Susan L. Clubb, DVM diplomat ABVP Avian
West Nile virus (WNV) was introduced into the United States in 1999 in New York. Initially described by Dr. Tracy McNamara and her colleagues at the Bronx Zoo. From that point, WNV has been the subject of intense monitoring and research as it moved southward along the Atlantic coast of the United States.
WNV was first reported in Florida in the fall of 2001. At that time cases consistent of WNV infection were observed in psittacine birds. Shortly after that we initiated a study to evaluate the use of the Fort Dodge Equine West Nile Virus Vaccine in psittacines.
In 2001, a total of 48 human cases of WNV encephalitis or meningitis were reported in 9 states. New York (12), Florida (10), New Jersey (7), Connecticut (6), Maryland (6), Pennsylvania (3), Massachusetts (2), Georgia (1), and Louisiana (1). Among these 48 cases, 27 (56%) were males (median age 70 years; range: 36 to 90); dates of illness onset ranged from 13 Jul to 15 Oct 2001; and 5 patients (10 per cent) died. (CDC Report-Tue 20 Nov 2001) Also in 2001, 189 WNV infections in horses were reported from 15 states (Alabama, Connecticut, Florida, Georgia, Illinois, Indiana, Kentucky, Louisiana, Massachusetts, Mississippi, New York, North Carolina, Pennsylvania, Tennessee, and Virginia). A total of 4604 crows and 1497 other birds with WNV infection were reported from 27 states and the District of Columbia.
WNV was first reported in Florida in the fall of 2001. The Florida Department of Health (DOH) and the Florida Department of Agriculture and Consumer Services (FDACS) closely monitor cases of WNV and announced that the first cases of 2002 in Florida occurred in February. A reporting system is located on the Florida Fish and Wildlife Conservation Commission's Web site at http://wld.fwc.state.fl.us/bird . If people do not have access to the Internet, they may report dead birds at 1-800-871-9703. Following is a brief discussion of cases of presumptive West Nile disease cases observed in psittacine birds in South Florida in the Fall of 2001 and spring of 2002. Initial results of the vaccine trial are also discussed.
Lesser Sulphur Crested Cockatoo – Adult male breeder was presented exhibiting severe neurological disease and seizures. He died shortly after presentation. Histopathology revealed lesions typical of a viral encephalitis consistent with either West Nile encephalitis or St. Louis encephalitis (SLE). WNV was not specifically identified on PCR or brain tissue.
Jardine's parrot – Presented with ataxia and uncoordination of approximately 2 weeks duration in October 2001. This bird was an indoor pet in Ft Lauderdale. Symptomatic treatment did not resolve signs although the bird was otherwise in good condition. Blood sample collected on the day of presentation was seropositive by serum neutralization test for West Nile antibodies.
Black lory housed outdoors in Miami developed sudden onset of ataxia and uncoordination, weight loss, and diarrhea in November 2001. Improved in general condition but retained intention tremors after symptomatic therapy. Continues to have neurological deficit but otherwise is stable to date. He was seropositive by serum neutralization (SN) test for West Nile antibodies.
A 38 year old male hybrid macaw in a zoological park in the Miami area developed sudden onset of neurological signs and enteritis. Tentative diagnosis was pancreatitis. Supportive treatment failed to stabilize him and he subsequently died after a 3-week course of illness. He was seropositive by SN test for West Nile antibodies. Histopathology revealed vague neurological lesions. Virus isolation was negative.
An adult female African grey parrot was presented with neurological signs. She subsequently died but was of advanced age and also had athlerosclerosis. She was seropositive by SN, negative on virus isolation but had histopathologic lesions suggestive of a viral encephalopathy. Virus isolation was negative.
Subsequent to these illnesses, in December 2001 and January 2002, 69 clinically normal birds were tested serologically for West Nile antibodies. Of the 69 birds, 33 were tested by hemagglutination inhibition (HI) and 36 were tested by SN. All of these birds were housed at the same facility as the lory and the hybrid macaw which had clinical disease and tested seropositive. Most of these birds were adult macaws with a few cockatoos and amazons. All tests were negative.
