Are crows the new plague carriers?

What's up with the West Nile virus, crows, and New York City?

Last updated 22-Feb-05


I'm afraid I haven't been able to spring time to update this page as I would like. The following information was current in Jan 2000. Lots has happened since then, so now it's more historical than current.

For more current information about the disease, check out the West Nile Virus site at the Environmental Risk Analysis Program, at Cornell University, or the Audubon Society West Nile page.


17 Dec 1999 issue of Science had two articles on West Nile:

Anderson, J.F., et al. 1999. Isolation of West Nile Virus form Mosquitoes, Crows, and a Cooper's Hawk in Connecticut. Science 286 (5448): 2331-2333.

Lanciotti, R.S., et al. 1999. Origin of the West Nile Virus Responsible for an Outbreak of Encephalitis in the Northeastern United States. Science 286 (5448): 2333-2337.

Information received from Dr. Lisa Reed, Rutgers University about monitoring for West Nile in New Jersey.

Received 27 October 1999 - USGS Wildlife Health Alert #9902B. As I expected, lots of species have the virus.


Crows have been receiving a lot of attention in the media in regards to being infected with the West Nile encephalitis virus in the New York City area. The entire story is not yet known, but here are a few facts related to the case. (Please note that I am not an active participant in this story, as I live about 4-5 hours away. I have been speaking with a number of people directly involved, but everything presented here is only second hand, or further removed. I have relied more on my conversations with people in the know than on reports in the media, however. If no citation is made, the statements are my own conclusions.)

My summary of the situation is that crows are the main victims, not the bad guys. They are the ones that are dying, and it is probably other birds (or horses) that will be the primary means by which the virus spreads (if it does).


The Disease

For information on the disease, check out the Center for Disease Control's web pages.

More information on the virus can be found at <>. The following information has been taken from that page, with a bit of translation and interpretation.

West Nile virus is a member of the Japanese encephalitis antigenic complex of the genus Flavivirus, family Flaviviridae. All known members of this complex (Alfuy, Japanese encephalitis, Kokobera, Koutango, Kunjin, Murray Valley encephalitis, St. Louis encephalitis, Stratford, Usutu, and West Nile viruses) are transmissible by mosquitoes and many of them can cause febrile, sometimes fatal, illnesses in humans. West Nile virus was first isolated in the West Nile district of Uganda in 1937 and was subsequently isolated from patients, birds, and mosquitoes in Egypt in the early 1950s. The virus was soon recognized as the most widespread of the flaviviruses, with geographic distribution including Africa and Eurasia. Until now, it has not been recorded in North America.

Mosquitoes, largely bird-feeding species, are the principal vectors of West Nile virus. The virus has been isolated from 43 mosquito species, predominantly of the genus Culex.

Wild birds are the principal hosts of West Nile virus. The virus has been isolated from a number of wetland and terrestrial avian species in diverse areas. High, long-term viremia, sufficient to infect vector mosquitoes, has been observed in infected birds, and the virus persists in the organs of inoculated ducks and pigeons for 20 to 100 days. Migratory birds are therefore hypothesized to be instrumental in the introduction of the virus to temperate areas of Eurasia during spring migrations. Rarely, West Nile virus has been isolated from mammals (several species of mice and hamsters, European Hare, an African fruit bat, camels, cattle, horses, dogs, a bushbaby, humans). Mammals are less important than birds in maintaining transmission cycles of the virus in ecosystems. Only horses and lemurs seem to support West Nile virus circulation locally. Frogs (Rana ridibunda) also can harbor the virus, and their donor ability for Cx. pipiens has been confirmed

In Europe, West Nile virus circulation is confined to two basic types of cycles and ecosystems: rural cycle (wild, usually wetland birds and bird-biting mosquitoes) and urban cycle (domestic birds and mosquitoes feeding on both birds and humans). Circulation of West Nile fever in Europe is similar to that of St. Louis encephalitis in North America, where the rural cycle of wild birds alternates with the urban cycle of domestic birds.

The Crow Connection

Crows, both American and Fish, are being associated with the disease outbreak because they are the obvious victims: they are dying in unusually large numbers.

I first received calls about oddly sick crows in the New York City area the last week of August 1999. Several people called me about finding sick and unusually approachable crows, as well as an unusual number of dead crows. Finding dead birds (other than roadkills or window strikes) is unusual, as most sick birds seem to find some quiet and hidden place to die. Finding one dead crow doesn't mean much, but finding several crows within a short period of time is highly unusual and indicates that something odd is going on.

I first suspected a poisoning event, especially since someone had been poisoning pigeons in Central Park just previous to the finding of dead crows. The state pathologist's lab, however, got some of the dead crows and tested them for a number of things, but found no indication of pesticide poisoning. Instead, the symptoms were consistent with a viral infection. When I spoke with them in mid September they had sent samples to a lab in Colorado, but had not yet received the results.

