West Nile Virus and Birds - ABC's Position Statement

Scientists do not yet fully understand the effects West Nile Virus will have on populations of native North American birds. Already, close to sixty species, numbering in the thousands in terms of individual birds, have been diagnosed as having died from the virus in northeastern United States. It remains to be seen if the virus will spread to other regions of the U.S. and Latin America. Recent laboratory tests have shown that West Nile is 100% lethal to crows, but the mortality rate is likely to vary among susceptible species. Historically, immunologically naive birds have suffered devastating population losses due to introduced disease in many parts of the world. In the end, there may be very little that wildlife and health officials can do to contain the spread of West Nile Virus in the U.S. to protect birds.


Mosquito control officials have responded responsibly in doing all that they can to stop the spread of the virus. However, most experts agree that West Nile has likely been established in North America and the time has come for the development of a standardized plan to combat the virus in the future. As a conservation organization, the mission of the American Bird Conservancy is to protect birds and their habitats throughout the Americas. To protect important bird habitat, a balanced perspective on mosquito control for West Nile Virus should reflect the important ecological role of non-target insects as natural mosquito predators, pollinators, and important food resources for birds.


Organophosphates, due to their acute and sub-lethal toxic effects to birds other wildlife, and humans, should not be used for mosquito control. Pyrethroids are highly acutely toxic to non-target and beneficial insets, and to practically all aquatic life. As pesticides are applied to kill a particular target insect, many other non-target insects are killed in the process. This can profoundly disturb the natural processes of the ecosystem. Imbalances in natural ecosystems make them susceptible to introduced plants, animals, and pathogens. In the case of West Nile Virus, the indiscriminate use of pesticides, while attempting to alleviate the risk of human disease, acts to further alter the ecosystems on which native birds and wildlife depend. All pesticides used today in controlling mosquitoes kill important natural predators of mosquitoes to some extent including frogs, fish, and dragonflies. Run-off and aerial drift of sprayed pyrethroids and organophosphates contaminate urban streams, and may persist in natural waters thereby contaminating ecosystems distant from the original site of pesticide application.

Experience with old-world West Nile Virus epidemics demonstrates that West Nile Virus poses similar, if not lesser, mortality risk to humans than the closely related, North America endemic, and bird-reservoired St. Louis Encephalitis virus. Control measures for West Nile should mirror preventive measures currently taken by public health and mosquito abatement officials for St. Louis Encephalitis virus. These control measures generally do not entail the spraying of adult mosquitoes in residential or suburban areas until surveyed mosquito pools and sentinel animals are shown to carry infectious virus. Case studies in New York and in Florida have shown that after many years of spraying adult mosquitoes to control Equine Encephalitis, populations of the targeted disease-carrying mosquito have actually increased. Controlling mosquitoes at the larval stage using relatively non-toxic and selective microbes and insect growth hormone regulators remains the most effective and least environmentally harmful methodology available. Larviciding of mosquitoes is the primary prevention strategy recommended by the U.S. Center for Disease Control and Prevention for St. Louis Encephalitis and West Nile Virus.


Further deliberations on West Nile associated mosquito eradication should examine the cost-benefit ratio of large-scale spraying, especially in light of a significant body of research that shows that mosquito insecticides are harmful to human health, as well as to birds and their surrounding environment. Pyrethroids, widely touted by government and mosquito control officials as harmless to humans and terrestrial wildlife, are suspected endocrine disruptors, and have also been shown to alter chromosomes in human white blood cells. In laboratory studies, many of the organophosphate insecticides can cause neurological disorders, immune dysfunction in children, and cancer in laboratory animals.


Birds are important sentinels for the overall health of the environment. Birds dying from West Nile Virus originally alerted public health officials in the United States to the presence of the virus. Experts agree that the practice of adulticiding for mosquito control in urban and suburban locales will not eradicate the West Nile Virus from wild populations of animal reservoirs and, in the vast majority of cases where insecticides have been sprayed in the recent past, is unwarranted for the protection of humans from the virus. American Bird Conservancy recommends:

  • Spraying with pyrethroids and organophosphates for the control of adult mosquitoes (adulticiding) in attempts to prevent human cases of West Nile virus should mirror those control measures undertaken for the control of St. Louis Encephalitis; a related, endemic North American arbovirus. Using these guidelines, in nearly all instances, adulticiding should not be undertaken as a mosquito control measure.

  • Localized use of registered larvicides should continue in attempt to control populations of mosquitoes likely to come into contact with infected West Nile animal reservoirs and humans. Larviciding should only be undertaken in disturbed or man-made bodies of water, which are of lesser ecological importance. Efforts to eliminate stagnant water and potential mosquito breeding areas around human habitation should be intensified. These efforts must include educating the public on measures they can take around their homes to reduce populations of mosquitoes.

  • lf declines in certain species or populations of birds are observed through traditional monitoring efforts (i.e. breeding bird surveys, banding stations, migration point-counts), it is absolutely vital for effective conservation planning that the reasons for noted declines be accurately identified. Therefore, active monitoring of wild birds for West Nile infection should be expanded to include birds outside of the New York area in order to map the spread of the virus to other areas. Government funds should be made available for monitoring efforts and research on critical questions regarding West Nile Virus in the Western Hemisphere and its potential effects on endemic wildlife.