Also, subsequent to these clinical cases, tissue samples for over 40 necropsy cases of psittacines were submitted to the Florida Dept of Health diagnostic laboratory for PCR testing and virus isolation. All of these cases were from birds which died in Miami-Dade or Palm Beach counties in Florida. Virus was not detected by PCR or isolated in any of these cases.
Recent cases (April 2001) are strongly suspicious for WNV. All were presented as clinical cases of neurological disease in Palm Beach County and one in Martin County.
A 2 year old blue & gold, owner presented due to suspected toxic reaction to antihistamines. Duration of signs were 4 days and bird showed severe ataxia and uncoordination. After flushing the crop and fluid therapy the signs dramatically improved. The bird tested positive for West Nile antibodies (1:40) by HI. The bird was clinically normal 3 weeks after the initial blood sample was collected. Retest serology is in progress at the time of writing.
An adult female breeder timneh grey parrot was presented with severe acute onset of neurological signs and again treated symptomatically. She was a laying hen and lipemic, so fatty embolism was suspected. Blood collected at presentation was seropositive by HI (1:20). Retest serology is in progress.
A number of suspicious cases have occurred in a flock of breeding cockatiels in Palm Beach County. At least 4 birds in a flock of approximately 100 have shown acute onset of neurological signs. Two of these birds were submitted for histopathological examination and lesions suggestive of viral encephalopathy were observed. Two birds showing clinical signs were seropositive by HI. Two birds were negative for virus isolation. Some affected birds have recovered spontaneously.
Control methods for WNV are aimed at vaccination and reduction of mosquito transmission. Continual monitoring of suspect cases is also important to develop a clear understanding of the disease.
Fogging of outdoor aviaries should be considered in areas where mosquitoes are prevalent and the disease has been diagnosed or suspected. We have been using this technique at Hurricane Aviaries for several years in times of threat of vector borne disease (specifically when pox virus has been diagnosed in the vicinity). No adverse effects have been recognized.
We use the the Burgess Bug Killer Fogger. We use the propane-powered model, but an electric one is available. We are using the Burgess Bug Killer Insect Fog and use it full strength in the fogger. The active ingredient is Resmethrin 0.2%, which is a synthetic pyrethrin and has very low toxicity. We fog under and around the cages in the early morning and late afternoon.
The operator wears a face mask to prevent excessive inhalation as the operator has a much longer period of exposure than the birds. The operator is also advised to wear long sleeves and long pants to prevent excessive accumulation of the chemicals on the skin. Some people are sensitive or allergic to the product.
Synthetic pyrethroids are considered safe by the US Environmental Protection Agency and do not pose unreasonable risks to wildlife or the environment. However, they are toxic to bees and to fish. For that reason the EPA has established specific precautions on the label to reduce such risks. For more information to go the EPA Web site http://www.epa.gov/pesticides.
Insect screening and insect trapping devices are also alternative methods for reducing the risk of mosquito transmission of WNV.
In December 2001 and January 2002 a study was undertaken to test the Fort Dodge Equine West Nile Vaccine in parrots.
The U.S. Department of Agriculture granted a conditional license for the first WNV vaccine for horses in August 2001 (Fort Dodge, Equine West Nile Virus Vaccine). The vaccine is labeled for vaccination of healthy horses as an aid in the prevention of disease caused by West Nile virus. The recommended dose is two 1-mL doses, 3 weeks apart, plus annual revaccination. The vaccine is available only from a licensed veterinarian.
The vaccine was safety tested in 649 horses of many breeds, ages and sizes in five states. It was shown to be 96.28% free of local or systemic reactions. Each serial is tested to ensure there is no surviving virus in the vaccine. It is adjuvanted with MetaStim™ for enhanced efficacy. Vaccinated horses in Fort Dodge Animal Health studies developed WNV-neutralizing antibodies, as measured by the standard plaque reduction neutralization test. Induction of significant neutralizing antibody levels gives a reasonable expectation of efficacy. The product license is conditional. Efficacy and potency studies are in progress.