At the same time the outbreak of "St. Louis encephalitis" was being reported in much the same areas where crows were being found dead. It seemed odd to me that crows would be dying from that disease, as most birds that are exposed to it get mildly sick, but do not die from it. A new disease of some other source of crow mortality made more sense.

On 23 September, the State of New York Department of Health released the information that several bird specimens from the NYC area had been diagnosed as being infected with a virus that most closely resembled West Nile virus. These birds included a crow from Westchester County, as well as birds from the Bronx Zoo.

Exact connections between the birds and the human illnesses have been difficult to establish (as is to be expected).

Are crows spreading the disease (or will they be)?

Probably not, for several reasons. First is that crows don't move all that far in the first place. Crows are only partially migratory (see my discussion of this topic on the crow faq page). Some of the crows breeding in the New York City area will probably move a bit farther south in the winter, but nearly all will stay put. They can make rather long daily movements, but not long distance migration. By the time the Canadian vacationers come down to mingle with the New York crows (late October/ November) most of the mosquitoes will be inactive and the transmission rate will be very low.

The second reason that I doubt crows will be major carriors of the disease is that many of them are dying from it. It is hard to judge from such a long distance away, but the number of dying crows reported is frighteningly large. It is far more likely that other species that get the disease but are not killed by it will be the most important source of the virus. (See the 27 October Wildlife Health Alert for a listing of species known to have the virus.)

If crows start dying in more southern states, it is likely that they will be local southern crows infected via other bird species.

Why are crows being hit so hard from this disease and not other birds?

Good question; no one knows. Every species is different, of course, and different responses to the same stressors are to be expected. As stated by the CDC web site: "Birds usually do not show any symptoms when infected with West Nile virus. However, natural disease due to the virus has been observed in a pigeon in Egypt (7), and inoculation of certain avian species (e.g., pigeons, chickens, ducks, gulls, and corvids) causes occasional encephalitis and death or long-term virus persistence (7,10,17,18). Chick embryos may be killed by the virus (8)."

Of course, this is a new disease never encountered by the birds in North America before. Any time a population is exposed to a brand new form of a disease, epidemics can happen because they have no defenses.

Reports of dead American Crows, Fish Crows, and Blue Jays indicate that the vulnerability is a family thing.

Where did the disease come from?

No one knows. It has been suggested that it came from a bird smuggled into the US from Africa or Asia. Because NYC is such a huge port of entry into this country, this possibility seems reasonable. Legally imported live birds must first undergo a month of quarantine, in theory to prevent just such an incursion of disease into the country. Illegally imported birds obviously do not pass through the quarantine. A migrant bird from Africa, Asia, or Europe is an even less likely possibility, but the probability if non-zero. Africa is pretty much out (or if not, the birders want to know where that African bird is!), but Europe is not. A very few European migrants show up on the East Coast every year, but only shorebirds would be candidates for an August arrival.

The 17 Dec 1999 issue of Science had two articles on West Nile. One [Lanciotti, R.S., et al. 1999. Origin of the West Nile Virus Responsible for an Outbreak of Encephalitis in the Northeastern United States. Science 286 (5448): 2333-2337] described the strain found in North America as matching a strain isolated from the brain of a dead "goose" in Israel in 1998 (no species given, nor any indication if it was a wild or domestic bird). The authors state "The WN virus could have entered the Western Hemisphere through a number of mechanisms, including travel by infected humans, imporation of illegal birds or other domestic pets, or unintentional introduction of virus-infected ticks or mosquitoes."

What should you do if you find a dead or dying crow

(or other bird)?

Because the crows appear to be dying from this disease, they can act as sentinels for its detection. Just like the canary in the mineshaft, crows may be the first to go and can alert us to the presence of the virus. Similarly, because domestic bird die-offs may be related to infection with West Nile-like virus, reports of bird die off may be an important surveillance mechanism for monitoring spread of the disease. If you find a dead bird you should contact your county health department, and they can advise you on further action (check the government listing of your telephone book). Counties will complete a report form for the State Health Department. All the health departments want very badly to track this virus, so our help will be appreciated.

Do not worry about the dead birds themselves. There is no evidence of bird to human transmission of the virus; rather, the virus is spread by infected mosquitoes (which only bite LIVE birds). Dead birds can be disposed of by burying (three feet deep) or by double-bagging them and disposing of them with regular trash. Gloves should be worn when handling them, but masks or other respiratory protection is not needed.

If you are outside the known disease area and you find a dead bird, DO NOT PANIC. Every bird dies at some time, and most discovered dead birds will not be infected. As I stated before, one dead bird doesn't mean much, but many dead ones might.


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Last updated 22-Feb-05