West Nile virus (WNV), an arthropod-borne virus belonging to the family Flaviviridae, had been recognized in Africa, Asia and the south of Europe for many decades. Only recently, it has been associated with an increasing number of outbreaks of encephalitis in humans and equines as well as an increasing number of infections in vertebrates of a wide variety of species. In this article, the data available on the incidence of WNV in vertebrates are reviewed. Moreover, the role of vertebrates in the transmission of WNV, the control of WNV infections in veterinary medicine as well as future perspectives are discussed. A wide variety of vertebrates, including more than 150 bird species and at least 30 other vertebrate species, are susceptible to WNV infection. The outcome of infection depends on the species, the age of the animal, its immune status and the pathogenicity of the WNV isolate. WNV infection of various birds, especially passeriforms, but also of young chickens and domestic geese, results in high-titred viremia that allows arthropod-borne transmission. For other vertebrate species, only lemurs, lake frogs and hamsters develop suitable viremia levels to support arthropod-borne transmission. The role of vertebrates in direct, non-arthropod-borne transmission, such as via virus-contaminated organs, tissues or excretions is less well characterized. Even though direct transmission can occur among vertebrates of several species, data are lacking on the exact amounts of infectious virus needed. Finally, the increased importance of WNV infections has led to the development of killed, live-attenuated, DNA-recombinant and chimeric veterinary vaccines.

In the present study, the outcome of an inoculation of equine peripheral blood mononuclear cells (PBMC) with equine herpesvirus type 1 (EHV-1) was studied in vitro. Cytoplasmic and plasma membrane expression of viral antigens, intra-and extracellular virus titres, and plaque formation in co-culture were determined. EHV-1 replicated in monocytes, although in a highly restricted way. Viral antigens were found at maximum levels (8n7 % of the monocytes) at 12 h post-infection. The infection was productive in 0n16 % of the monocytes. The virus yield was 10 0n7 TCID 50 per productive cell. In a population of resting lymphocytes, 0n9 % of cells were infected and less than 0n05 % produced infectious virus. After prestimulation with different mitogens, the number of infected lymphocytes increased four to twelve times. The susceptible lymphocytes were T-lymphocytes. In mitogenstimulated lymphocytes, clear expression of viral antigens was found on the plasma membrane.Equine herpesvirus type 1 (EHV-1), a member of the Alphaherpesvirinae, is a major pathogen of horses, responsible for respiratory disorders, abortion, neonatal foal disease and neurological disorders. Starting from 4-6 days after EHV-1 infection, an extensive cell-associated viraemia is detected, which lasts until 9-14 days after infection (Gibson et al., 1992). T-lymphocytes seem to be the most susceptible of the peripheral blood mononuclear cells (PBMC) (Scott et al., 1983). Viraemia is associated with, for example, T-cell lymphopenia and appearance of blastic cells (McCulloch et al., 1993) and may occur in the presence of virus-neutralizing antibodies (Doll & Bryans, 1963 ;Mumford et al., 1987). Carried by infected leukocytes, EHV-1 spreads to internal organs.Information on the interaction between EHV-1 and leuko- et al. (1983). In vitro-infected, non-stimulated PBMC gave a higher number of plaques than mitogen-stimulated PBMC, in contrast to the results obtained in vivo.The main purpose of this study is to obtain more detailed information about the replication of EHV-1 in freshly isolated, equine PBMC and to investigate the effect of mitogen stimulation on the replication kinetics of EHV-1 in lymphocytes.PBMC were isolated by density centrifugation of heparinized blood from adult, infection-immune horses on Ficoll-Paque and afterwards separated into two subpopulations by plasma-mediated adhesion as described by Nauwynck & Pensaert (1994). Adherent cells consisted predominantly of monocytes and non-adherent cells consisted predominantly of lymphocytes. Remaining monocytes in the lymphocyte-enriched population were removed by plastic adhesion during 1 h at 37 mC. The composition of the PBMC and subpopulations was determined by flow cytometry (FACSCalibur ; Becton Dickinson) using MAbs HB88A and DH59B (VMRD) to stain T-lymphocytes and monocytes, respectively (Tumas et al., 1994), and a hyperimmune goat serum against horse IgM (Kirkegaard and Perry Laboratories) to stain B-lymphocytes. In the population of PBMC, the percentages of T-lymphocytes, B-lymphocytes...

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Absence of viral antigens on the surface of equine herpesvirus-1-infected peripheral blood mononuclear cells: a strategy to avoid complement-mediated lysis Equine herpesvirus-1 (EHV-1) may cause abortion in vaccination-and infection-immune horses. EHV-1-infected peripheral blood mononuclear cells (PBMCs) play an important role in virus immune evasion. The mechanisms by which infected PBMCs can avoid destruction by EHV-1-specific antibody and equine complement were examined. The majority of EHV-1-infected PBMCs (68?6 %) lacked surface expression of viral antigens and these cells were not susceptible to complement-mediated lysis. In infected PBMCs with surface expression of viral antigens, 63 % showed focal surface expression, whereas 37 % showed general surface expression. General surface expression rendered infected PBMCs susceptible to lysis by antibody and complement (from 5?4 to 31?2 % lysed cells depending on the concentration of antibody and complement). Infected PBMCs with focal surface expression showed significant lysis only in the presence of high concentrations of antibody and complement. Thus, the absence of surface expression protects infected PBMCs against complement-mediated lysis.