The West Nile virus may no longer be a death sentence for crows. In a new study from the College of Veterinary Medicine, wildlife experts describe the successful treatment and release of five American crows infected with the deadly disease — the first known crows to survive the West Nile virus.
“Their survival may indicate a change in crow’s immunity or a shift in the behavior of the virus,” said Dr. Cynthia Hopf-Dennis, clinical assistant professor at Janet L. Swanson Wildlife Hospital and first author of the article.
The paper, published May 2 in the Journal of Avian Medicine and Surgery, details the crows’ survival and the species’ potential adaptation to the virus. The authors monitored the crows’ immune response while they were in the hospital and found that the birds release the virus in respiratory secretions for at least 93 days after being infected.
This is an important discovery that shows that they can both pass it on to others and be tested through this method during those 13 weeks after infection, the researchers said.
“It’s an interesting interaction that we know is unique across species. I suspect there are more across the country that survive,” Hopf-Dennis said. “It’s likely that our North American species are adapting to the virus, and I wouldn’t be surprised if the virus also adapts to the species it has encountered over the past 20 years.”
The first known cases of West Nile virus in the United States were discovered in 1999, when an outbreak in New York City caused a 45% decline in regional American crows. Since the virus’s initial spread, its severity has fluctuated in North America due to a multitude of vector, host, and environmental factors that are still not fully understood.
The West Nile virus, which is transmitted by mosquitoes, can infect more than 250 species of birds, but is deadliest to corvids and birds of prey. These groups include American crows, blue jays, ravens, eagles, hawks and owls.
The virus causes severe dehydration, acid-base and electrolyte imbalances, cell damage, multi-organ inflammation and necrosis. Infected birds may appear fuzzy, weak and unable to fly. Infections usually occur in the spring, summer, and early fall, when mosquitoes are more common.
Among corvids, West Nile virus is most lethal to American crows, which previously had a 100% mortality rate. A contributing factor is the bird’s common resting place. After a mosquito has infected the crow, the crow secretes large amounts of the virus in its feces, which can affect bird-to-bird transmission during dormancy.
West Nile virus can infect both mammals and birds, especially horses and humans, which are considered dead-end hosts. Human cases vary from year to year and can be fatal. In 2020, 33 people of the known 540 infections died from West Nile virus.
The study covers American crows admitted to Janet L. Swanson Wildlife Hospital from 2013-20, when 32 crows were hospitalized, 25 of which tested positive for West Nile virus. Of those 25, only the five American crows described in this study survived to be released.
While there is no cure or prescription treatment for birds suffering from West Nile virus, the five survivors at Cornell received supportive care, including fluid therapy, anti-inflammatory medication, vitamin B supplementation, prophylactic antifungals and antiparasitic medications. This is the gold standard approach for birds suffering from the disease, although it can vary depending on a hospital’s resources. The birds were regularly tested for the virus after infection, which showed “presumably positive” in one bird 93 days later.
“This 93-day period is a longer moulting interval than previously reported for American crows,” Hopf-Dennis said. “More frequent testing would be useful in future cases to track how long birds shed West Nile virus.”
Although there is a vaccine for captive birds, the level of immunity varies by species, and widespread vaccination of wild birds is not feasible. “Their main protection will be the maternal immunity that the crows can pass on to their offspring,” Hopf-Dennis said. “My hope is that when we bring crows back to the wild after treatment and rehabilitation, they can contribute to a stronger population that is able to survive the virus and provide some degree of protection to their offspring.”
The study’s co-authors include Dr. Elizabeth Bunting, associate professor of practice in the Department of Public and Ecosystem Health; dr. Sara Childs-Sanford, DVM ’99, assistant professor in the Zoological Medicine Section and Chief of Section at Janet L. Swanson Wildlife Hospital; and Anne Clark, Bartle Professor in the Department of Biological Sciences at Binghamton University.
Said Hopf-Dennis, “For other wildlife rehabilitators and veterinarians who have experienced the 100% mortality rate historic for crows infected with West Nile virus, this study reminds us that they are worth treating, can survive to release – and can thrive in the wild, even after infection.”
Melanie Greaver Cordova is deputy director of communications at the College of Veterinary Medicine.