A private horse farm in Montgomery County, Maryland, has been under a voluntary quarantine since March 25 while veterinarians investigate the cause of illness in 12 horses, four of which have been euthanized, the Maryland Department of Agriculture said May 25.Veterinarians are testing for several diseases, including but not limited to Equine Herpesvirus. No cases have been identified at any other facilities.The farm wasn't identified. Montgomery County is located southwest of Baltimore and northwest of Washington, D.C."There has been a great deal of concern among horse owners in the Poolesville area that Equine Herpesvirus may be present and may have been transmitted from the quarantined farm," Maryland state veterinarian Dr. Phyllis Cassano said in the release. "We want them to know that this is not the case. There are no other farms involved in this situation, and there has been no confirmed diagnosis."Horse owners can be assured that there will be no movement of equines to or from the affected farm until veterinarians are certain that there is no risk that an infection could be transmitted. The owners of the farm have been extremely cooperative at this very difficult time."Veterinarians involved in the case will be release information as it becomes available. Information about best management practices for livestock and poultry health can be found online at www.mda.state.md.us.