In late summer 1999, an outbreak of human encephalitis occurred in the northeastern United States that was concurrent with extensive mortality in crows (Corvus species) as well as the deaths of several exotic birds at a zoological park in the same area. Complete genome sequencing of a flavivirus isolated from the brain of a dead Chilean flamingo (Phoenicopterus chilensis), together with partial sequence analysis of envelope glycoprotein (E-glycoprotein) genes amplified from several other species including mosquitoes and two fatal human cases, revealed that West Nile (WN) virus circulated in natural transmission cycles and was responsible for the human disease. Antigenic mapping with E-glycoprotein-specific monoclonal antibodies and E-glycoprotein phylogenetic analysis confirmed these viruses as WN. This North American WN virus was most closely related to a WN virus isolated from a dead goose in Israel in 1998.

West Nile virus (WNV) was isolated in a flock of 1,200 migrating white storks that landed in Eilat, a town in southern Israel, on August 26, 1998. Strong, hot westerly winds had forced the storks to fly under considerable physical stress before reaching the agricultural land surrounding the town. Most of the flock were fledglings, <1 year old, which had hatched in Europe. Thirteen dead or dying storks were collected 2 days after arrival and submitted to the laboratory for examination. Four WNV isolates were obtained from their brains. Out of 11 storks tested six days after arrival, three had WNV-neutralizing antibodies. Comparative analysis of full-length genomic sequences of a stork isolate and a 1999 flamingo isolate from the USA showed 28 nucleotide (nt) (0.25%) and 10 amino acid (0.3%) changes. Sequence analysis of the envelope gene of the stork isolate showed almost complete identity with isolates from Israeli domestic geese in 1998 and 1999 and from a nonmigrating, white-eyed gull in 1999. Since these storks were migrating southwards for the first time and had not flown over Israel, we assume that they had become infected with WNV at some point along their route of migration in Europe.

During a recent epizootic, losses due to West Nile virus (WNV) infection in young goose flocks were estimated to be far greater than expected if mosquito-borne transmission was the principal route of infection. Contact transmission was investigated experimentally as an alternative explanation. A group of 10, 3-week-old geese were inoculated subcutaneously and placed in one insect-proof room with 20 geese of the same age. A group of 10 geese were housed in an adjacent insect-proof room to serve as an environmental control. All geese in the inoculated group produced antibodies, eight became viraemic and five died between 7 and 10 days after infection. Virus was shed from the cloaca and oral cavity by three geese. Two of the in-contact birds died on days 10 and 17 after infection, and WNV was recovered from another three birds. None of the environmental control group became infected. This result strongly suggests that horizontal transmission of WNV can occur in commercial flocks and may be aggravated if cannibalism and feather-picking of sick geese occur.

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Marek's disease (MD) is a highly contagious lymphoproliferative disease of chickens caused by a herpesvirus, while reticuloendotheliosis (REV) virus is an avian C-type retrovirus that causes bursal and nonbursal lymphomas which often closely resemble MDV lymphomas. To provide a rapid and sensitive means of diagnosing and differentiating between these two neoplastic conditions, we have applied the PCR. The primers chosen to detect MDV sequences flank the 132 bp tandem repeat of the BamHI-H fragment, whose PCR product is specific for serotype 1 MDVs. The primers selected for REV are based on proviral spleen necrosis virus-long terminal repeat (SNV-LTR) sequences, and can identify both defective and non-defective REVs. The PCR for field isolates of both viruses was standardized with DNA extracted from chick embryo fibroblasts (CEF) infected with various Israeli isolates of MDV and REV. All chickens were examined for REV antibodies by ELISA, REV isolation was performed in CEF and the tumours were examined histopathologically. In some instances, MDV- and REV-PCRs were performed on whole blood DNA and tumour DNA from the same bird. Normal and tumour-bearing birds from a total of 16 chicken and turkey flocks were studied by PCR. These included three laying flocks with MD, two pullet flocks with neurolymphomatosis, four laying flocks that were the progeny of a REV antibody positive flock, four other REV antibody positive flocks, one turkey flock and two chicken flocks with an immunodeficiency syndrome. Some flocks were bled two or three times at 1- to 3-month intervals. We also analysed 30 normal grandparent light breeding birds. Compared to virus isolation, PCR was more sensitive in detecting MDV and REV sequences. It appears, therefore, that PCR is an efficacious and sensitive method for differential diagnosis of tumour-bearing and immunodeficient birds.