Abstract

West Nile virus (WNV) causes a potentially fatal disease in birds with mortalities reported in over 250 avian species.¹ Experimental vaccine protocols using an equine approved vaccine have been attempted with mixed or unknown efficacy.^3-5 Serum neutralization assays are one way to assess the immune response to WNV vaccine in birds;² it is unknown at this time whether a given antibody titer correlates with protective immunity. The arrival of WNV at the Los Angeles Zoo was monitored by on-site mosquito and wild bird sampling which established 7 July 2004 as the exact date of arrival of the virus on zoo grounds. Prior to this date, 63 birds representing 28 species had been vaccinated using 1.0 ml of equine vaccine (Fort Dodge West Nile Innovator) on a schedule of days 0, 21 and 60, followed by an annual booster. Blood was collected at the time of each vaccination with plasma sent to Cornell University’s diagnostic lab for neutralizing antibody testing.

Out of 63 birds vaccinated for WNV, 31 maintained negative titers and 32 birds seroconverted. Three birds that initially seroconverted had negative titers on future rechecks. Two birds had antibodies to St. Louis encephalitis virus. Two birds of African origin had positive titers to WNV before vaccination. No consistent trend was seen between vaccination and antibody response either in general or within any taxonomic group. These data suggest that antibody titers, as detected by neutralizing antibody tests, are not a reliable means to assess vaccination status or protective immunity. No vaccinated birds have contracted WNV at the LA Zoo, while two unvaccinated psittacines developed clinical WNV and survived with medical management. Vaccination may impart protective immunity despite inconsistent antibody titers.

Literature Cited

1.  Centers for Disease Control and Prevention. Vertebrate ecology. The following 284 bird species have been reported to CDC's West Nile virus avian mortality database from 1999-present. Available at: www.cdc.gov/ncidod/dvbid/westnile/birdspecies.htm. Accessed April 9th 2005.

2.  D’Agostino J.J., and R. Isaza. 2004 Clinical Signs and results of specific diagnostic testing among captive birds housed at zoological institutions and infected with West Nile virus. J. Am. Vet. Med. Assoc. 224:1640–43.

3.  Llizo, S.Y. 2004. An update of West Nile virus vaccine trials in a bird collection. Proc. Am. Assoc. Zoo. Vet. Annu. Meet. 245–247.

4.  Llizo, S.Y. 2003 Management of West Nile virus in zoo birds. Proc. Am. Assoc. Zoo. Vet. Annu. Meet. 117–121.

5.  Nusbaum, K.E., J.C. Wright, W.B. Johnston, A.B. Allison, C.D. Hilton, L.A. Staggs, D.E. Stallknecht, and J.L. Shelnutt. 2003. Absence of humoral response in flamingos and red-tailed hawks to experimental vaccination with a killed West Nile virus vaccine. Avian Dis. 47: 750–52.