas at all. We began to notice sickly sparrows all around the zoo; hopping around but unable to fly. I had a real mystery on my hands: The Mystery of the Sick Sparrows.
I mentioned my concerns to one of the veterinarians. She said that she heard that West Nile Virus was experiencing a surge in the Pittsburgh area.
West Nile Virus was first identified in Uganda in 1937 and in the U.S. in 1999. Since its introduction to America it has spread quickly. How does it spread? In a word: mosquitoes. Mosquitoes carry the highest amounts of virus in the early fall, which is why the rate of the disease increases in late August to early September. The risk of disease decreases as the weather becomes colder and mosquitoes die off. Although many people are bitten by mosquitoes that carry West Nile virus, most do not know they've been exposed. Few people develop severe disease or even notice any symptoms at all. In fact, approximately 90% of West Nile Virus infections in humans are without any symptoms at all.
Let’s not put all the blame on the infamous mosquito though; birds hold their share of the blame. Though mosquitoes spread the disease, birds make it all possible. The birds are amplifying hosts, developing sufficient viral levels to transmit the infection to other biting mosquitoes which go on to infect other birds.
By now, with the cold weather settling in around Pittsburgh, I have seen many less sick birds. By the beginning of October I estimated that the West Nile Virus had infected about 20% of the sparrows around the zoo; now it is probably less than 1%.
Case closed.
