Woodrow Wilson's speech to Congress requesting for a declaration of war on Germany referred to the other Central Powers in this way (in bold for my emphasis):

I have said nothing of the Governments allied with the Imperial Government of Germany because they have not made war upon us or challenged us to defend our right and our honour. The Austro-Hungarian Government has, indeed, avowed its unqualified endorsement and acceptance of the reckless and lawless submarine warfare adopted now without disguise by the Imperial German Government, and it has therefore not been possible for this Government to receive Count Tarnowski, the Ambassador recently accredited to this Government by the Imperial and Royal Government of Austria-Hungary; but that Government has not actually engaged in warfare against citizens of the United States on the seas, and I take the liberty, for the present at least, of postponing a discussion of our relations with the authorities at Vienna. We enter this war only where we are clearly forced into it because there are not other means of defending our rights. It will be all the easier for us to conduct ourselves as belligerents in a high spirit of right and fairness because we act without animus, not in enmity towards a people or with the desire to bring any injury or disadvantage upon them, but only in armed opposition to an irresponsible government which has thrown aside all considerations of humanity and of right and is running amuck.

Since the casus belli was Germany's unrestricted submarine warfare, Wilson's initial request to Congress dealt purely with that.

By December Wilson went back to Congress requesting a declaration of war on Austria-Hungary with the justification that it was essentially a vassal of Germany and therefore needed to be dealt with . It also explains why the US did not end up declaring war on Bulgaria and the Ottoman Empire. (Pertinent sentences bolded by me for emphasis)

What shall we do, then, to push this great war of freedom and justice to its righteous conclusion? We must clear away with a thorough hand all impediments to success and we must make every adjustment of law that will facilitate the full and free use of our whole capacity and force as a fighting unit. One very embarrassing obstacle that stands in our way is that we are at war with Germany but not with her allies. I therefore very earnestly recommend that the Congress immediately declare the United States in a state of war with Austria-Hungary. Does it seem strange to you that this should be the conclusion of the argument I have just addressed to you? It is not. It is in fact the inevitable logic of what I have said. Austria-Hungary is for the time being not her own mistress but simply the vassal of the German Government. We must face the facts as they are and act upon them without sentiment in this stern business. The government of Austria-Hungary is not acting upon its own initiative or in response to the wishes and feelings of its own peoples but as the instrument of another nation. We must meet its force with our own and regard the Central Powers as but one. The war can be successfully conducted in no other way. The same logic would lead also to a declaration of war against Turkey and Bulgaria. They also are the tools of Germany. But they are mere tools and do not yet stand in the direct path of our necessary action. We shall go wherever the necessities of this war carry us, but it seems to me that we should go only where immediate and practical considerations lead us and not heed any others

Ref:

Scott, James Brown. War Between Austria-Hungary and the United States The American Journal of International Law, vol. 12, no. 1, 1918, pp. 165–172

Woodrow Wilson's Address to a Joint Session of Congress Requesting a Declaration of War Against Germany, April 2, 1917

Woodrow Wilson's Fifth Annual Message, December 4 1917