Facts about the Virus

  • West Nile Virus (WNV) belongs to the family of viruses known as Flaviviruses. Other flaviviruses include the well-know viruses that cause Yellow Fever and Dengue lever. West Nile is most closely related to Japanese Equine Encephalitis and St. Louis Encephalitis, also flaviviruses West Nile Virus is transmitted through a mosquito vector and is classified as an arbovirus (arthropod-borne virus). Person-to-person, or bird-to-person direct transmission is not possible except through blood transfusion or by introducing infectious material into the blood stream. For example, a penetrating injury such as a contaminated needle-stick to a laboratory worker may transmit the virus.

  • St. Louis Encephalitis virus (SLE) is endemic to the Southern United States SLE is reservoired in birds asymptomatically, and is occasionally transmitted to humans. Both West Nile Virus and Saint Louis Encephalitis virus can cause encephalitis in humans, but Saint Louis has a higher mortality rate. Most infections in humans of both viruses occur without symptoms, and only a very small percentage of infected people develop encephalitis. Usually, older persons, who may be immunocompromised develop the encephalitis form of the disease. Approximately thirty people each year die from St. Louis Encephalitis, annually in the U.S.

  • West Nile outbreaks occasionally occur among humans in Europe, Asia, and Africa. These outbreaks usually correspond to a particular set of climate, vector, and viral amplification factors.

  • Birds are the primary reservoir for the virus; however, mammals, notably rodents and horses, are susceptible to West Nile Virus, and can also act as reservoirs.

Facts about the Epidemic in the United States

  • Scientists do not know how West Nile Virus was introduced into the United States Normal interhemispheric migration of certain bird species (e.g. Eurasian Widgeon, Ruff, Black-headed Gull) could have introduced the virus into the U.S. however, birds that would have migrated to North America would most likely harbor the West African strain of the virus, whereas the New York strain is nearly identical to a strain found in the Middle East. Other sources of introduction include imported mammals, such as horses, or imported birds, such as poultry or pet birds. Infected humans could have also acted as carriers.

  • West Nile Virus has been found in 59 wild species of birds in New York. While most often dead birds recovered have been crows (620 American Crows), the virus does not appear to be confined within any taxonomic boundary: Merlins, Cooper's Hawks, Snowy- Owls, Ruffed Grouse, Cormorants, Black Skimmer, Great Blue Herons, five species of warbler, American Robins, bluebirds, and many other species of birds have died. Over 1000 dead birds have been analyzed and found to be positive for the virus. Birds from the Bronx Zoo that have died from to virus include: Chilean Flamingoes, Guanay Cormorants, Black-billed Magpies, Bronze-winged Ducks, lmpeyan Pheasants, Blyth's Tragopan, Snowy Owls, and Bald Eagles.

  • Raccoon, squirrel, chipmunk, rabbits, and three species of bat tested positive for West Nile virus in the New York Area in year 2000.

Facts about Mosquito Abatement Measures: Their effects on West Nile occurrence, wildlife, and humans

  • SLE is mainly transmitted by a common, ornithophilic species-complex of mosquitoes known as Culex pipiens. WNV is also transmitted by Culex pipiens but certain species of Anopheles and Aedes mosquitoes serve as competent vectors, as well. Culex pipiens can breed in polluted water, and is most active at night. Stagnant water sources in and around urban and residential settings are likely breeding areas for C. pipiens. Reduction of sources of non-ecologically important standing water can substantially reduce numbers of C. pipiens.

  • Larviciding is a term to describe the practice of killing mosquitoes when they are in the aquatic, larval stage. It is accomplished most commonly by using Bacillus thuringiensis (Bti) , a naturally occurring soil bacterium. Mosquito larvae eat the product that is made up of the dormant spore form of the bacterium and the associated pure toxin. The toxin disrupts the gut in the mosquito by binding to receptor cells present in insects, but not in vertebrates. Insect growth hormone regulators, such as Methoprene, have been used since 1975 for the control of mosquito larva. Growth hormone regulators work by preventing the maturation of insect larva. Larviciding in ditches and small ponds near human habitation is one of the most effective measures in reducing population numbers of mosquitoes that come into contact with humans. It is also the most cost-effective control measure.

  • Adulticiding refers to the use of a pesticide to kill adult mosquitoes. A variety of chemicals have been used to kill adult mosquitoes. Pyrethrin is a naturally occurring compound that can be extracted from plants of the Chrysanthemum family. Currently, synthetic pyrethrins, called pyrethroids, are sprayed more often in states concerned about West Nile Virus than other pesticides for adult mosquito control. Pyrethroids interfere with sodium channel function in insects, and are very effective. However mosquitoes can develop resistance to them. Also, pyrethroids are extremely toxic to fish arid other aquatic invertebrates and should not be sprayed near natural water sources. Pyrethroids are suspected endocrine disruptors, although their relative low level of persistence in the environment potentially downplays this fact. Malathion and Naled, organophosphates registered for mosquito adulticide use, have been used sparingly, if at all, to combat West Nile Virus. Organophosphates are nerve toxins that affect a broad spectrum of life forms including insects, fish, mammals, and birds. Naled can be highly toxic to birds when used as an adulticide. Naled is highly corrosive to metal surfaces and tends to irritate the eyes and mucous membranes of exposed humans. Malathion is the less toxic of the two, and poses less of a hazard to birds.