A vaccination study including 59 psittacine birds (and 2 turacos and 1 toucanette) was initiated in December 2001. Birds used in this study were housed outdoors in Palm Beach and Miami-Dade counties. Species included macaws, cockatoos, amazons, conures, cockatiels, African greys, and quaker parakeets. All birds were bled at the time of the initial vaccination to establish a baseline of exposure. Each bird served as its own control. Three weeks later they were bled a second time when the second dose of vaccine was administered. A third blood sample was collected 3 weeks after the second dose of vaccine. Dosage and route of administration was varied to get an impression of dosage requirement. Vaccinated psittacines developed WNV-neutralizing antibodies, as measured by the standard, plaque-reduction, neutralization test. Induction of significant neutralizing antibody levels gives a reasonable expectation of efficacy. However, challenge studies were not preformed.
At the time of this writing the results are being analyzed. A full report will follow, however initial impressions are good. It appears that the best results were with a low dose (0.25ml) administered subcutaneously. No adverse reactions were observed in any birds. Each bird was carefully examined for granuloma formation or other adverse reactions at the site of vaccination and none were found.
In 1999, an epidemic of WNV encephalitis occurred in New York City (NYC) and two surrounding New York counties. Simultaneously, an epizootic among American crows and other bird species occurred in four states. Indigenous transmission of WNV had never been documented in the western hemisphere until this epidemic. In 2000, the epizootic expanded to 12 states and the District of Columbia, and the epidemic continued in NYC, 5 New Jersey counties, and 1 Connecticut county. In addition to these outbreaks, several large epidemics of WNV have occurred in other regions of the world where this disease was absent or rare >5 years ago. Many of the WNV strains isolated during recent outbreaks demonstrate an extremely high degree of homology that strongly suggests widespread circulation of potentially epidemic strains of WNV. The high rates of severe neurologic illness and death among humans, horses, and birds in these outbreaks are unprecedented and unexplained. (Martin & Gruber)
WNV was found throughout New York State in year 2000. The epicenter was located in New York City with a high level of activity in the immediately surrounding counties, including Rockland, Westchester, Nassau, and Suffolk. During 2000, WNV testing was performed by the Wadsworth Center on 3,687 dead birds, representing 153 species, 46 families, and 18 orders. There were 1,203 WNV-positive birds, representing 63 species, 30 families, and 14 orders. The percentage of WNV-positive birds was 33% for all birds tested throughout the state, with no significant difference in infection rates in migratory versus resident birds, although significantly more resident birds were submitted for testing. The highest apparent mortality for the entire season was observed in American crows in Staten Island, a location that also showed the highest minimal infection rate in Culex pipiens complex mosquitoes. Studies examining tissue tropism of WNV in corvids and noncorvids from the epicenter and from remote locations indicated that the kidney was the most consistently infected tissue in birds, regardless of level of infection. The brain was the next most consistently positive tissue. The differences in infection among the tissues were most apparent when low levels of virus were present. Experimental mouse inoculation demonstrated a classical flavivirus infection pattern. (Kramer)
Steele et al. studied birds from two wildlife facilities in New York City, New York, that died or were euthanized and were suspected to have WNV infections. Using standard histologic and ultrastructural methods, virus isolation, immunohistochemistry, in situ hybridization and reverse-transcriptase polymerase chain reaction, we identified WNV as the cause of clinical disease, severe pathologic changes, and death in 27 birds representing eight orders and 14 species. Virus was detected in 23/26 brains (88%), 24/ 25 hearts (96%), 15/18 spleens (83%), 14/20 livers (70%), 20/20 kidneys (100%), 10/13 adrenals (77%), 13/ 14 intestines (93%), 10/12 pancreata (83%), 5/12 lungs (42%), and 4/8 ovaries (50%) by one or more methods.
