White spot syndrome virus (WSSV) has caused substantial global economic impact on aquaculture, and it has been determined that strains can vary in virulence. In this study, the effect of viral load was evaluated by infecting Litopenaeus vannamei with 10-fold serial dilution of tissue infected with strain WSSV Mx-H, and the virulence of four WSSV strains from north-western Mexico was assessed along with their variable number of tandem repeat (VNTR) genotypes in ORF75, ORF94 and ORF125. The LD of the Mx-H strain was a dilution dose of 10 ; the mortality titre was 10 LD per gram. In shrimp injected with 10 to 10 LD , no significant virulence differences were evident. Using mortality data, the four WSSV strains grouped into three virulence levels. The Mx-F strain (intermediate virulence) and the Mx-C strain (high virulence) showed more genetic differences than those observed between the Mx-G (low-virulence) and Mx-H (high-virulence) strains, in ORF94 and ORF125. The application of high-viral-load inocula proved useful in determining the different virulence phenotypes of the WSSV strains from the Eastern Pacific.

Introducción En México, el cultivo de camarón es una industria muy importante ya que genera empleos en zonas no productivas para la agricultura. Actualmente existen 482 granjas con un área estimada en 41,750 ha y una producción de 78,000 TM por año. La mayoría de las granjas se concentran en los estados de Sonora y Sinaloa (Gutiérrez-Venegas 2005).