Changes

======Serology======

======Serology======

The presence of viral Abs within 24hours of the initial viraemia typically precedes clinical signs.  Ab titre increases sharply then deteriorates over 6 months.  Samples taken when clinical signs appear are likely to miss the Ab peak and may thus demonstrate a decreasing titre.  Serological confirmation of EEE or WEE virus infection requires a four-fold or greater increase or decrease in antibody titre in paired serum samples collected 10-14 days apart.  A presumptive diagnosis can be made on a single sample if an unvaccinated horse with appropriate clinical signs has antibody against only EEE or WEE virus.  Maternal-derived Ab may interfere with diagnosis in foals.  The serum half-life of colostral Ab in foals is around 20days.

Ab titre increases sharply within 24 hours of the initial viraemia, before clinical signs are apparent.  It then deteriorates over 6 months.  Samples taken when clinical signs appear are likely to miss the Ab peak and will demonstrate a decreasing titre.  Thus, serological confirmation of Eastern or Western EEV infection requires a four-fold or greater increase OR decrease in Ab titre in paired serum samples taken 10-14 days apart.  A presumptive diagnosis can be made on a single sample if an unvaccinated horse with suggestive clinical signs has Ab against only Eastern or Western EEV.  Colostral-derived Ab has a serum half-life of around 20days and may interfere with diagnosis in foals.

   

   

*Complement fixation (CF): to avoid anti-complementary effects, serum should be separated from blood as soon as possible. CF Ab against both Eastern and Western EEV is less useful for serological diagnosis because it appears relatively late and does not persist.

*Complement fixation (CF): to avoid anti-complementary effects, serum should be separated from blood as soon as possible. CF Ab against both Eastern and Western EEV is less useful for serological diagnosis because it appears relatively late and does not persist.

*Haemagglutination inhibition (HAI): titres of 1/10 and 1/20 are indicative, titres of 1/40 and above are positive.

*Haemagglutination inhibition (HAI): titres of 1/10 and 1/20 are indicative, titres of 1/40 and above are positive.

*ELISA may be used to detect viral-specific IgM to the surface glycoprotein of Venezuelan EEV, from 3 days post-onset of clinical signs up to 21 days post-infection.  This is useful in acute infections where convalescent serum samples are unobtainable.   

*ELISA may be used to detect viral-specific IgM to the surface glycoprotein of Venezuelan EEV, from 3 days post-onset of clinical signs up to 21 days post-infection.  This is useful in acute infections where convalescent serum samples are unobtainable.   

*The PRN test is very specific and can differentiate EEE and WEE infections.  It is performed in duck embryo fibroblast, Vero, or BHK-21 cell cultures.  Serum is tested against 100 plaque-forming units of virus.  Endpoints are based on a 90% reduction in the number of plaques compared with the virus control.

*The plaque reduction neutralization (PRN) test is very specific and can differentiate EEE and WEE infections.  It is performed in duck embryo fibroblast, Vero, or BHK-21 cell cultures.  Serum is tested against 100 plaque-forming units of virus.  Endpoints are based on a 90% reduction in the number of plaques compared with the virus control.

Most commonly, the PRN test or a combination of PRN and HAI tests is used to detect Ab against Eastern and Western EEV. Cross-reactivity occurs between Ab against these viruses in the CF and HAI tests.

Most commonly, the PRN test or a combination of PRN and HAI tests is used to detect Ab against Eastern and Western EEV. Cross-reactivity occurs between Ab against these viruses in the CF and HAI tests.