Cellular targets included neurons and glial cells in the brain, spinal cord, and peripheral ganglia; myocardial fibers; macrophages and blood monocytes; renal tubular epithelium; adrenal cortical cells; pancreatic acinar cells and islet cells; intestinal crypt epithelium; oocytes; and fibroblasts and smooth muscle cells. Purkinje cells were especially targeted, except in crows and magpies. Gross hemorrhage of the brain, splenomegaly, meningoencephalitis, and myocarditis were the most prominent lesions.
Immunohistochemistry was an efficient and reliable method for identifying infected cases, but the polyclonal antibody cross-reacted with SLE virus and other flaviviruses. In contrast, the in situ hybridization probe pWNV-E (WN-USAMRIID99) reacted only with WNV. (Steele, et al.)
A total of 12 human WNV cases was reported to the Florida State Health Office during 2001. The number of dead birds that were found with WNV in Florida was over 1000, along with close to 200 sentinel chickens testing positive for the disease. FDACS reports over 400 horses in the state contracted WNV. Due to the heavy load of WNV in the environment and our inexperience with both this new emerging disease and the new vaccine just released last year, The Florida State Veterinarian is recommending that all horse owners vaccinate their horses with the full two-dose series again this year.
WNV has similarities to the native flavivirus, SLE virus, but has unique features not observed with SLE virus or with WNV in the old world. The primary route of transmission for most of the arboviruses in North America is by mosquito, and infected native birds usually do not suffer morbidity or mortality. An exception to this pattern is eastern equine encephalitis virus, which has an alternate direct route of transmission among nonnative birds, and some mortality of native bird species occurs. (McLean, et al.)
The strain of WNV which is now in the United States is unique in that it causes significant mortality in exotic and native bird species, especially in the American crow (Corvus brachyrhynchos). Experimental studies were conducted at the USGS National Wildlife Health Center in which crows were inoculated with a 1999 New York strain of WNV, and all experimentally infected crows died. Control crows in regular contact with experimentally inoculated crows in the same room but not inoculated with WNV also succumbed to infection. The direct transmission between crows was most likely by the oral route. Inoculated crows were viremic before death, and high titers of virus were isolated from a variety of tissues. (McLean, et al.)
WNV encephalitis in horses, previously reported in Africa, Asia, and Europe, occurred for the first time in the Western Hemisphere in 1999. The causative agent, WNV, is a flavivirus maintained in nature by a bird-mosquito cycle. The disease in horses is manifested primarily by ataxia of variable severity. Outbreaks of encephalitis may have a case fatality rate in excess of 40%, although this virus infection is inapparent in some horses. Early evidence indicates that WNV has overwintered in the northeastern United States and poses a threat for future disease occurrences in horses. (Ostlund) The use of vaccination to protect horses from West Nile encephalitis is now widespread.
A number of zoos and other facilities have tested or are currently testing West Nile vaccinations for use in birds using the recently licensed Equine WNV Vaccine.
