A myriad of avian pathogens exist, but several in particular are noteworthy for their effects on birds.
- In 1994, House Finches began exhibiting conjunctivitis (severely swollen eyes) from a previously unknown bacterial strain of Mycoplasma gallisepticum. By 1998 it had decimated House Finch populations in Vermont. Other species found with infections include American Goldfinch, Purple Finch, and Evening Grosbeak.
- West Nile virus (WNV), an old world pathogen found in New York City in 1999, was first documented in the state in 2000 when a hermit thrush, found dead in southern Vermont, tested positive.
- From 2003 – 2005 the Vermont Department of Health found 116 dead birds of 14 species with WNV in 12 counties. Over 78 percent were American Crows and Blue Jays.
- The mosquito Culex pipiens is the dominant vector, and feeds predominantly on American robins, which are believed to be a reservoir host for WNV.
- WNV declined in virulence during its rapid spread, and a diverse host community apparently dampened and slowed transmission.
- Other pathogens that have gained attention in the media pose little if any effect to birds or humans.
- Past outbreaks of avian influenza type ‘A’ virus (H5N1) caused worldwide concern, but has not been detected in the Americas. All subtypes of type ‘A’ influenza viruses occur naturally in wild birds, especially waterfowl, most causing little or no threat to bird or human health.ß
- Malaria parasites also appear to be innocuous to most infected birds, yet it is quite prevalent. A sample of nearly 2,500 Vermont birds of 110 species found nearly 44 percent infected.
- It is unknown if negative synergistic health effects from other pathogens or stressors may occur with some infected individuals.