The Emergence of Major Avian Diseases in North America: West Nile Virus and More


USDA National Wildlife Research Center - Staff, Wildlife Damage Management, Internet Center for Publications (2003).




Some major diseases of free-ranging wild birds have invaded, emerged or reemerged during the last few decades in North America. These diseases have increased in frequency of occurrence, prominence and geographical distribution; and are affecting a wide variety of avian species. Diseases in wildlife have changed during the last few decades from sporadic, self-limiting outbreaks with minor losses to that of frequently occurring mortality events with major losses of wildlife. Some zoonotic diseases of wildlife that affect humans have emerged dramatically worldwide during this same time period. Some of the causes of invasive and emergent diseases are the unprecedented worldwide population growth that has resulted in enormous destruction, fragmentation, and deterioration of natural habitats and in increased human encroachment into wilderness areas. At the same time, international travel and worldwide transport of animals, food products, and movement of disease pathogens have increased dramatically. People and animals can travel rapidly between continents during the incubation period of diseases and introduce new pathogens into natïve populations. Changes in animal production and handling and food processing along with the development of resistance to antibiotics and other antimicrobial drugs have allowed bacterial pathogens to expand and affect wildlife. Finally, changes in major ecosystems and climate changes that affect the distribution and population sizes of wildlife species and disease vectors have enhanced transmission of disease agents and influenced wildlife losses from diseases.




Some of the important emerging diseases are urban geese associated
diseases, antibiotic resistant bacteria, house finch conjunctivitis, trematode infections of waterbirds, avian botulism, avian cholera, avian vacuolar myelinopathy, and West Nile virus (WNV) (Table 1).



Conjunctivitis, an infectious disease caused by the bacteria Mycoplasma
gallisepticum, was identified in house finches (Carpodacus mexicanus) in Washington D.C. in 1994 and then rapidly spread westward to the
Mississippi River within 3 years causing substantial finch mortality throughout their range in the eastern U.S. . Avian vacuolar myelinopathy, an emerging neurological disease syndrome, was discovered killing bald eagles (Haliaeetus leucocephalus) and American coots (Fulica americana) at DeGray Lake, Arkansas, in 1994. The disease is likely caused by a toxin(s) and also affects waterfowl, other avian species, and a few aquatic mammals. The disease distribution has expanded and now occurs at man-made lakes in numerous locations in the southeastern U.S.A trematode parasite, Leyogonimus polyoon, known previously in Eastern Europe was likely introduced in infected snails into Shawano Lake, Wisconsin. The parasite amplified in the intermediate aquatic snail hosts in the lake and was transmitted to American coots causing epizootics in 1996-97 that killed over 12,000 birds. The disease appears unique to that lake environment and has
not yet spread to other locations. Avian cholera, caused by the bacteria
Pasteurella multocida, and avian botulism, caused by type C toxin
produced by the bacteria Clostridium botulinum, are the most common
diseases of North American waterfowl. These two diseases have been
reported for a number of years but have reemerged as a major problem
throughout many areas in the western U.S. causing frequent epizootics
with major bird losses. The bird mortality has also affected a large number of species. An aberrant form of type C avian botulism has emerged at the Salton Sea in southern California where fish are affected and serve as a source of toxin for fish-eating birds. An epizootic in these birds occurred there in 1996 and was responsible for a major loss of 15 to 20% of the western population of white pelicans (Pelacanus erythrorhynchos) and of a large number of the endangered brown pelican (Pelacanus occidentalis). Avian botulism now causes annual mortality of pelicans at the Salton Sea. The rapidly increasing populations of urban Canada geese (Branta canadensis) are contaminating recreational waters and land use areas used by the public with pathogenic bacteria that may pose some human health risks. Birds may also be the source of and disseminators of emerging pathogenic bacteria that affect livestock at confined animal feeding operations and dairies.




West Nile virus was previously known only from Africa, the Middle East, Europe, and western Asia until it was introduced into the U.S. in New York City (NYC) in 1999. The virus quickly became established causing a regional epizootic in the bird population, mostly in Accs (Corvus brachyrhynchos), followed by a localized human epidemic of 62 cases and seven deaths. The strain of WNV introduced was especially virulent for many North American bird species and caused significant mortality in corvids (crows, jays, magpies, and related species). Mortality in the avian host species for the other arboviruses in the U.S. (St. Louis encephalitis, eastern equine encephalitis, and western equine encephalitis viruses) is unusual, but the significant bird mortality from WNV quickly became useful to public health officials to detect the presence of the virus.


