Yep, Bittrex has only BTC/Altcoins pairs, It hasn't Altcoins/Altcoins pairs, which is something I don't understand at all why not, So as Erikalui said, you need to play with Bitcoin each time (convert to BTC and then use BTC t buy an Altcoin)
It does have altcoin to altcoin pairs.
For example, there are Ethereum and USDT markets on bittrex. You essentially are selling whatever altcoin that you have directly to Ethereum/USDT(both altcoins themselves, and in the process you are pretty much avoiding a ton of fees.
however a thing to consider is that there are very few pairs when doing alt to alt. Also, the market spread may be higher since there is a lesser amount of traders actively trading on these pairs.