The Commissioner of Agriculture shall cause to be slaughtered all animals found to be suffering from infectious or contagious disease, or suspected of being so infected, when in his opinion it is necessary to have such animals slaughtered for the purpose of preventing the spread of the disease with which the animals are infected or suspected of being infected. No animal, however, shall be slaughtered because it is believed to be suffering from glanders or tuberculosis, or is suspected of being so infected, until the mallein test has been applied to determine the presence of glanders and the tuberculin test has been applied to determine the presence of tuberculosis and such disease is shown to be present, unless the symptoms of such diseases are plainly and visibly present.