State-wide surveillance was started in 2000 in the Atlantic and Gulf coastal states by the Centers for Disease Control and Prevention (CDC) to track the expansion of WNV from the affected area in the northeast. The surveillance system used multiple techniques: dead bird surveillance (virus testing of
dead wild birds), monitoring antibody conversions in sentinel chickens,
mosquito capture and testing, and veterinary and human surveillance. New molecular technology to detect virus/antigen in specimens (polymerase chain reaction, PCR) provided rapid testing of birds and mosquitoes and aided in tracking the movement of the virus, and sensitive and specific serologic testing enable the rapid detection and subsequent confirmation of human and equine cases. A rapid field diagnostic assay (VecTest) was developed to test mosquitoes for WNV antigen and is being evaluated as a rapid field test for corvids who excrete large amounts of virus when infected. If successful, this field testing capability will allow field biologists to obtain rapid information about WNV infections in dead birds. A national system (ArboNET) for surveillance data was established and maintained by CDC to allow weekly reporting and updating of WNV surveillance information by all states involved in the surveillance and maps of the reported information were prepared weekly by the U.S. Geological Survey.


The surveillance network detected the early reappearance of the virus in
crows in May 2000 within the area previous affected by the virus (epi-
center) indicating its survival through the winter of 1999-2000. By the end of the summer transmission season in 2000, WNV in birds was detected in 12 states and a total of 12,961 dead birds were submitted for virus testingmand 4,305 (33%) were virus positive. Accs comprised 58%
of the birds tested and 89% (3,824) of the positive bird. In 2001, WNV
reappeared even earlier in the spring in a larger area in the northeastern
states and a new infected area was detected in northern Florida that was
likely establish- ed there in the fall of 2000 by southward migrating birds
traveling from the infected zone in the northeast. The virus distribution
quickly expanded to 27 eastern states and Ontario, Canada accompanied
by increases in human and equine cases, dead birds, and positive mosquitoes. The rapid geographical expansion of WNV to the southeastern and midwestern states in 2001 was likely aided by the seasonal movement of migratory birds that move along four major migratory corridors in North America each season between breeding areas in the north and wintering areas in the south.


The increased disease intensity and dramatic geographical expansion of
WNV in 2002 was remarkable. Virus activity began early in the season in Louisiana and was detected subsequently in a number of the previously affected eastern states and Canada. New foci of WNV transmission were soon discovered in Texas followed by states in the Great Plains and Rocky Mountain region, and in the central provinces of Canada. Virus activity became diffused across a large region of the North American continent affecting 44 states, Washington D.C., and five Canadian provinces, and caused a more than fifty-fold increase in the number of human cases (58 to >4100) and nearly twenty-fold increase in equine cases (738 to >14,000) in the U.S. as a consequence of similar increases in wild bird infections and mortality (Figure 1). In 2002, new methods of direct transmission between humans through organ transplants, blood transfusions, infant nursing, and intrauterine infection became evident. The large number of vertebrate host species affected increased and included captive rocky mountain goats, sheep, reindeer, additional exotic bird species, and domestic pets. A large die-off of owls and hawks may have been caused by WNV and captive-reared alligators died from WNV infection. Dissemination of WNV by migratory birds appeared more apparent as the virus spread throughout the Mississippi Flyway and the Central Flyway to the west in 2002. Further evidence of the role of migratory birds in disseminating WNV is the recent discovery of WNV transmission on Caribbean Islands and in Mexico.


The extensive wild bird mortality associated with WNV and its occurrence within backyards in suburban/urban neighborhoods is unique among most avian diseases (except for house finch conjunctivitis) and is alarming to the public. There is also a raising concern among avian biologists as a possible serious threat to populations of some migratory and nonmigratory avian species.




The geographical extent of the emergent diseases of wild birds and their
rapid expansion, variety of species affected, novel hosts involved, transitory nature of many bird species, difficulties in diagnosing diseases, and the lack of adequate resources for wildlife are some of the factors limiting the management and control of these diseases. The need for surveillance, monitoring, diagnosis, and timely reporting of wildlife diseases, particularly those with domestic animal and human health risks, has frequently been expressed. The recent establishment of a surveillance, monitoring and research program within Wildlife Services of APHIS, USDA, will address this need and should provide better information in the future to appropriately manage some of the major wildlife diseases.



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