title: Characterization of Puerto Rican West Nile Virus isolates in mice.
abstract: BACKGROUND
  West Nile virus (WNV) is a neurotropic arbovirus that was first isolated in 1937 in the West Nile District of Uganda. The virus emerged in New York in 1999 and is now endemic in North America (2007). The first virus isolates from Puerto Rico were obtained in 2007 from a chicken (PR20wh) and a mosquito pool (PR423). Our study further characterized these viral isolates using in vitro plaque morphology assays and in vivo using a Balb/c mice pathogenesis model.
  
  
  RESULTS
  In the in vitro experiments, PR WNV isolates produced significantly smaller plaques in Vero cells compared to the New York 1999 strain (NY99). For the in vivo experiments, PR WNV isolates were propagated in mammalian (Vero) and insect (C6/36) cell lines and then inoculated in Balb/c mice. When WNV was propagated in Vero cells, we observed a trend towards significance in the survival rate with PR20wh compared to NY99 (log rank, p = 0.092). Regardless of whether the viral isolates were propagated in Vero or C6/36 cells, we found a significantly greater survival in mice infected with PR20wh strain, when compared to NY99 (log rank, p = 0.04), while no statistical difference was detected between PR423 and NY99 (p = 0.84). The average survival time (AST) in mice was significantly lower in C6/36-derived PR423 when compared to C6/36-derived NY99 (t-test, p = 0.013), and Vero-derived PR423 (t-test, p < 0.001). Eight days post infection in mice the viral load in brain tissue for Vero-derived PR423 was significantly higher when compared to NY99 and PR20wh.
  
  
  CONCLUSIONS
  These results suggest that the PR WNV isolate, PR20wh, is a less pathogenic strain in mice than NY99. Moreover, we found that PR423 is a pathogenic isolate that causes faster mortality than NY99, when propagated in C6/36.
authors.affiliation: Department of Microbiology and Medical Zoology, University of Puerto Rico, Room A-355 UPR-Medical Sciences Campus, PO Box 365067, San Juan, 00936-5067, Puerto Rico. hjl0@cdc.gov.
authors.forename: Elba V
authors.initials: EV
authors.lastname: Caraballo
authors.affiliation: Centers for Disease Control, Division of Vector Borne Diseases, Dengue Branch San Juan, San Juan, Puerto Rico. enh4@cdc.gov.
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Hunsperger
authors.affiliation: Department of Microbiology and Medical Zoology, University of Puerto Rico, Room A-355 UPR-Medical Sciences Campus, PO Box 365067, San Juan, 00936-5067, Puerto Rico. idali.martinez@upr.edu.
authors.forename: Idalí
authors.initials: I
authors.lastname: Martínez
mesh_terms: D000818:Animals; D002460:Cell Line; D002645:Chickens / Q000821:virology; D009033:Culicidae / Q000821:virology; D004195:Disease Models, Animal; D008807:Mice, Inbred BALB C; D011647:Puerto Rico; D016019:Survival Analysis; D013997:Time Factors; D010948:Viral Plaque Assay; D014774:Virulence; D014901:West Nile Fever / Q000473:pathology* / Q000821:virology*; D014902:West Nile virus / Q000302:isolation & purification* / Q000472:pathogenicity*