Immunization of pigs with a type 2 modified live PRRSV vaccine prevents the development of a deadly long lasting hyperpyrexia in a challenge study with highly pathogenic PRRSV JX143.

Porcine reproductive and respiratory syndrome virus (PRRSV) has been confirmed to be the underlying cause of the so-called 'porcine high fever disease' (PHFD), a disease that emerged in China in 2006 and subsequently spread over South East Asia. The aim of this study was to investigate whether animals challenged with the Chinese highly pathogenic PRRSV JX143 would be protected by vaccination with single dose of a type 2 modified live virus (MLV) vaccine. Forty-four pigs 17-19 days of age were weighed and randomly assigned to either vaccination with subsequent challenge (V/C, n=20), challenge only (NV/C, n=12) and no vaccination and no challenge (strict controls, n=12). Pigs of the challenged groups (V/C and NV/C) were inoculated intranasally 27 days post-vaccination with PRRSV JX143. Animals were monitored during the subsequent 21 days post challenge and were necropsied at the end of the experiment on day 49. Observations and measurements included body temperature, clinical scores for behavior/general condition, cough and breathing pattern, mortality, serological response and PRRSV viremia via RNA detection. Challenge in the NV/C pigs resulted in 100% morbidity and 67% mortality whereas all vaccinated pigs survived. There was a close association between hyperpyrexia (fever over 41°C) and incidence in mortality, which was completely prevented by vaccination. Clinical symptoms were less severe, and of transient nature only, in the vaccinated pigs. Vaccination did not prevent infection, but reduced the impact of clinical disease and prevented hyperpyrexia associated mortality.