We evaluated loop-mediated isothermal amplification (LAMP) as a means of detecting equine herpesvirus type 1 (EHV-1) DNA directly from nasal swabs. To increase the sensitivity, we added a step in which the samples were heat-treated to the original LAMP procedure. The detection limit of the LAMP assay with heat treatment was 10 times more sensitive than the original LAMP assay even when the DNA extraction step was omitted. In addition, the LAMP assay with heat treatment was more sensitive than the original LAMP assay and the polymerase chain reaction using clinical samples. The LAMP assay with heat treatment is easy to perform and so should be applicable to the diagnosis of EHV-1 infections in clinical laboratories.