His arguments for the pointlessness of these experiments were original; they followed from his experiences with traditional medicine and his well-developed critique of civilization. His contemporary allies, however, ignored these arguments and simply used the Wagner name.
The open letter led directly to Wagner’s much-discussed essay Religion und Kunst, in which, among other things, he paints a horrific scenario of the unimpeded development of science and technology.
This according to “Richard Wagner als Gegner von Tierversuchen: Ein visionärer Zivilisationskritiker” by Ulrich Tröhler and Joachim Thiery (WagnerSpectrum XI/1  pp. 73–104). This journal, along with many others, is covered in our new RILM Abstracts of Music Literature with Full Text collection.
Above, the composer with his dog Pohl; below, no horses were annoyed during this performance.