Animal viral diseases and global change: bluetongue and West Nile fever as paradigms

Jiménez-Clavero, Miguel Á

Viremia is transient, starting at 3-5 days after infection in sheep somewhat later in other ruminants, peaking at about 7-10 days, declining slowly thereafter (Darpel et al., 2007) . The duration of viremia is variable, depending on the species affected, and can last for 5-6 weeks in sheep, up to 8 weeks in cattle Bonneau et al., 2002) . Virus detection in blood is commonly used as a proof of infection in diagnostic tests based in specific reverse transcription-polymerase chain reaction (RT-PCR) methods (Jimenez-Clavero et al., 2006) . The host's immune system responds to infection by generating serotypespecific neutralizing antibodies, which are widely recognized as key factors in the long-term protective response against infection (Schwartz-Cornil et al., 2008) . Seroconversion occurs between 1 and 2 weeks after infection. Serogroup-specific antibodies are directed primarily to the VP7 protein of the virus capsid, which are commonly detected by enzyme immunoassay (ELISA) employing recombinant VP7 antigen (Afshar et al., 1992; Mecham and Wilson, 2004) , while serotype-specific neutralizing antibodies (which are detected by specific neutralization tests) recognize epitopes in the VP2 capsid protein (DeMaula et al., 1993; Pierce et al., 1995) . The presence of specific antibodies in serum samples is another common diagnostic test, which indicates that infection has occurred, or that the animal has been vaccinated. There are live attenuated and inactivated vaccines, which are specific for each serotype.