Researchers in Israel conducted a series of vaccination trials in which 3-week old geese were immunized with an attenuated, commercial, flavivirus vaccine derived from Israel turkey meningoencephalitis virus (TME). Birds were challenged 2 weeks later with a low Vero cell passage of WNV by the intracerebral route. Another vaccination trial was based on WNV harvested from infant mouse brain, inactivated with formalin, and oil adjuvanted. (Malkinson)
A single injection given either subcutaneously or intramuscularly resulted in 75% protection of the vaccinated groups, while two injections spaced two weeks apart resulted in 94% protection. Groups of geese, vaccinated at the farms and challenged under controlled conditions in the laboratory, showed levels of protection ranging from 39% to 72% for TME vaccine and 52% and 80% for WNV vaccine. The lower levels of protection are attributable to flocks being affected with intercurrent infections at the time of vaccination. (Malkinson)
Senne studied the pathogenicity of WNV in domestic chickens and concluded that they are a potential reservoir for WNV. Seven-week-old chickens derived from a specific-pathogen-free flock were inoculated subcutaneously with 1.8 x 10(3) 50% tissue culture infectious dose of a crow isolate of WNV in order to observe clinical signs and evaluate the viremic phase, gross and microscopic lesions, contact transmission, and immunologic response. There were no observable clinical signs in the WNV-inoculated chickens during the 21-day observation period. However, histopathologic examination of tissues revealed myocardial necrosis, nephritis, and pneumonitis at 5 and 10 days postinoculation (DPI); moderate to severe nonsuppurative encephalitis also was observed in brain tissue from one of four inoculated birds examined at 21 DPI. WNV was recovered from blood plasma for up to 8 DPI. Virus titers as high as 10(5)/ml in plasma were observed at 4 DPI. Fecal shedding of virus was detected in cloacal swabs on 4 and 5 DPI only. The WNV also was isolated from myocardium, spleen, kidney, lung, and intestine collected from chickens euthanized at 3, 5, and 10 DPI. No virus was isolated from inoculated chickens after 10 DPI. Antibodies specific to WNV were detected in inoculated chickens as early as 5 DPI by the plaque reduction neutralization test and 7 DPI by the indirect fluorescent antibody test. Chickens placed in contact with inoculated chickens at 1 DPI lacked WNV-specific antibodies, and no WNV was isolated from their blood plasma or cloacal swabs throughout the 21 days of the experiment. (Senne)
Very preliminary observations suggest that WNV is a health risk for psittacine birds but the morbidity and mortality rates appear to be very low. Additionally the case mortality rate appears to be low and older birds seem to be more severely affected. Establishing a definitive diagnosis for West Nile disease has been challenging. Virus isolation has been unrewarding which might indicate transient infections occur in psittacines. Several cases exhibiting acute onset of clinical signs were seropositive indicating possibly rapid seroconversion. It appears by the findings of this study that psittacines may in fact react similarly to chickens as described by Senne et al.
Initial impressions of vaccine efficacy are promising. No adverse reactions were observed upon vaccination. The majority of the birds vaccinated seroconverted. Further analysis will be reported in future publications. In cases where indicated, it is the authors opinion that vaccination is a viable option.
Lean RG, Ubico SR, Docherty DE, Hansen WR, Sileo L, McNamara TS, West Nile virus transmission and ecology in birds. Ann N Y Acad Sci 951:54-7 2001 Dec.
Kramer LD, Bernard KA, West Nile virus infection in birds and mammals, Ann N Y Acad Sci 951:84-93 2001 Dec.
Steele KE, Linn MJ, Schoepp RJ, Komar N, Geisbert TW, Manduca RM, Calle PP, Raphael BL, Clippinger TL, Larsen T, Smith J, Lanciotti RS, Panella NA, McNamara TS, Pathology of fatal West Nile virus infections in native and exotic birds during the 1999 outbreak in New York City, New York.Vet Pathol 37:208-24 2000 May.
Malkinson M, Banet C, Khinich Y, Samina I, Pokamunski S, Weisman Y, Use of live and inactivated vaccines in the control of West Nile fever in domestic geese. Ann N Y Acad Sci 951:255-61 2001 Dec.
Centers for Disease Control and Prevention (CDC) through ArboNET and verified by states and other jurisdictions as of Tue 20 Nov 2001.
Martin AA, Gubler DJ, West Nile encephalitis: an emerging disease in the United States. Clin Infect Dis 33:1713-9 2001 Nov 15.
Ostlund EN, Andresen JE, Andresen M, West Nile encephalitis. Vet Clin North Am Equine Pract 16:427-41 2000 Dec.
Senne DA, Pedersen JC, Hutto DL, Taylor WD, Schmitt BJ, Panigrahy B Pathogenicity of West Nile virus in chickens. Avian Dis 44:642-9 2000 Jul-Sep.
Additional information about WNV activity is available at