Antiviral Activity of Tilmicosin against Highly Pathogenic Porcine Reproductive and Respiratory Syndrome (HP-PRRS) isolated during 2009 to 2015

Highly Pathogenic Porcine Reproductive and Respiratory Syndrome (HP-PRRS) is currently widespread in China and several Southeast Asian countries. Management, biosecurity and vaccination have been used to encountered with PRRS virus outbreaks. However, immune evasion strategies and various antigenic heterogeneities of the causative viruses could hamper those strategies. Nowadays, successful controlling procedures are still limitED. One of the alternative ways to control the virus in the field is chemotherapeutic. Tilmicosin, a tylosin derivative macrolide antibiotic which is recommended for treatment and prevention of respiratory disease associated with bacterial infection in pig, can induce positive impacts in PRRS infected herds and reduce type 2 PRRS virus infection in vitro. However, it is known that some viral gene may experience mutagenesis or antigenic change under antibodies or drug pressure. In this study, minimum inhibitory concentration of tilmicosin to HP-PRRS virus which were isolated from several provinces in Northeastern part of Thailand during 2009 to 2015 were tested.

MARC-145 cell lines were maintained in Dulbecco’s Modified Eagle Medium. The virus used in this study were HP-PRRS virus which were isolated from infected pigs in the Northeastern part of Thailand in year 2009, 2010, 2011, 2012, 2014 and 2015. Some of those farms added tilmicosin in pig feed to control or prevent PRRS virus in their farm. Tilmicosin used in this study was obtained from Huvepharma Thailand.

Twenty thousand MARC-145 cells/well were seeded in 96-well micro-titer plates and infected with PRRS virus at MOI of 0.05. Tilmicosin was then treated to infected cells 24 hours later. The minimum inhibitory concentration (MIC) of tilmicosin which protected 50 percent of cells to produce CPE were recorded and calculated using Reed and Muench’s formula.

The mean MIC of tilmicosin to HP-PRRS virus isolated in year 2009, 2010, 2011, 2012, 2014 and 2015 were 26.33±0.47 (n=2), 26.46±0.20 (n=8), 26.17±0.28 (n=3), 26.5 (n=1), 26.50(n=1) and 26±0 (n=2) µg/ml respectively.

Tilmicosin has been added to pig feed aiming to control clinical sign of PRRS/HP-PRRS virus in pig farm ever since. However, tilmicosin inhibit PRRS/HP-PRRS virus in a dose-dependent manner. Under low concentration of tilmicosin, viral replication would be inhibited partially. The genetic mutation of PRRS virus could occur in that condition and could lead to the resistance of the virus to tilmicosin. Our result indicates that no resistance of HPPRRS virus to tilmicosin have been developed. However, further study in genetic level is still needed.

Disclosure of Interest: None Declared.